Chapter 5
ChApter 5 Lipids
ChApter oBJeCtiVeS Chapter 5 is designed to allow you to:
1. List four classes of lipids (fats) and the role of each in nutritional health.
4. Explain how lipids are digested and absorbed. 2. Distinguish between fatty acids and triglycerides. 5. Name the classes of lipoproteins and classify them according to their functions.
3. Differentiate among saturated, monounsaturated, and polyunsaturated fatty acids in terms of structure and food sources.
6. List the function of lipids, including the two essential fatty acids.
7. Discuss the implications of various fats, including omega-3 fatty acids, with respect to cardiovascular disease.
8. Characterize the symptoms of cardiovascular disease and highlight some known risk factors. ChApter outLine
Chapter Objectives
Refresh Your Memory
Broader Roles for Fatty Acids and
Triglycerides in the Body
Case Study: Planning a Heart Healthy
Diet
Lipids: Common Properties
Phospholipids in the Body
Lipids: Main Types
Cholesterol in the Body
Medical Interventions to Lower Blood
Lipids
Fats and Oils in Foods
Recommendations for Fat Intake
Making Lipids Available for Body Use
Nutrition and Your Health: Lipids and
Cardiovascular Disease
Carrying Lipids in the Bloodstream
Summary/Study Questions/Check Your
Knowledge/Further Readings
Rate Your Plate
Essential Functions of Fatty Acids
YOUR
BILL FROM A MEDICAL LABORATORY READS
“BLOOD
LIPID PROFILE—
$95.”
Your doctor informs you that your “triglycerides are too high.” A health-food advertisement suggests using Benecol margarine to lower blood cholesterol.
Advertisers plug foods “lowest in saturated fat.” All of these substances— triglycerides, cholesterol, and saturated fat—are lipids, a collective term referring to fats and oils.
Lipids contain more than twice the calories per gram (on average, 9 kcal) as proteins and carbohydrates (on average, 4 kcal each). Consumption of common saturated fatty acids also contributes to the risk of cardiovascular disease. As the comic in this chapter suggests, for these reasons, some concern about certain lipids is warranted, but lipids also play vital roles both in the body and in foods. Their presence in the diet is essential to good health, and in general, lipids such as those found in vegetable oils should comprise
20% to 35% of an adult’s total calorie intake.
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Let’s look at lipids in detail—their forms, functions, metabolism, and food sources. Chapter 5 will also look at the link between various lipids and the major “killer” disease in North America: cardiovascular disease, which involves the heart, including the coronary arteries (coronary heart disease), as well as other arteries in the body.
Refresh Your Memory
As you begin your study of lipids in Chapter 5, you may want to review:
• Legal definitions for various label descriptors, such as low-fat and fat-free in
Chapter 2
• The concept of energy density in Chapter 2
• The processes of digestion and absorption in Chapter 3
• The metabolic syndrome in Chapter 4
Lipids: Common Properties
Humans need very little fat in their diet to maintain health. In fact, the body’s need for the essential fatty acids can be met by daily consumption of about 2 to 4 tablespoons of plant oil incorporated into foods and consumption of fatty fish such as salmon or tuna at least twice weekly. If fish is not consumed, the essential fatty acids in canola oil, soybean oil, and walnuts contribute much of the same health benefits as those found in fish. Thus, one could follow a purely vegetarian diet containing about 10% of calories from fat and still maintain health. However, as long as saturated fat, cholesterol, and partially-hydrogenated fat (which contains trans fat) is minimized, fat intake can safely be higher than that 10% allotment. The Food and Nutrition Board suggests that fat intake can be as high as 35% of calories consumed for an adult. Some experts sug-
High fat, low fat, no fat—which is best? And why is there such a debate? Would it be easier just to avoid fat altogether? Doesn’t a high-fat diet lead to obesity? To cardiovascular disease? Overall, which are the “best” fats, and why are French fries, doughnuts, stick margarine, and crackers getting such a bad rap? Chapter 5 provides some answers.
© 2001 Batiuk, Inc. Distributed by North American Syndicates, Inc. All rights reserved.
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Chapter 5: Lipids 163
gest that an intake as high as 40% of calories is appropriate. After learning more about lipids—fats, oils, and related compounds—in Chapter 5, you can decide for yourself how much fat you want to consume, as well as how to track your daily intake.
Lipids are a diverse group of chemical compounds. They share one main characteristic: They do not readily dissolve in water. Think of an oil and vinegar salad dressing.
The oil is not soluble in the water-based vinegar; on standing, the two separate into distinct layers, with oil on top and vinegar on the bottom.
triglyceride The major form of lipid in the body and in food. It is composed of three fatty acids bonded to glycerol, an alcohol.
glycerol A three-carbon alcohol used to form triglycerides.
phospholipid Any of a class of fat-related substances that contain phosphorus, fatty acids, and a nitrogen-containing base. The phospholipids are an essential part of every cell.
Lipids: Main Types
The chemical structure of lipids is diverse. Triglycerides are the most common type of lipid found in the body and in foods. Each triglyceride molecule consists of three fatty acids bonded to glycerol. Phospholipids and sterols, including cholesterol, are also classified as lipids, although their structures can be quite different from the structure of triglycerides. All of these lipid compounds are described in Chapter 5.
Food experts, such as chefs, call lipids that are solid at room temperature fats, and lipids that are liquid oils. Most people use the word fat to refer to all lipids because they don’t realize there is a difference. However, lipid is a generic term that includes triglycerides and many other substances. To simplify our discussion, Chapter 5 primarily uses the term fat. When necessary for clarity, the name of a specific lipid, such as cholesterol, will be used. This word use is consistent with the way many people use these terms.
sterol A compound containing a multi-ring
(steroid) structure and a hydroxyl group
(–OH). Cholesterol is a typical example.
cholesterol A waxy lipid found in all body cells. It has a structure containing multiple chemical rings that is found only in foods that contain animal products.
saturated fatty acid A fatty acid containing no carbon-carbon double bonds.
monounsaturated fatty acid A fatty acid containing one carbon-carbon double bond.
polyunsaturated fatty acid A fatty acid containing two or more carbon-carbon double bonds.
Fatty Acids: The Simplest Form of Lipids
In the body and in foods, fatty acids are found in the main form of lipids, triglycerides. A fatty acid is basically a long chain of carbons bonded together and flanked by hydrogens. At one end of the molecule (the alpha end), is an acid group. At the other end (the omega end), is a methyl group (Fig. 5-1).
Fats in foods are not composed of a single type of fatty acid. Rather, each dietary fat, or triglyceride, is a complex mixture of many different fatty acids, the combination of which provides each food its unique taste and smell.
Recall from Chapter 1 that fatty acids can be saturated or unsaturated. Chemically speaking, a carbon atom can form four bonds. Within the carbon chain of a fatty acid, the carbons bond to other carbons and to hydrogens. The carbons that make up the chain of a saturated fatty acid are all connected to each other by single bonds. This allows for the maximum number of hydrogens to be bound. Just as a sponge can be saturated (full) with water, a saturated fatty acid such as stearic acid is saturated with hydrogen (Fig. 5-1a).
As noted earlier, most fats high in saturated fatty acids, such as animal fats, remain solid at room temperature. A good example is the solid fat surrounding a piece of uncooked steak. Chicken fat, semisolid at room temperature, contains less saturated fat than beef fat. However, in some foods, saturated fats are suspended in liquid, such as the butterfat in whole milk, so the solid nature of these fats at room temperature is less apparent.
If the carbon chain of a fatty acid contains a double bond, those carbons in the chain have fewer bonds to share with hydrogen, and the chain is said to be unsaturated.
A fatty acid with only one double bond is monounsaturated (Fig. 5-1b). Canola and olive oils contain a high percentage of monounsaturated fatty acids. Likewise, if two or more of the bonds between the carbons are double bonds, the fatty acid is even less saturated with hydrogens, and so it is polyunsaturated (Fig. 5-1c, d). Corn, soybean, sunflower, and safflower oils are rich in polyunsaturated fatty acids.
Unsaturated fatty acids can exist in two different structural forms, the cis and trans forms. The form of fatty acids known as trans fatty acids was briefly described
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Saturated Fatty Acid (stearic acid)
(Methyl group)
(a)
H
(Acid group)
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Monounsaturated Fatty Acid (oleic acid; ω-9)
(b)
(c)
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Polyunsaturated Fatty Acid (alpha-linolenic acid; ω-3)
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Polyunsaturated Fatty Acid (linoleic acid; ω-6)
(d)
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O
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Figure 5-1 c Chemical forms of saturated, monounsaturated, and polyunsaturated fatty acids. Each of the depicted fatty acids contains
18 carbons, but they differ from each other in the number and location of double bonds. The linear shape of saturated fatty acids, as shown in (a), allows them to pack tightly together and so form a solid at room temperature. In contrast, unsaturated fatty acids have “kinks” where double bonds interrupt the carbon chain (see Figure 5-2). Thus, unsaturated fatty acids pack together only loosely, and are usually liquid at room temperature.
in Chapter 1. In their natural form, monounsaturated and polyunsaturated fatty acids usually are in the cis form (Fig. 5-2). By definition, the resulting cis fatty acid has the hydrogens on the same side of the carbon-carbon double bond. During certain types of food processing (discussed later in this chapter), some hydrogens are transferred to opposite sides of the carbon-carbon double bond, creating the trans form, or a trans fatty acid. As seen in Figure 5-2, the cis bond causes the fatty acid backbone to bend, whereas the trans bond allows the backbone to remain straighter.
This makes it similar to the shape of a saturated fatty acid. The Food and Nutrition
Board suggests limiting intake of trans fatty acids in processed foods (also referred to as trans fats) as much as possible. Later you will see why.
You may be surprised to learn that some trans fatty acids, known as conjugated linoleic acid (CLA), occur naturally. CLA is a family of derivatives of the fatty acid linoleic acid. The bacteria that live in the rumens of some animals (cows, sheep, and goats, for example) produce trans fatty acids that eventually appear in foods such as beef, milk, and butter. These naturally occurring trans fats are currently under study for possible health benefits, including prevention of cancer, decreasing body fat, and improvement in insulin levels in diabetics. About 20% of trans fatty acids in our diets come from this source,
Dietary supplements of CLA are available but are highly variable in their quality.
Overall, a fat or an oil is classified as saturated, monounsaturated, or polyunsaturated based on the type of fatty acids present in the greatest concentra-
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Chapter 5: Lipids 165
cis fatty acid A form of an unsaturated fatty acid that has the hydrogens lying on the same side of the carbon-carbon double bond. trans fatty acid A form of an unsaturated
Cis form
(Causes
backbone of molecule to bend) fatty acid, usually a monounsaturated one when found in food, in which the hydrogens on both carbons forming the double bond lie on opposite sides of that bond.
Trans form
long-chain fatty acid A fatty acid that contains 12 or more carbons.
omega-3 (ω-3) fatty acid An unsaturated fatty acid with the first double bond on the third carbon from the methyl end (—CH3).
omega-6 (ω-6) fatty acid An unsaturated fatty acid with the first double bond on the sixth carbon from the methyl end (—CH3).
alpha-linolenic acid An essential omega-3 fatty acid with 18 carbons and three double bonds. Elaidic acid
Oleic acid
linoleic acid An essential omega-6 fatty
FiguRe 5-2 c Cis and trans fatty acids. In the cis form at carbon-carbon double bonds in a
acid with 18 carbons and two double bonds.
fatty acid, the hydrogens (in blue) lie on the same side of the double bond. This causes a “kink” at that point in the fatty acid, typical of unsaturated fatty acids in nature. In contrast, in the trans form at carbon-carbon double bonds in a fatty acid, the hydrogens lie across from each other at the double bond. This causes the fatty acid to exist in a linear form, like a saturated fatty acid.
Cis fatty acids are much more common in foods than trans fatty acids. The latter are primarily found in foods containing partially-hydrogenated fats, notably stick margarine, shortening, and deep-fat fried foods.
essential fatty acids Fatty acids that must
tion (Fig. 5-3). Fats in foods that contain primarily saturated fatty acids are solid at room temperature, especially if the fatty acids have long carbon chains (i.e., a long-chain fatty acid), as opposed to shorter versions. In contrast, fats containing primarily polyunsaturated or monounsaturated fatty acids (long chain or shorter) are usually liquid at room temperature. Almost all fatty acids in the body and in foods are long-chain varieties.
An important characteristic of unsaturated fatty acids is the location of the double bonds. If the first double bond starts three carbons from the methyl (omega) end of the fatty acid, it is an omega-3 (ω-3) fatty acid (review Fig. 5-1c). If the first double bond is located six carbons from the omega end, it is an omega-6 (ω-6) fatty acid (review Fig. 5-1d). Following this scheme, an omega-9 fatty acid has its first double bond starting at the ninth carbon from the methyl end (review Fig. 5-1b). In foods, alphalinolenic acid is the major omega-3 fatty acid; linoleic acid is the major omega-6 fatty acid. These are also the essential fatty acids we need to consume (more on this in the later section on putting lipids to work in the body). Oleic acid is the major omega-9 fatty acid.
Triglycerides
Fats and oils in foods are mostly in the form of triglycerides. The same is true for fats found in body structures. Although some fatty acids are transported in the bloodstream attached to proteins, most fatty acids are formed into triglycerides by cells in the body.
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be supplied by the diet to maintain health.
Currently, only linoleic acid and alphalinolenic acid are classified as essential.
oleic acid An omega-9 fatty acid with
18 carbons and one double bond.
m Plant oils vary in their content of specific
fatty acids. Oils similar in appearance may vary significantly in fatty acid composition. Olive and canola oils are rich in monounsaturated fat; olive oil has been awarded much attention in recent years. Canola oil, however, is a much less expensive choice of monounsaturated fat. Safflower oil is rich in polyunsaturated fat.
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Rich Sources of :
Saturated
Fatty Acids
Monounsaturated
Fatty Acids
Primarily Trans
Fatty Acids
Polyunsaturated
Fatty Acids
Saturated Fatty Acids
Coconut oil
**
Butter
Palm oil
Lard or beef fat
Monounsaturated
Fatty Acids
Olive oil
Canola oil*
Peanut oil
Soybean oil*
Polyunsaturated
Fatty Acids
Safflower oil
Sunflower oil
Corn oil
Trans Fatty Acids
Tub margarine
Stick margarine
Shortening
0
20
40
60
80 100 0
20
40
60
80 100 0
20
40
60
80 100 0
20
40
60
80 100
% of all Fatty Acids in the Product
*Rich source of the omega-3 fatty acid alpha-linolenic acid (7% and 12% of total fatty acid for soybean oil and canola oil, respectively).
**The natural trans fatty acids in butter are not harmful and may even have health-promoting properties, such as preventing certain forms of cancer.
Figure 5-3 c Saturated, monounsaturated, polyunsaturated, and trans fatty acid composition of common fats and oils (expressed as % of all fatty acids in the product).
diglyceride A breakdown product of a triglyceride consisting of two fatty acids bonded to a glycerol backbone.
monoglyceride A breakdown product of a triglyceride consisting of one fatty acid attached to a glycerol backbone.
lecithin A group of compounds that are major components of cell membranes.
As noted before, triglycerides contain a simple three-carbon alcohol, glycerol, which serves as a backbone for the three attached fatty acids (Fig. 5-4a). Removing one fatty acid from a triglyceride forms a diglyceride. Removing two fatty acids from a triglyceride forms a monoglyceride. Later you will see that before most dietary fats are absorbed in the small intestine, the two outer fatty acids are typically removed from the triglyceride. This produces a mixture of fatty acids and monoglycerides, absorbed into the intestinal cells. After absorption, the fatty acids and monoglycerides are mostly re-formed into triglycerides.
Phospholipids
Phospholipids are another class of lipid. Like triglycerides, they are built on a backbone of glycerol. However, at least one fatty acid is replaced with a compound containing phosphorus (and often other elements, such as nitrogen) (Fig. 5-4b). Many types of phospholipids exist in the body, especially in the brain. They form important parts of cell membranes. Lecithin is a common example of a phospholipid. Various forms are found in body cells and they participate in fat digestion in the small intes-
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Chapter 5: Lipids 167
H
H
O
Figure 5-4 c Chemical forms of com-
O
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fatty acid
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fatty acid
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fatty acid
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fatty acid
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Glycerol
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mon lipids: (a) triglyceride, (b) phospholipid
(in this case, lecithin), and (c) sterol (in this case, cholesterol).
Choline
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CH3
C
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CH3
CH3
Glycerol
Phosphate
Triglyceride
Lecithin—A Phospholipid
(a)
(b)
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Cholesterol—A Sterol
(c)
tine. The body is able to produce all the phospholipids it needs. Even though lecithin is sold as a dietary supplement and is present as an additive in many foods, phospholipids such as this one are not essential components of the diet.
Sterols
Sterols are the last class of lipids Chapter 5 covers. Their characteristic multi-ringed structure makes them different from the other lipids already discussed (Fig. 5-4c). The most common example of a sterol is cholesterol. This waxy substance doesn’t look like a triglyceride—it doesn’t have a glycerol backbone or any fatty acids. Still, because it doesn’t readily dissolve in water, it is a lipid. Among other functions, cholesterol is used to form certain hormones and bile acids and is incorporated into cell structures.
The body can make all the cholesterol it needs.
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Concept Check
Lipids are a group of compounds that do not dissolve readily in water. Included in this group are fatty acids, triglycerides, phospholipids, and sterols. Fatty acids can be distinguished from one another by the length of the carbon skeleton and the number and position of double bonds along that skeleton. Saturated fatty acids contain no double bonds within their carbon skeleton; that is, they are fully saturated with hydrogens. Monounsaturated fatty acids contain one carbon-carbon double bond, and polyunsaturated fatty acids contain two or more of these bonds. Certain omega-3 and omega-6 fatty acids are essential in the human diet.
Triglycerides are the major form of fat in the body and in foods. These consist of three fatty acids bonded to a glycerol backbone. Phospholipids are similar to triglycerides in structure, but at least one fatty acid is replaced by another compound containing phosphorus. Phospholipids play an important structural role in cell membranes. Sterols, another class of lipids, do not resemble either triglycerides or phospholipids, but instead have a multi-ringed structure.
Cholesterol, one example of a sterol, forms parts of cells, some hormones, and bile acids.
Whereas certain essential fatty acids (as components of triglycerides) are needed in the diet, the body produces all the triglycerides, phospholipids, and cholesterol it needs. Next we will discuss the sources of fat in foods (Fig. 5-5).
MyPyramid: Sources of Fats
Food Group
Sources of Fat
Grams per Serving
Grains
• Crackers
• Pasta dishes with added fat
0 –18
Vegetables
• French fried potatoes 0 –27
Fruits
• Fruit pies
• Avocados
0 –11
Oils
• All
12 –14
Milk
• Whole milk
• Some yogurts
• Many cheeses
• Premium ice cream 0 –10
Meat & Beans
• Marbled meat
• Bacon
• Poultry (skin)
• Deep-fat fried meat • Nuts
7 –17
Figure 5-5 c Sources of fats from MyPyramid. The height of the background color (none, 1/3, 2/3, or completely covered) within each group in the pyramid indicates the average nutrient density for fat in that group. The fruit group and vegetable groups are generally low in fat. In the other groups, both high-fat and low-fat choices are available. Careful reading of food labels can help you choose lower-fat versions of some foods. In general, any type of frying adds significant amounts of fat to a product, as with French fries and fried chicken. With regard to physical activity, fats are a key fuel in prolonged events, such as long-distance cycling, long-distance running, and prolonged slow to brisk walking.
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Chapter 5: Lipids 169
Fats and Oils in Foods
Lipids in the form of triglycerides are abundant in the North American diet. The foods highest in fat (and therefore energy dense) include salad oils and spreads such as butter, margarine, and mayonnaise. All of these foods contain close to
100% of calories as fat. In reduced-fat margarines, water replaces some of the fat.
Whereas regular margarines are 80% fat by weight (11 grams per tablespoon), some reduced-fat margarines are as low as 30% fat by weight (4 grams per tablespoon). When used in recipes, the extra water added to these margarines can cause texture and volume changes in the finished product. Cookbooks can provide guidance for appropriate use of these products by suggesting alterations in recipes to compensate for the increased water content.
Still considering the overall fat content, whole foods highest in fat include nuts, bologna, avocados, and bacon, which have about 80% of calories as fat. Next, peanut butter and cheddar cheese have about 75%. Marbled steak and hamburgers
(ground chuck) have about 60%, and chocolate bars, ice cream, doughnuts, and whole milk have about 50% of calories as fat. Eggs, pumpkin pie, and cupcakes have 35%, as do lean cuts of meat, such as top round (and ground round) and sirloin. Bread contains about 15%. Finally, foods such as cornflakes, sugar, and fat-free milk have essentially no fat. Careful label reading is necessary to determine the true fat content of food—these are only rough guidelines (Fig. 5-6).
m The North American diet contains many high-fat foods—
including typical cookie choices. Portion control with these foods is thus important, especially if one is trying to control calorie intake.
Figure 5-6 c Food sources of fat com-
Food Sources of Fat
Food Item
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Fat
(grams)
Calories from Fat %
% AHA
Recommendation
AHA
Recommendation
70
30%
T-bone steak, 3 ounces
17
66%
24%
Mixed nuts, 1 ounce
16
78%
23%
Canola oil, 1 tablespoon
14
100%
Hamburger with bun,
1 each
12
39%
17%
Margarine, 1 tablespoon
12
100%
17%
Avocado, 1⁄2 cup
11
86%
16%
Cheddar cheese, 1 ounce
10
74%
14%
Whole milk, 1 cup
8
49%
11%
Chicken breast with skin, 3 ounces
7
36%
10%
Whole milk yogurt,
8 ounces
7
28%
10%
Snack crackers, 1 ounce
7
45%
10%
Baked beans, 1⁄2 cup
7
31%
10%
M&M chocolate candies, 1 ounce
6
39%
9%
Flax seeds, 1 tablespoon
3
62%
4%
Fig Newton cookies,
2 each
3
23%
4%
100%
pared to the American Heart Association
(AHA) recommendation of 70 grams per day, or 30% of calories from fat for a
2100 kcal diet.
20%
Key:
Grains
Vegetables
Fruits
Oil
Milk
Meat & Beans
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m Peanuts are a source of lecithins, as are wheat
germ and egg yolks.
emulsifier A compound that can suspend fat in water by isolating individual fat droplets, using a shell of water molecules or other substances to prevent the fat from coalescing.
The type of fat in food is important to consider along with the total amount of fat.
Animal fats are the chief contributors of saturated fatty acids to the North American diet. About 40% to 60% of total fat in dairy and meat products is in the form of saturated fatty acids. In contrast, plant oils contain mostly unsaturated fatty acids, ranging from 73% to 94% of total fat. A moderate to high proportion of total fat (49% to 77%) is supplied by monounsaturated fatty acids in canola oil, olive oil, and peanut oil. Some animal fats are also good sources of monounsaturated fatty acids (30% to 47%) (review Fig. 5-3). Corn, cottonseed, sunflower, soybean, and safflower oils contain mostly polyunsaturated fatty acids (54% to 77%). These plant oils supply the majority of the linoleic and alpha-linolenic acid in the North American food supply.
Wheat germ, peanuts, egg yolk, soy beans, and organ meats are rich sources of phospholipids. Phospholipids such as lecithin, a component of egg yolks, are often added to salad dressing. Lecithin is used as an emulsifier in these and other products because of its ability to keep mixtures of lipids and water from separating (see Fig. 5-7). Emulsifiers are added to salad dressings to keep the vegetable oil suspended in water. Eggs added to cake batters likewise emulsify the fat with the milk.
Cholesterol is found only in animal foods (Table 5-1). An egg yolk contains about 210 milligrams of cholesterol. Eggs are our main dietary source of cholesterol, along with meats and whole milk. Manufacturers who advertise their brand of peanut butter, vegetable shortening, margarines, and vegetable oils as “cholesterol-free” are taking advantage of uninformed consumers—all of these products are naturally cholesterol-free. Some plants contain other sterols similar to cholesterol, but they do not pose the heart health risks associated with cholesterol. In fact, some plant sterols have blood cholesterollowering properties (see the later section on medical interventions to lower blood lipids). Fat Is Hidden in Some Foods
Some fat discussed so far is obvious: butter on bread, mayonnaise in potato salad, and marbling in raw meat. Fat is harder to detect in other foods that also contribute significant amounts of fat to our diets. Foods that contain hidden fat include whole milk, pastries, cookies, cake, cheese, hot dogs, crackers, French fries, and ice cream.
Figure 5-7 c Emulsifiers in action.
Emulsifiers prevent many brands of salad dressings and other condiments from separating into layers of water and fat.
Emulsifiers attract fatty acids inside and have a water-attracting group on the outside. Add them to salad dressing, shake well, and they hold the oil in the dressing away from the water. Emulsification is important in both food production and fat digestion/absorption. Emulsifiers and Agitation in Salad Dressing
Water
Emulsified fat
Oil
Water
Oil droplet attracted to fatty acid core
Water attracted to shell
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Chapter 5: Lipids 171
Table 5-1 Cholesterol Content of Selected Foods in Ascending Order Food
Amount
Skim milk
1 cup
Mayonnaise
Cholesterol in Milligrams
Food
Amount
Cholesterol in Milligrams
4
Oysters, salmon
3 ounces
40
1 tablespoon
10
Clams, halibut, tuna
3 ounces
55
Butter
1 pat
11
Chicken, turkey* (white meat)
3 ounces
70
Lard
1 tablespoon
12
Beef,* pork
3 ounces
75
Cottage cheese
½ cup
15
Lamb, crab
3 ounces
85
Low-fat milk (2%)
1 cup
22
Shrimp, lobster
3 ounces
110
Half-and-half
¼ cup
23
Heart (beef)
3 ounces
165
Hot dog*
1
29
Egg (egg yolk)*†
1
210
Ice cream, ∼10% fat
½ cup
30
Liver (beef)
3 ounces
410
Cheese, cheddar
1 ounce
30
Kidney
3 ounces
540
Whole milk
1 cup
34
Brains
3 ounces
2640
*Leading contributors of cholesterol to the North American diet.
Egg whites are cholesterol-free.
†
When we try to cut down on fat intake, hidden fats need to be considered, along with the more obvious sources.
A place to begin searching for hidden fat is on the Nutrition Facts labels of foods you buy. Some signals from the ingredient list that can alert you to the presence of fat are animal fats, such as bacon, beef, ham, lamb, pork, chicken, and turkey fats; lard; vegetable oils; nuts; dairy fats, such as butter and cream; egg and egg-yolk solids; and partially-hydrogenated shortening or vegetable oil. Conveniently, the label lists ingredients by order of weight in the product. If fat is one of the first ingredients listed, you are probably looking at a high-fat product. Use food labels to learn more about the fat content of the foods you eat (Fig. 5-8).
Figure 5-8 c Reading labels helps lo-
Nutrition Facts
cate hidden fat. Who would think that wieners (hot dogs) can contain about 85% of food calories as fat? Looking at the hot dog does not suggest that almost all of its food calories comes from fat, but the label shows otherwise. Let’s do the math: 13 grams total fat × 9 kcal per gram of fat = 120 kcal from fat; 120 kcal/140 kcal per link = 0.84 or 84% kcal from fat.
Serving Size 1 Link (45g)
Servings Per Container 10
Amount Per Serving
Calories 140 Calories from Fat 120
% Daily Value*
Total Fat 13g
20%
Saturated Fat 5g
23%
Trans Fat 0g
**
Cholesterol 20mg
7%
Sodium 420mg
17%
1%
Total Carbohydrate 2g
Dietary Fiber 0g
0%
Sugars 1g
Protein 5g
Vitamin A 0% • Vitamin C 0%
Calcium 0% • Iron 2%
*Percent Daily Values are based on a 2,000 calorie diet.
**Intake of trans fat should be as low as possible. war43742_ch05_160_201.indd 171
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Definitions for Nutrient Claims About Fat and Cholesterol on Food Labels
Fat
• Fat free: less than 0.5 g of fat per serving
• Saturated fat free: less than 0.5 g per serving, and the level of trans fatty acids does not exceed 0.5 g per serving
• Low fat: 3 g or less per serving and, if the serving is 30 g or less or 2 tablespoons or less, per 50 g of the food. 2% milk can no longer be labeled low fat, as it exceeds 3 g per serving. Reduced fat will be the term used instead.
• Low saturated fat: 1 g or less per serving and not more than 15% of kcal from saturated fatty acids
• Reduced or less fat: at least 25% less per serving than reference food
• Reduced or less saturated fat: at least
25% less per serving than reference food
Cholesterol
• Cholesterol free: less than 2 milligrams
(mg) of cholesterol and 2 g or less of saturated fat per serving
• Low cholesterol: 20 mg or less cholesterol and 2 g or less of saturated fat per serving and, if the serving is 30 g or less or
2 tablespoons or less, per 50 g of the food
• Reduced or less cholesterol: at least 25% less cholesterol and 2 g or less of saturated fat per serving than reference food
The definitions for various fat descriptors on food labels, such as “low-fat,” “fat-free,” and “reduced-fat,” were listed in Table 2-9 in Chapter 2 and are reprinted in the margin here. Recall that “low-fat” indicates, in most cases, that a product contains no more than
3 grams of fat per serving. Products marketed as “fat-free” must have less than one-half of a gram of fat per serving. A claim of “reduced-fat” means the product has at least 25% less fat than is usually found in that type of food. When there is no Nutrition Facts label to inspect, controlling portion size is a good way to control fat intake.
When many North Americans think of a low-fat diet, they include reduced-fat versions of pastries, cookies, and cakes. When health professionals refer to a low-fat diet, they often have a different plan in mind: focusing primarily on fruits, vegetables, and whole-grain breads and cereals. Whether to choose a fat-rich food should depend on how much fat you have eaten or will eat during that particular day. So, if you plan to eat high-fat foods at your evening meal, you could reduce your fat intake at a previous meal to balance overall fat intake for the day.
Fat in Food Provides Some Satiety, Flavor, and Texture
Fat in foods has generally been considered to be the most satiating of all the macronutrients. However, studies show that protein and carbohydrate probably lead to the most satiety (gram for gram). High-fat meals do provide satiety, but primarily because one consumes a lot of calories in the process. A high-fat meal is likely to be a high-calorie meal.
Various fats play important roles in foods, so much ingenuity must go into the production of reduced-fat products to preserve flavor and texture. In some cases, “fat-free” also means tasteless. Fat components in foods provide important textures and carry flavors. If you’ve ever eaten a high-fat yellow cheese or cream cheese, you probably agree that fat melting on the tongue feels good. The fat in reduced-fat and whole milk also gives body, which fat-free milk lacks. The most tender cuts of meat are high in fat, visible as the marbling of meat. In addition, many flavorings dissolve in fat. Heating spices in oil intensifies the flavors of an Indian curry or a Mexican dish, carried to the sensory cells that discriminate taste and smell in the mouth.
Making Decisions
Low-Fat Diets
A person who has been following a typical North American diet will probably need some time to adjust to the taste of a lower-fat diet. Emphasizing flavorful fruits, vegetables, and whole grains will help one to adapt to a low-fat diet. Interestingly, after an adjustment period, higher-fat foods may not be as palatable or may lead to gastrointestinal discomfort. For example, after switching from whole to 1% low-fat milk for a few weeks, whole milk begins to taste more like cream than milk. It is certainly possible to make the change from a higher-fat diet to a lower-fat diet. The benefits of weight control and reduced risk for several chronic diseases make the adjustment worth the effort.
Wise Use of Reduced-Fat Foods Is Important
m Fat is an important component of the flavor
and overall appeal of cheese.
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Manufacturers have introduced reduced-fat versions of numerous food products. The fat content of these alternatives ranges from 0% in fat-free Fig Newtons to about 75% of the original fat content in other products. However, the total calorie content of most fat-reduced products is not substantially lower than that of their conventional versions. Generally, when fat is removed from a product, something must be added— commonly, sugars—in its place. It is difficult to reduce both the fat and sugar contents of a product at the same time and maintain flavor and texture. For this reason, many reduced-fat products (e.g., cakes and cookies) are still energy dense. Use the Nutrition
Facts label to choose the portion size with the desired calories.
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Chapter 5: Lipids 173
Fat Replacement Strategies for Foods
CRitiCaL thinking
Five types of fat replacements are available to food manufacturers. Addition of these substances during manufacturing yields products that, to varying degrees, satisfy consumers’ desire for reduced-fat products that are still tasty.
Water, starch derivatives, and Fibers. The simplest fat replacement is water. The addi-
tion of water yields a product, such as diet margarine, with less fat per serving than the normal product. Starch derivatives that bind water to form a gel are a second type of fat replacement. The resulting gel replaces some of the mouth feel lost by the removal of fat.
Starch derivatives are made from the hulls or bran of various plants, including oats, peas, soybeans, rice, corn, and wheat. Starch derivatives commonly used by food manufacturers include the fiber cellulose, Maltrin, Stellar, Z-trim, and Oatrim. These substances are used in a variety of foods, including luncheon meats, salad dressings, frozen desserts, table spreads, dips, baked goods, and candies. Most starch derivatives contain some calories, but at least half the amount in fat. These starch derivatives cannot be used in fried foods.
Gum fiber extracted from plants can also be used to replace fat. This thickens a product and replaces some of the body that fat provides. Diet salad dressings and fatreduced ice cream have gums added for this reason.
Allison has decided to start eating a lowfat diet. Allison has mentioned to you that all she needs to do is add less butter, oil, or margarine to her foods and she will dramatically lower her fat intake. How can you explain to Allison that she needs to be aware of the hidden fats in her diet as well?
protein-derived Fat Replacements. Both egg and milk proteins can be used as fat
replacements. These proteins are treated to produce microscopic, mistlike protein globules. When these substances replace fat in a food product, they feel like fat in the mouth, although the product does not contain any fatty acids. These proteins are used in milk and other dairy products, baked goods, frostings, salad dressings, and mayonnaise-type products. Such fat replacements yield some calories—but only about 1 to 2 kcal per gram. They have this low-calorie value for two reasons: Proteins contain only 4 kcal per gram, and the products have a high water content. engineered Fats and Related products. The newest form of fat replacement is en-
gineered fat. This type of product is synthesized in the laboratory from various food constituents. Olestra (Olean) is a good example. It is made by chemically bonding fatty acids to sucrose (table sugar). The resulting product cannot be digested by either the human digestive enzymes or bacteria that live in the intestinal tract. Therefore, olestra cannot be absorbed and so provides no calories for the body.
Olestra can replace much of the fat in salad dressing and cakes. It was the first fat replacement that could function like fat in fried foods. Olestra is approved by FDA for use in fried snack foods, such as potato chips.
The major problem associated with the use of olestra is that it binds the fatsoluble vitamins A, D, E, and K, and other fat-soluble phytochemicals such as the carotenoids, thus reducing their absorption. To compensate, food manufacturers add these vitamins to food products containing olestra. Other suspected problems, such as GI tract discomfort, have not been supported by careful research. Thus, warnings about use of olestra and GI tract disturbances, which used to be required on labels for olestra-containing foods, are no longer mandatory.
Food manufacturers are working on other types of engineered fats, which either wholly or partially escape absorption by the body. One example is salatrim, marketed under the name
Benefat, which yields only about 5 kcal per gram. It is composed of some saturated fatty acids poorly absorbed by the body. This product has been used in reduced-fat chocolate.
So far, fat replacements have had little impact on our diets, partly because the approved forms either are not very versatile or have not been used extensively by manufacturers. The public has shown little interest in the use of fat replacers, such as olestra. In addition, fat replacements are not practical to use in the foods that contribute the greatest quantity of fat to our diets—beef, cheese, whole and reduced-fat milk, and pastries.
The main benefit of fat replacements will be in helping us cut some fat and calories from our diets. The reduction in overall calorie intake, however, will probably be small because we tend to compensate for the fewer calories per serving by eating more servings.
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m Fat replacements such as gum fiber are typi-
cally seen in soft serve ice cream.
c Canada has not approved the use of olestra in food products; the United States is the sole country that permits the use of this fat substitute in foods.
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Concept Check
Fat-dense foods—those with more than 60% of total calories as fat—include plant oils, butter, margarine, mayonnaise, nuts, bacon, avocados, peanut butter, cheddar cheese, steak, and hamburger. Of the foods we typically eat, cholesterol is found naturally only in those of animal origin, with eggs being a primary source. Emulsifiers, such as the phospholipids, lecithins, are added to salad dressings and other fat-rich products to keep the vegetable oils and other fats suspended in the water. Hidden fat exists in foods such as whole milk, pastries, cookies, cake, cheese, hot dogs, crackers, French fries, and ice cream. Fat has a variety of roles in foods, including that of contributing to flavor and texture. Fat also provides the pleasurable mouth feel of many of our favorite foods, intensifies the taste of many spices, and tenderizes many popular cuts of meat. Fat free doesn’t mean calorie free; moderation in the use of reduced-fat products is still important.
Fat Rancidity Limits Shelf Life of Foods
BHA, BHT Butylated hydroxyanisole and butylated hydroxytoluene—two common synthetic antioxidants added to foods.
hydrogenation The addition of hydrogen to a carbon-carbon double bond, producing a single carbon-carbon bond with two hydrogens attached to each carbon. Hydrogenation of unsaturated fatty acids in a vegetable oil increases its hardness, so this process is used to convert liquid oils into more solid fats, used in making margarine and shortening. Trans fatty acids are a by-product of hydrogenation of vegetable oils.
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Decomposing oils emit a disagreeable odor and taste sour and stale. Stale potato chips are a good example. The double bonds in unsaturated fatty acids break down, producing rancid by-products. Ultraviolet light, oxygen, and certain procedures can break double bonds and, in turn, destroy the structure of polyunsaturated fatty acids. Saturated fats and trans fats can much more readily resist these effects because they contain fewer carbon-carbon double bonds.
Rancidity is not a major problem for consumers because the odor and taste generally discourage us from eating enough to become sick. However, rancidity is a problem for manufacturers because it reduces a product’s shelf life. To increase shelf life, manufacturers often add partially-hydrogenated plant oils to products. Foods most likely to become rancid are deep-fried foods and foods with a large amount of exposed surface. The fat in fish is also susceptible to rancidity because it is highly polyunsaturated.
Antioxidants such as vitamin E help protect foods against rancidity by guarding against fat breakdown. The vitamin E naturally occuring in plant oils reduces the breakdown of double bonds in fatty acids. When food manufacturers want to prevent rancidity in polyunsaturated fats, they often add the synthetic antioxidants BHA and
BHT or vitamin C to products that contain fat such as salad dressings and cake mixes.
Manufacturers also tightly seal products and use other methods to reduce oxygen levels inside packages.
Hydrogenation of Fatty Acids in Food Production
Increases Trans Fatty Acid Content
As mentioned previously, most fats with long-chain saturated fatty acids are solid at room temperature, and those with unsaturated fatty acids are liquid at room temperature. In some kinds of food production, solid fats work better than liquid oils.
In pie crust, for example, solid fats yield a flaky product, whereas crusts made with liquid oils tend to be greasy and more crumbly. If oils with unsaturated fatty acids are used to replace solid fats, they often must be made more saturated (with hydrogen), as this solidifies the vegetable oils into shortenings and margarines. Hydrogen is added by bubbling hydrogen gas under pressure into liquid vegetable oils in a process called hydrogenation (Fig. 5-9). The fatty acids aren’t fully hydrogenated to the saturated fatty acid form, as this would make the product too hard and brittle.
Partial hydrogenation—leaving some monounsaturated fatty acids—creates a semisolid product.
The process of hydrogenation produces trans fatty acids as was described earlier in this chapter. Most natural monounsaturated and polyunsaturated fatty acids exist in the cis form, causing a bend in the carbon chain, whereas the straighter carbon forms of trans fat more closely resemble saturated fatty acids. This may be
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Chapter 5: Lipids 175
FiguRe 5-9 c How liquid oils become
(a)
solid fats. (a) Unsaturated fatty acids are present in liquid form. (b) Hydrogens are added (hydrogenation), changing some carbon-carbon double bonds to single bonds and producing some trans fatty acids. (c) The partially-hydrogenated product is likely to be used in margarine, shortening, or for deep-fat frying.
Hydrogen source (b)
(c)
Unsaturated vegetable oil (liquid)
Adding hydrogen under pressure
Partially-hydrogenated fat
(semisolid)
the mechanism whereby trans fat increases the risk for heart disease. Studies also indicate that trans fats increase overall inflammation in the body, which is not healthful. Thus, people should limit intake of partially-hydrogenated fat and thus trans fat. This may not be such a concern for the average person, as long as trans fat intake is not excessive and the diet is adequate in polyunsaturated fat. However, because trans fatty acids serve no particular role in maintaining body health, the latest Dietary Guidelines for Americans, the American Heart Association, and the
Food and Nutrition Board each recommend minimal trans fat intake.
Not so long ago, public pressure persuaded manufacturers to eliminate the tropical oils rich in saturated fat (palm, palm olein, and coconut) from food processing. Partially-hydrogenated soybean oil—rich in trans fat—became the major replacement. Currently, trans fat intake in North America is estimated to contribute about 3% to 4% of total calories, amounting to 10 grams per day, on average. Table 5-2 lists typical sources.
FDA is now requiring the trans fat content of foods on food labels (review
Fig 5-8). The food labels in Canada also must list trans fat content. FDA hopes to make consumers more aware of the amounts of trans fat in foods as well as the negative health consequences associated with their excessive consumption. North American companies are already responding to this issue by creating products free of trans fat. For example, Promise, Smart Beat, and some
Fleischmann’s margarines are lower in or free of trans fat (less than 0.5 grams per serving) compared to typical margarines.
This addition of the trans fat listing on labels helps consumers at the supermarket, but when dining out, consumers are “left in the dark” as to which foods contain trans fat. Knowing which foods are low in trans fat when ordering at a restaurant is difficult because information about preparation methods and precise fat composition is rarely available. To minimize trans fat intake, a general guideline is to limit consumption of
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m Tub margarine is much lower in trans fat than
stick margarine or shortenings. Some brands of tub margarines are even free of trans fatty acids.
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tabLe 5-2 Total Fat, Saturated Fat, and Trans Fat Content of Typical Sources of
Trans Fat (in descending order of trans fat) Food Item
Serving
Size
Kcal
Fat (grams)
Saturated
Fat (grams)
Trans Fat (grams)
French fried potatoes (fast-food variety)
Medium size
540
26.9
6.7
7.8
Doughnut
1
330
18.2
4.7
5.0
Cake, pound
1 slice
290
16.4
3.4
4.3
Shortening
1 tbsp
110
13.0
3.4
4.2
Potato chips
Small bag
170
11.2
1.9
3.2
Margarine, stick
1 tbsp
100
11.0
2.1
2.8
Cookies (cream-fi lled)
3
150
6.1
1.2
1.9
Margarine, tub
1 tbsp
60
6.7
1.2
0.6
Butter
1 tbsp
100
10.8
7.2
0.3
Milk, whole
1 cup
140
6.6
4.3
0.2
100
10.8
1.6
0
Mayonnaise (soybean oil) 1 tbsp
The fi ve major sources of trans fat are (in order); cakes, cookies, crackers, pies, and bread; margarine; fried potatoes; potato chips, corn chips, and popcorn; and shortening used in the home.
Source: http://www.cfsan.fda.gov/~dms/qatrans2.html
fried (especially deep-fat fried) food items, any pastries or flaky bread products (such as pie crusts, crackers, croissants, and biscuits), and cookies.
Until all foods are labeled with trans fat content, consumers can also make educated guesses on the trans fat content of foods by examining the list of ingredients on the food label. If partially-hydrogenated vegetable oil is one of the first three ingredients on the label, you can assume there is a significant amount of trans fat in the product.
Limiting trans fat at home is a much easier task. Most importantly, use little or no stick margarine or shortening. Instead, substitute vegetable oils and softer tub margarines (whose labels list vegetable oil or water as the first ingredient). Avoid deep-fat frying any food in shortening. Substitute baking, panfrying, broiling, steaming, grilling, or deep-fat frying in unhydrogenated vegetable oils. Replace nondairy creamers with reduced-fat or fat-free milk, since most nondairy creamers are rich in partiallyhydrogenated vegetable oils. Finally, read the ingredients on food labels, using the previous tips to estimate trans fat content.
ConCept CheCk
m French fries and other fried foods are a com-
mon source of fat and trans fatty acids for many adults. For those who choose to consume these products on a regular basis, a small serving size is recommended, especially if a person has elevated blood lipids.
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Hydrogenation of unsaturated fatty acids is the process of adding hydrogen to carbon-carbon double bonds to produce single bonds. This results in the creation of some trans fatty acids.
Hydrogenation changes vegetable oil to solid fat. It is wise to monitor trans fat intake, as this form of fat increases the risk for heart disease.
The carbon-carbon double bonds in polyunsaturated fatty acids are easily broken, yielding products responsible for rancidity. The presence of antioxidants, such as vitamin E in oils, naturally protects unsaturated fatty acids against oxidative destruction. Manufacturers can use hydrogenated fats and add natural or synthetic antioxidants to reduce the likelihood of rancidity.
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Making Lipids Available for Body Use
It’s no secret that fats and oils make foods more appealing. Their presence in foods adds flavor, lubrication, and texture. What happens to lipids once they are eaten?
Let’s take a closer look at the digestion, absorption, and physiological roles of lipids in the body.
Digestion
In the first phase of fat digestion, the stomach (and salivary glands to some extent) secretes lipase. This enzyme acts primarily on triglycerides that have fatty acids with short chain lengths, such as those found in butterfat. The action of salivary and stomach lipase, however, is usually dwarfed by that of the lipase enzyme released from the pancreas and active in the small intestine. Triglycerides and other lipids found in common vegetable oils and meats have longer chain lengths and are generally not digested until they reach the small intestine (Fig. 5-10).
lipase Fat-digesting enzyme produced by the salivary glands, stomach, and pancreas.
Fat Digestion and Absorption
1
2
2
1
Stomach
Liver
3
3
Pancreas
4
4
Small intestine 5 Anus
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5
Figure 5-10 c A summary of fat digestion and absorption.
Chapter 3 covered general aspects of this process.
Only minor digestion of fat takes place in the stomach through the action of lipase enzymes.
The liver produces bile,stored in the gallbladder and released through the bile duct into the small intestine. Bile aids in fat digestion and absorption by emulsifying lipids in the digestive juices. The pancreas secretes a mixture of enzymes, including lipase, into the small intestine.
The small intestine is the primary site for digestion and absorption of lipids. Once absorbed, long-chain fatty acids are packaged for transport through the lymph and bloodstream. (Shorter-chain fatty acids are absorbed directly into portal circulation.)
Less than 5% of ingested fat is normally excreted in the feces.
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c If the gall bladder is surgically removed
(e.g., in cases of gall stone formation), bile will enter the small intestine directly from the liver.
In the small intestine, triglycerides are broken down by lipase into smaller products, namely monoglycerides (glycerol backbones with a single fatty acid attached) and fatty acids. Under the right circumstances, digestion is rapid and thorough. The
“right” circumstances include the presence of bile from the gallbladder. Bile acids present in the bile act as emulsifiers on the digestive products of lipase action, suspending the monoglycerides and fatty acids in the watery digestive juices. This emulsification improves digestion and absorption because as large fat globules are broken down into smaller ones, the total surface area for lipase action increases.
With regard to phospholipid digestion, certain enzymes from the pancreas and cells in the wall of the small intestine digest phospholipids. The eventual products are glycerol, fatty acids, and remaining parts. With regard to cholesterol digestion, any cholesterol with a fatty acid attached is broken down to free cholesterol and fatty acids by certain enzymes released from the pancreas.
Making Decisions
Bile Acids
Gallbladder
Substances sent from the liver to the small intestine
Liver
as ch d su an d ces
S u b st a n blo o into Enterohepatic
Circulation
b se ile a nt c ba id s r ea ck to t b s o r b e d he li ver Small intestine During meals, bile acids circulate in a path that begins in the liver, goes on to the gallbladder, and then moves to the small intestine. After participating in fat digestion, most bile acids are absorbed and end up back at the liver. Approximately 98% of the bile acids are recycled. Only
1% to 2% ends up in the large intestine to be eliminated in the feces. Using medicines that block some of this reabsorption of bile acids is one way to treat high blood cholesterol. The liver takes cholesterol from the bloodstream to form replacement bile acids. Viscous fiber in the diet can also bind to bile acids to produce the same effect (see a later section on medical interventions related to cardiovascular disease for details).
Absorption
The products of fat digestion in the small intestine are fatty acids and monoglycerides.
These products diffuse into the absorptive cells of the small intestine. About 95% of dietary fat is absorbed in this way. The chain length of fatty acids affects the ultimate fate of fatty acids and monoglycerides after absorption. If the chain length of a fatty acid is less than 12 carbon atoms, it is water soluble and will therefore probably travel as such through the portal vein that connects directly to the liver. If the fatty acid is a more typical long-chain variety, it must be reformed into a triglyceride in the intestinal absorptive cell and eventually enter circulation via the lymphatic system (review
Chapter 3 for an overview of this process).
Concept Check
In the small intestine, a lipase enzyme released from the pancreas digests dietary triglycerides into monoglycerides (glycerol backbones with single fatty acids attached) and fatty acids.
These breakdown products then diffuse into the absorptive cells of the small intestine. Longchain fatty acids are transported through the lymphatic system, whereas fatty acids with shorter carbon chains are absorbed directly into the portal vein that connects directly to the liver. Other lipids are prepared for absorption by different enzymes.
Carrying Lipids in the Bloodstream lipoprotein A compound found in the bloodstream containing a core of lipids with a shell composed of protein, phospholipid, and cholesterol.
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As noted earlier, fat and water don’t mix easily. This incompatibility presents a challenge for the transport of fats through the watery media of the blood and lymph.
Lipoproteins serve as vehicles for transport of lipids from the small intestine and liver to the body tissues (Table 5-3).
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Table 5-3 Composition and Roles of the Major Lipoproteins in the Blood
Lipoprotein
Primary Component
Key Role
Chylomicron
Triglyceride Carries dietary fat from the small intestine to cells
VLDL
Triglyceride Carries lipids made and taken up by the liver to cells LDL
Cholesterol Carries cholesterol made by the liver and from other sources to cells
HDL
Protein Contributes to cholesterol removal from cells and, in turn, excretion of it from the body
Lipoproteins are classified into four groups—chylomicrons, VLDL, LDL, and HDL– based on their densities. Lipids are less dense than proteins. Therefore, lipoproteins that contain a large percentage of lipids in comparison to protein are less dense than those depleted of lipids.
Dietary Fats Are Carried by Chylomicrons
As you learned in the previous section, digestion of dietary fats results in a mixture of glycerol, monoglycerides, and fatty acids. Once these products are absorbed by the cells of the small intestine, they are reassembled into triglycerides. Then, the intestinal cells package the triglycerides into chylomicrons, which enter the lymphatic system and eventually the bloodstream (review Fig. 3-5 in Chapter 3 for a depiction of lymphatic circulation). Chylomicrons contain dietary fat and originate only from the intestinal cells. Like the other lipoproteins described in the next section, chylomicrons are composed of large droplets of lipid surrounded by a thin, water-soluble shell of phospholipids, cholesterol, and protein (Fig. 5-11). The water-soluble shell around a chylomicron allows the lipid to float freely in the water-based blood. Some of the proteins present may also help other cells identify the lipoprotein as a chylomicron.
Once a chylomicron enters the bloodstream, the triglycerides in its core are broken down into fatty acids and glycerol by an enzyme called lipoprotein lipase, attached to the inside walls of the blood vessels (Fig. 5-12). As soon as the fatty acids
chylomicron Lipoprotein made of dietary fats surrounded by a shell of cholesterol, phospholipids, and protein. Chylomicrons are formed in the absorptive cells of the small intestine after fat absorption and travel through the lymphatic system to the bloodstream.
lipoprotein lipase An enzyme attached to the cells that form the inner lining of blood vessels; it breaks down triglycerides into free fatty acids and glycerol.
Figure 5-11 c The structure of a lipo-
Protein
protein, in this case an LDL. This structure allows fats to circulate in the water-based bloodstream. Various lipoproteins are found in the bloodstream. The primary component of LDL is cholesterol.
Triglycerides
Phospholipids
Free cholesterol
Cholesterol bound to fatty acids
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Various fats in a meal
Source of Fat
Lipoprotein
Function
The chylomicrons deliver fatty acids to cells.
HDL made by the liver and intestine
Packaged into
VLDL by the liver
Packaged into chylomicrons in the intestine
Lipoprotein
Production
Cholesterol that arises from dying cells or during lipoprotein metabolism
Various lipids collected (or made) by the liver
HDL pick up these lipids and transfer them to other lipoproteins or to the liver.
The VLDL deliver fatty acids to cells
LDL arise from the
VLDL particle as the fatty acids are removed. Ultimate
LDL
Production
LDL deliver primarily cholesterol to cells.*
LDL
Function
* Cholesterol not taken up by body cells can be taken up by scavenger cells in the arteries.
The eventual cholesterol buildup leads to atherosclerosis.
VLDL: Very-Low-Density Lipoprotein
LDL: Low-Density Lipoproteinn
HDL: High-Density Lipoprotein
Figure 5-12 c Lipoprotein production and function. Chylomicrons carry absorbed fat to body cells. VLDL carries fat taken up from the bloodstream by the liver, as well as any fat made by the liver, to body cells. LDL arises from VLDL and carries mostly cholesterol to cells.
HDL arises mostly from the liver and intestine. HDL carries cholesterol from cells to other lipoproteins and to the liver for excretion.
are released to the bloodstream, they are absorbed by cells in the vicinity, while much of the glycerol circulates back to the liver. Muscle cells can immediately use the absorbed fatty acids for fuel. Adipose cells, on the other hand, tend to re-form the fatty acids into triglycerides for storage. After triglycerides have been removed, a chylomicron remnant remains. Chylomicron remnants are removed from circulation by the liver and their components are recycled to make other lipoproteins and bile acids.
Other Lipoproteins Transport Lipids from the Liver to the Body Cells
very-low-density lipoprotein (VLDL)
The lipoprotein created in the liver that carries cholesterol and lipids that have been taken up or newly synthesized by the liver.
low-density lipoprotein (LDL) The lipoprotein in the blood containing primarily cholesterol; elevated LDL is strongly linked to cardiovascular disease risk.
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The liver takes up various lipids from the blood. The liver also is the manufacturing site for lipids and cholesterol. The raw materials for lipid and cholesterol synthesis include free fatty acids taken up from the bloodstream, as well as carbon and hydrogen derived from carbohydrates, protein, and alcohol. The liver then must package these synthesized lipids as lipoproteins for transport in the blood to body tissues.
First in our discussion of lipoproteins made by the liver are very-low-density lipoproteins (VLDL). These particles are composed of cholesterol and triglycerides surrounded by a water-soluble shell. VLDLs are rich in triglycerides and thus are very low in density. Once in the bloodstream, lipoprotein lipase on the inner surface of the blood vessels breaks down the triglyceride in the VLDL into fatty acids and glycerol. Fatty acids and glycerol are released into the bloodstream and taken up by the body cells.
As its triglycerides are released, the VLDL becomes proportionately denser. Much of what eventually remains of the VLDL fraction is then called low-density lipoprotein
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(LDL); this is composed primarily of the remaining cholesterol. The primary function of
LDL is to transport cholesterol to tissues. LDL particles are taken up from the bloodstream by specific receptors on cells, especially liver cells, and are then broken down. The cholesterol and protein components of LDL provide some of the building blocks necessary for cell growth and development, such as synthesis of cell membranes and hormones.
The final group of lipoproteins, high-density lipoproteins (HDL), is a critical and beneficial participant in this process of lipid transport. Its high proportion of protein makes it the densest lipoprotein. The liver and intestine produce most of the HDL in the blood. It roams the bloodstream, picking up cholesterol from dying cells and other sources. HDL donates the cholesterol primarily to other lipoproteins for transport back to the liver to be excreted. Some HDL travels directly back to the liver.
high-density lipoprotein (HDL) The lipoprotein in the blood that picks up cholesterol from dying cells and other sources and transfers it to the other lipoproteins in the bloodstream, as well as directly to the liver; low HDL increases the risk for cardiovascular disease. menopause The cessation of the menstrual cycle in women, usually beginning at about
50 years of age.
scavenger cells Specific form of white
“Good” and “Bad” Cholesterol in the Bloodstream
HDL and LDL are often described as “good” and “bad” cholesterol, respectively. Many studies demonstrate that the amount of HDL in the bloodstream can closely predict the risk for cardiovascular disease. Risk increases with low HDL because little cholesterol is transported back to the liver and excreted. Women tend to have high amounts of HDL, especially before menopause, compared to men. High amounts of HDL slow the development of cardiovascular disease, so any cholesterol carried by HDL can be considered “good” cholesterol.
On the other hand, LDL is sometimes considered “bad” cholesterol. In our discussion of LDL, you learned that LDL is taken up by receptors on various cells. If LDL is not readily cleared from the bloodstream, scavenger cells in the arteries take up the lipoprotein, leading to a buildup of cholesterol in the blood vessels. This buildup, known as atherosclerosis, greatly increases the risk for cardiovascular disease (see the following
“Nutrition and Your Health” section). LDL is only a problem when it is too high in the bloodstream because low amounts are needed as part of routine body functions.
Making Decisions
LDL Cholesterol
The cholesterol in foods is not designated as “good” or “bad.” It is only after cholesterol has been made or processed by the liver that it shows up in the bloodstream as LDL or HDL. Dietary patterns can affect the metabolism of cholesterol, however. Diets low in saturated fat, trans fat, and cholesterol encourage the uptake of LDL by the liver, thereby removing LDL from the bloodstream and decreasing the ability of scavenger cells to form atherosclerotic plaques in the blood vessels. Likewise, diets high in saturated fat, trans fat, and cholesterol reduce the uptake of LDL by the liver, increasing cholesterol in the blood and the risk for cardiovascular disease. What foods in your diet are high in saturated fat, trans fat, or cholesterol?
blood cells that can bury themselves in the artery wall and accumulate LDL. As these cells take up LDL, they contribute to the development of atherosclerosis.
atherosclerosis A buildup of fatty material (plaque) in the arteries, including those surrounding the heart.
See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
c It appears that saturated fatty acids promote an increase in the amount of free cholesterol (not attached to fatty acids) in the liver, whereas unsaturated fatty acids do the opposite. As free cholesterol in the liver increases, it causes the liver to reduce cholesterol uptake from the bloodstream, contributing to elevated LDL in the blood.
(Trans fatty acids are thought to act in the same ways as saturated fatty acids.)
Concept Check
Lipids generally move through the bloodstream as part of lipoproteins. Dietary fats absorbed from the small intestine are packaged and transported as chylomicrons, whereas lipids synthesized in the liver are packaged as very-low-density lipoproteins (VLDL). Lipoprotein lipase removes triglycerides from the interiors of both chylomicrons and VLDL, breaking the triglycerides down into glycerol and fatty acids, which are taken up by tissues for energy needs or storage. What remains after the action of lipoprotein lipase are chylomicron remnants (from chylomicrons), the components of which are recycled by the liver, or low-density lipoproteins (LDL, from VLDL), rich in cholesterol. LDL is picked up by receptors on body cells, especially liver cells. Scavenger cells in the arteries may do the same, speeding the development of atherosclerosis. High-density lipoprotein (HDL), also produced in part by the liver, picks up cholesterol from cells and transports it primarily to other lipoproteins for eventual transport back to the liver. Risk factors for cardiovascular disease include an elevated level of LDL and/or low amounts of HDL in the blood.
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essentiaL FunCtions oF FattY aCids
Omega-3 Fatty Acids in Fish
(grams per 3 ounce serving)
Atlantic salmon
Anchovy
Sardines
Rainbow trout
Coho salmon
Bluefi sh
Striped bass
Tuna, white, canned
Halibut
Catfi sh, channel
The various classes of lipids have diverse functions in the body. All are necessary for health, but, as mentioned earlier, many can be made by the body and therefore are not needed in our diet. Of all the classes of lipids, only certain polyunsaturated fatty acids are essential parts of a diet.
1.8
1.7
1.4
1.0
0.9
0.8
0.8
0.7
0.4
0.2
The Essential Fatty Acids
Recommended omega-3 fatty acid
(alpha-linolenic acid) intake per day:
Men
1.6 grams
Women
1.1 grams
eicosapentaenoic acid (EPA) An omega-3 fatty acid with 20 carbons and five carboncarbon double bonds. It is present in large amounts in fatty fish and is slowly synthesized in the body from alpha-linolenic acid.
docosahexaenoic acid (DHA) An omega-3 fatty acid with 22 carbons and six carbon-carbon double bonds. It is present in large amounts in fatty fish and is slowly synthesized in the body from alpha-linolenic acid. DHA is especially present in the retina and brain.
We must obtain linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) from foods to maintain health, so they are called essential fatty acids (Fig. 5-13). These omega-6 and omega-3 fatty acids form parts of vital body structures, perform important roles in immune system function and vision, help form cell membranes, and produce hormone-like compounds. Omega-6 and omega-3 fatty acids must be obtained through the diet because human cells lack the enzymes needed to produce these fatty acids. Other fatty acids, such as omega-9 fatty acids, can be synthesized in the body, and therefore are not essential components of the diet.
Still, we need to consume only about 5% of our total calories per day from essential fatty acids. That corresponds to about 2 to 4 tablespoons of plant oil each day. We can easily get that much—from mayonnaise, salad dressings, and other foods—without much effort. Regular consumption of vegetables and whole-grain breads and cereals also helps to supply enough essential fatty acids.
Research also suggests that we include a regular intake of the omega-3 fatty acids, eicosapentaenoic acid (ePa) and docosahexaenoic acid (DHa) , which can be made from alpha-linolenic acid. EPA and DHA are naturally high in fatty fish such as salmon, tuna, sardines, anchovies, striped bass, catfish, herring, mackerel, trout, or halibut. Consumption of one or more of these fish at least twice a week is recommended to obtain EPA and DHA. Additional sources of omega-3 fatty acids include canola and soybean oils, walnuts, flax seeds, mussels, crab, and shrimp.
Making deCisions
arachidonic acid An omega-6 fatty acid made from linoleic acid with 20 carbon atoms and 4 carbon-carbon double bonds.
m The American Heart Association recommends
eating fatty fish such as salmon at least twice a week. As a source of omega-3 fatty acids, fish is a heart-healthy alternative to other animal sources of protein, which can be high in saturated fat and cholesterol.
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Mercury in Fish
Consumption of fatty fish at least twice a week is recommended as a good source of the omega-3 fatty acids. Some fish can be a source of mercury, toxic in high amounts, especially swordfish, shark, king mackerel, and tile fish (see Chapter 13). Albacore tuna also is a potential source, while other forms of tuna are much lower in mercury. Those fish low in mercury are salmon, sardines, bluefish, and herring. Shrimp is also low in mercury. For others, varying your choices rather than always eating the same species of fish and limiting overall intake to 12 ounces per week (on average 2 to 3 meals of fish or shellfish per week) is recommended to reduce mercury exposure, especially for pregnant women and children. Existing research indicates that the benefits of fish intake, especially in reducing the risk of cardiovascular disease, outweigh the possible risks of mercury contamination.
The recommendation to consume omega-3 fatty acids stems from the observation that compounds made from omega-3 fatty acids tend to decrease blood clotting and inflammatory processes in the body. The omega-6 fatty acids, notably arachidonic acid made from linoleic acid, generally increase clotting and inflammation, and saturated fatty acids also increase blood clotting.
Some studies show that people who eat fish at least twice a week (total weekly intake: 8 ounces [240 grams]) run lower risks for heart attack than do people who rarely eat fish. In these cases, the omega-3 fatty acids in fish oil are probably acting to reduce blood clotting. As will be covered in detail in the “Nutrition and Your Health”
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FiguRe 5-13 c The essential fatty acid
Polyunsaturated Fatty Acids
(PUFA)
Omega-3 Family
Omega-6 Family
Alpha-linolenic acid
Linoleic acid
Docosahexaenoic acid (DHA)
Eicosapentaenoic acid (EPA)
(EFA) family. All these fatty acids are available from dietary sources; linoleic acid and alpha-linolenic acid must be consumed as body synthesis does not take place. These are the essential fatty acids. The other fatty acids in this figure can be synthesized from the essential fatty acids.
Arachidonic acid section, blood clots are part of the heart attack process. In addition, these omega-3 fatty acids have a favorable effect on heart rhythm. Consequently, the risk of heart attack decreases with the consumption of omega-3 fatty acids from fish, especially for people already at high risk.
We need to remember, however, that blood clotting is a normal body process. Certain groups of people, such as Eskimos in Greenland, eat so much seafood that their normal blood-clotting ability can be impaired. An excess of omega-3 fatty acid intake can allow uncontrolled bleeding and may cause hemorrhagic stroke. However, no increase in risk of stroke has been observed in studies using moderate amounts of omega-3 fatty acids.
Studies also have shown that large amounts of omega-3 fatty acids from fish
(2 to 4 grams per day) can lower blood triglycerides in people with high triglyceride concentrations. In addition, these omega-3 fatty acids are suspected to be helpful in managing the pain of inflammation associated with rheumatoid arthritis by suppressing immune system responses. This may also help with certain behavioral disorders and cases of mild depression.
In some instances, fish oil capsules can be safely substituted for fish consumption if a person does not like fish. Generally, about 1 gram of omega-3 fatty acids (about three capsules) from fish oil per day is recommended, especially for people with evidence of cardiovascular disease. (Freezing fish oil capsules before consumption or using enteric coated capsules will reduce the fishy aftertaste.)
The American Heart Association also recently suggested that fish oil supplements
(providing 2 to 4 grams of omega-3 fatty acids per day) could be employed to treat elevated blood triglycerides, as previously noted. However, fish oil capsules should be limited for individuals who have bleeding disorders, take anticoagulant medications, or anticipate surgery, because they may increase risk of uncontrollable bleeding and hemorrhagic stroke. Thus for fish oil capsules, as well as other dietary supplements, it is important to follow a physician’s recommendations. Remember that fish oil supplements are not regulated by the FDA. The quality of these supplements, therefore, is not standardized, and contaminants naturally present in the fish oil may not have been removed.
In summary, the regular consumption of fatty fish is advised. Consuming whole fish is thought to have greater benefits and be safer than using fish oil supplements. Fish is not only a rich source of omega-3 fatty acids, but is also a valuable source of protein and trace elements that may also provide protective effects for the cardiovascular system. Broiled or baked fish is recommended rather than fried fish because frying may decrease the ratio of omega-3 to omega-6 fatty acids and may produce trans fatty acids and oxidized lipid products that may increase cardiovascular disease risk.
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See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
hemorrhagic stroke Damage to part of the brain resulting from rupture of a blood vessel and subsequent bleeding within or over the internal surface of the brain.
m Walnuts are one of the richest plant sources of
the omega-3 fatty acid, alpha-linolenic acid, and are also a good source of plant sterols.
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CRitiCaL thinking
Making deCisions
Advertisements often claim that fats are bad. Your classmate Mike asks, “If fats are so bad for us, why do we need to have any in our diets?” How would you answer him?
Flax Seeds or Walnuts?
Flax seeds and walnuts are getting attention today because they are rich vegetable sources of the omega-3 alpha-linolenic acid. About 2 tablespoons per day is typically recommended if used as an omega-3 fatty acid source. Flax seeds can be purchased in many natural food stores rather inexpensively. These need to be chewed thoroughly or they will pass through the GI tract undigested. Many people find it easier to grind them in a coffee grinder before eating them. Flax seed oil is also available, but it turns rancid very quickly, especially if not refrigerated. Compared to other nuts and seeds, walnuts are one of the richest sources of alpha-linolenic acid (2.6 grams per 1-ounce serving or 14 walnut halves). The DRI for alphalinolenic acid is 1.6 grams/day for men and 1.1 grams/day for women. In addition, walnuts are a rich source of plant sterols known to inhibit intestinal absorption of cholesterol.
rancid Containing products of decomposed fatty acids that have an unpleasant flavor and odor.
total parenteral nutrition The intravenous feeding of all necessary nutrients, including the most basic forms of protein, carbohydrates, lipids, vitamins, minerals, and electrolytes.
Effects of a Deficiency of Essential Fatty Acids
If humans fail to consume enough essential fatty acids, their skin becomes flaky and itchy, and diarrhea and other symptoms such as infections often are seen. Growth and wound healing may be restricted. These signs of deficiency have been seen in people fed intravenously by total parenteral nutrition containing little or no fat for 2 to 3 weeks, as well as in infants receiving formulas low in fat. However, because our bodies need the equivalent of only about 2 to 4 tablespoons of plant oils a day, even a lowfat diet will provide enough essential fatty acids if it follows a balanced plan such as
MyPyramid and includes a serving of fatty fish at least twice a week.
ConCept CheCk
Because humans can’t make either omega-3 or omega-6 fatty acids, which perform vital functions in the body, they must be obtained from the diet and therefore are called essential fatty acids. Plant oils are generally rich in omega-6 fatty acids. Eating fatty fish at least twice a week is a good way to meet omega-3 fatty acid needs. Fish oil supplementation (about 1 gram per day of the related omega-3 fatty acids) is generally acceptable under a physician’s guidance if a person does not like fish; however, people with certain medical conditions (e.g., taking anticoagulant medications) should be cautious with use of fish oil supplements due to increased risk of hemorrhagic stroke. Essential fatty acid deficiency can occur after 2 to 3 weeks if fat is omitted from total parenteral nutrition solutions, which in turn can lead to skin disorders, diarrhea, and other health problems.
bRoadeR RoLes FoR FattY aCids and tRigLYCeRides in the bodY
Many key functions of fat in the body require the use of fatty acids in the form of triglycerides. Triglycerides are used for energy storage, insulation, and transportation of fat-soluble vitamins.
Providing Energy
m When at rest or during light activity, the body
uses mostly fatty acids for fuel.
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Triglycerides contained in the diet and stored in adipose tissue provide the fatty acids that are the main fuel for muscles while at rest and during light activity. Muscles use carbohydrate for fuel in addition to fatty acids supplied by triglycerides only in endurance exercise, such as long-distance running and cycling, or in short bursts of intense activity, such as a 200-meter run. Other body tissues also use fatty acids for
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energy needs. Overall, about half of the energy used by the entire body at rest and during light activity comes from fatty acids. When considering the whole-body, the use of fatty acids by skeletal and heart muscle is balanced by the use of glucose by the nervous system and red blood cells. Recall from Chapter 4 that cells need a supply of carbohydrate to efficiently process fatty acids for fuel. The details about how we burn fat as a fuel will be discussed in Chapter 7.
Storing Energy for Later Use
We store energy mainly in the form of triglycerides. The body’s ability to store fat is essentially limitless. Its fat storage sites, adipose cells, can increase about 50 times in weight. If the amount of fat to be stored exceeds the ability of the existing cells to expand, the body can form new adipose cells.
An important advantage of using triglycerides to store energy in the body is that they are energy dense. Recall that these yield, on average, 9 kcal per gram, whereas proteins and carbohydrates yield only about 4 kcal per gram. In addition, triglycerides are chemically stable, so they are not likely to react with other cell constituents, making them a safe form for storing energy. Finally, when we store triglycerides in adipose cells, we store little else, especially water. Adipose cells contain about 80% lipid and only 20% water and protein. In contrast, imagine if we were to store energy as muscle tissue, which is about 73% water. Body weight linked to energy storage would increase dramatically. The same would be true if we stored energy primarily as glycogen, as about 3 grams of water are stored for every gram of glycogen.
Nucleus
Cell
membrane
Fat droplet
Adipose cell
Insulating and Protecting the Body
The insulating layer of fat just beneath the skin is made mostly of triglycerides. Fat tissue also surrounds and protects some organs—kidneys, for example—from injury.
We usually don’t notice the important insulating function of fat tissue, because we wear clothes and add more as needed. A layer of insulating fat is important in animals living in cold climates. Polar bears, walruses, and whales all build a thick layer of fat tissue around themselves to insulate against cold-weather environments. The extra fat also provides energy storage for times when food is scarce.
Transporting Fat-Soluble Vitamins
Triglycerides and other fats in food carry fat-soluble vitamins to the small intestine and aid their absorption. People who absorb fat poorly, such as those with the disease cystic fibrosis, are at risk for deficiencies of fat-soluble vitamins, especially vitamin
K. A similar risk comes from taking mineral oil as a laxative at mealtimes. The body cannot digest or absorb mineral oil, so the undigested oil carries the fat-soluble vitamins from the meal into the feces, where they are eliminated. Unabsorbed fatty acids can bind minerals, such as calcium and magnesium, and draw them into the stool for elimination. This can harm mineral status (see Chapter 9). Recall that the main problem with the fat replacer, olestra, is that it can bind the fat-soluble vitamins and reduce their absorption.
Phospholipids in the Body
Many types of phospholipids exist in the body, especially in the brain. They form important parts of cell membranes. Phospholipids are found in body cells, and they participate in fat digestion in the intestine. Recall that the various forms of lecithin
(discussed earlier) are common examples of phospholipids (review Fig. 5-4b).
Cell membranes are composed primarily of phospholipids. A cell membrane looks much like a sea of phospholipids with protein “islands” (Fig. 5-14). The proteins
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Figure 5-14 c Phospholipids are the main components of cell membranes, forming a double or bilayer of lipid.
Carbohydrate chains Protein chain
Membrane protein Globular protein Cholesterol
Phospholipid bilayer form receptors for hormones, function as enzymes, and act as transporters for nutrients. The fatty acids on the phospholipids serve as a source of essential fatty acids for the cell. Some cholesterol is also present in the membrane.
In some foods, phospholipids function as emulsifiers (as covered earlier) allowing fat and water to mix. By breaking fat globules into small droplets, emulsifiers enable a fat to be suspended in water. They act as bridges between the oil and water that in turn lead to the formation of tiny oil droplets surrounded by thin shells of water. In an emulsified solution, such as salad dressing or mayonnaise, millions of tiny oil droplets are separated by shells of water (review Fig. 5-7).
The body’s main emulsifiers are the lecithins and bile acids, produced by the liver and released into the small intestine via the gallbladder during digestion.
Cholesterol in the Body
See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
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Cholesterol plays many vital roles in the body. It forms part of some important hormones, such as estrogen, testosterone, and a form of the active vitamin D hormone.
Cholesterol is also the building block of bile acids, needed for fat digestion. Finally, cholesterol is an essential structural component of cells and the outer layer of the lipoprotein particles that transport lipids in the blood. The cholesterol content of the heart, liver, kidney, and brain is high, reflecting its critical role in these organs.
About two-thirds of the cholesterol circulating through your body is made by body cells; the remaining one-third is consumed in the diet. Each day, our cells produce approximately 875 milligrams of cholesterol. Of the 875 milligrams of cholesterol made by the body, about 400 milligrams are used to make new bile acids to replenish those lost in the feces, and about 50 milligrams are used to make hormones. In addition to all the cholesterol cells make, we consume about 180 to 325 milligrams of cholesterol per day from animal-derived food products, with men consuming the higher amount compared to women. Absorption of cholesterol from food ranges from about 40% to 65%. The effect of blood cholesterol, especially LDL cholesterol, on cardiovascular disease risk will be discussed in the “Nutrition and Your Health” section.
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ConCept CheCk
Triglycerides are the major form of fat in the body. They are used for energy and stored in adipose tissue, they insulate and protect body organs, and they transport fat-soluble vitamins.
Phospholipids are emulsifiers—compounds that can suspend fat in water. Phospholipids also form parts of cell membranes and various compounds in the body. Cholesterol, a sterol, forms part of cell membranes, hormones, and bile acids. If sufficient amounts are not consumed, the body makes what phospholipids and cholesterol it needs.
ReCoMMendations FoR Fat intake
There is no RDA for total fat intake for adults, although there is an Adequate
Intake set for total fat for infants (see Chapter 15). The 2005 Dietary Guidelines for Americans recommend that total fat intake should not exceed 20% to 35% of total calories, which equates to 44 to 78 grams per day for a person who consumes
2000 kcal daily. The most specific recommendations for fat intake come from the
American Heart Association (AHA). Many North Americans are at risk for developing cardiovascular disease, so AHA promotes dietary and lifestyle goals aimed at reducing this risk. The AHA diet and lifestyle goals for cardiovascular disease risk reduction for the general public are presented in Table 5-4. These goals include aiming for an overall healthy eating pattern; appropriate body weight; and a desirable blood cholesterol profile, blood pressure, and blood glucose level. In Table 5-5, a more detailed list of recommendations is provided for those who currently are at high risk or have cardiovascular disease.
To reduce risk for cardiovascular disease, the AHA recommends that no more than 7% of total calories come from saturated fat and no more than 1% from trans fat. These are the primary fatty acids that raise LDL. In addition, cholesterol should amount to a maximum of 300 mg per day. This often happens along with the reduction in saturated fat and trans fat intake. Table 5-6 is an example of a diet that adheres to 20% or 30% of calories as fat. Compare these recommendations to the actual dietary intake patterns of these fats by North Americans: 33% of calories from total fat, about 13% of calories from saturated fat, and 180 to 320 milligrams of cholesterol each day.
Both the National Cholesterol Education Program (NCEP) and the Food and
Nutrition Board are in agreement with the advice of the AHA. One exception in the tabLe 5-4 American Heart Association 2006 Diet and Lifestyle Goals for Cardiovascular Disease Risk Reduction
m Trim the fat from meats before cooking to
help reduce your saturated fat intake. Also, limit use of meat that is highly marbled with fat (seen as streaks of fat).
c One goal of Healthy People 2010 is to increase the proportion of persons age 2 years and older who consume less than 10% of calorie intake from saturated fat.
• Consume an overall healthy diet.
• Aim for a healthy body weight.
• A im for recommended levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides.
• Aim for a normal blood pressure.
• Aim for a normal blood glucose level.
• Be physically active.
• Avoid use of and exposure to tobacco products.
From: Lichtenstein AH et al: Diet and lifestyle recommendations revision 2006. A scientifi c statement from the
American Heart Association Nutrition Committee. Circulation 114:82, 2006.
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c Some advice regarding fat intake from the
2005 Dietary Guidelines:
Table 5-5 American Heart Association 2006 Diet and Lifestyle Recommendations for
Cardiovascular Disease Risk Reduction
• Limit intake of fats and oils high in saturated and/or trans fatty acids, and choose products low in such fats and oils.
• Balance calorie intake and physical activity to achieve or maintain a healthy body weight.
• Consume fats high in polyunsaturated and monounsaturated fatty acids, such as from fish, nuts, and vegetable oils.
• Choose whole-grain, high-fiber foods.
• When selecting and preparing meat, poultry, dry beans, and milk or milk products, make choices that are lean, low-fat, or fat-free. • Limit your intake of saturated fat to less than 7% of energy, trans fat to less than 1% of energy, and cholesterol to less than 300 milligrams per day by
— choosing lean meats and vegetable alternatives;
— selecting fat-free (skim), 1%-fat, and low-fat dairy products; and
— minimizing intake of partially hydrogenated fats.
• Consume a diet rich in vegetables and fruits.
• Consume fish, especially oily fish, at least twice a week.
• Minimize your intake of beverages and foods with added sugars.
• Choose and prepare foods with little or no salt.
• If you consume alcohol, do so in moderation.
• When you eat food prepared outside of the home, follow the AHA Diet and Lifestyle
Recommendations.
From: Lichtenstein AH and others: Diet and lifestyle recommendations revision 2006. A scientific statement from the American Heart Association Nutrition Committee. Circulation 114:82, 2006.
Table 5-6 Daily Menu Examples Containing 2000 kcal and 30% or 20% of Calories as Fat
30% of Calories as Fat
Food
m Whole grains (shredded wheat, whole-wheat
bread, oatmeal cookies, popcorn), fruits (orange juice, apple, banana, raisins), vegetables (carrots, lettuce, tomato), lean meats (roast beef, turkey, chicken), and fat-free milk are the primary components of the low-fat menus in Table 5-6.
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20% of Calories as Fat
Fat
(grams)
Food
Fat
(grams)
Breakfast
Orange juice, 1cup
Shredded wheat, ¾ cup
Toasted bagel
Tub margarine, 3 teaspoons
1% low-fat milk, 1 cup
0.5
0.5
1.1
11.4
2.5
Same
Shredded wheat, 1 cup
Same
Tub margarine, 2 teaspoons
Fat-free milk, 1 cup
Lunch
Whole-wheat bread, 2 slices
Roast beef, 2 ounces
Mayonnaise, 3 teaspoons
Lettuce
Tomato
Oatmeal cookie, 1
2.4
4.9
11.0
—
—
3.3
Same
Light turkey roll, 2 ounces
Mayonnaise, 2 teaspoons
Same
Same
Oatmeal cookie, 2
2.4
0.9
7.3
—
—
6.6
Snack
Apple
—
Same
—
Dinner
Chicken tenders frozen meal
Carrots, ½ cup
Dinner roll, 1
Margarine, 1 teaspoon
Banana
1% low-fat milk, 1 cup
18.0
—
2.0
3.8
0.6
2.5
Fat-free chicken tenders
Same
Same
Same
Same
Fat-free milk, 1 cup
—
—
2.0
3.8
0.6
0.6
Snack
Raisins, 2 teaspoons
Air-popped popcorn, 3 cups
Margarine, 2 teaspoons
—
1.0
7.6
Raisins, ½ cup
Air-popped popcorn, 6 cups
Same
—
2.0
7.6
Totals
73.1
44.3
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Chapter 5: Lipids 189
tabLe 5-7 Food and Nutrition Board Recommendations for Omega-6 and Omega-3
Fatty Acids per Day
Men (grams per day)
Linoleic acid (omega-6)
Women (grams per day)
17
Alpha-linolenic acid (omega-3)
12
1.6
1.1
latest guidelines from the NCEP is that fat intake could be as high as 35% of total calories as long as intakes of saturated fat, cholesterol, and trans fat are minimized.
The Food and Nutrition Board combines the AHA and NCEP recommendations, suggesting that fat provide a range of 20% to 35% of calories. The 2005 Dietary
Guidelines also support this advice. In addition to fat intake, controlling total calorie intake is also significant, as weight control is a vital component of cardiovascular disease prevention.
Regarding essential fatty acids, the Food and Nutrition Board has issued recommendations for both omega-6 and omega-3 fatty acids.
The amounts listed in Table 5-7 work out to about 5% of calorie intake for the total of both essential fatty acids. Infants and children have lower needs (again, see Chapter 15). Consumption of fish at least twice a week is one step toward meeting requirements for essential fatty acids.
The typical North American diet derives about 7% of calories from polyunsaturated fatty acids, and thus meets essential fatty acid needs. An upper limit of 10% of calorie intake as polyunsaturated fatty acids is often recommended, in part because the breakdown
(oxidation) of those present in lipoproteins is linked to increased cholesterol deposition in the arteries (see the “Nutrition and Your
Health” section in this chapter). Depression of immune function is also suspected to be caused by an excessive intake of polyunsaturated fatty acids.
In recent years, the Mediterranean diet has attracted a lot of attention as a result of lower rates of chronic diseases seen in people following such a diet plan. The major sources of fat in the Mediterranean diet include liberal amounts of olive oil compared to a small amount of animal fat (from animal flesh and dairy products). In contrast, major sources of fat in the typical North American diet include animal flesh, whole milk, pastries, cheese, margarine, and mayonnaise. While dietary fat sources definitely play a role in prevention of chronic disease, it is important to remember that other aspects of one’s lifestyle also contribute to disease risk. People who follow a Mediterranean diet also tend to consume moderate alcohol (usually in the form of red wine, which contains many antioxidants), eat plenty of whole grains and few refined carbohydrates, and are also more physically active than typical North Americans.
An alternative plan for reduction of cardiovascular disease is Dr. Dean Ornish’s purely vegetarian (vegan) diet plan. This diet is very low in fat, including only a scant quantity of vegetable oil used in cooking and the small amount of oils present in plant foods. Individuals restricting fat intake to 20% of calories should be monitored by a physician, as the resulting increase in carbohydrate intake can increase blood triglycerides in some people, which is not a healthful change. Over time, however, the initial problem of high blood triglycerides on a low-fat diet may self-correct. Among people following the Ornish plan, blood triglycerides initially increased, but within a year, fell to normal values as long as they emphasized highfiber carbohydrate sources, controlled (or improved) body weight, and followed a regular exercise program.
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c The advice to consume 20% to 35% of calories as fat does not apply to infants and toddlers below the age of 2 years. These youngsters are forming new tissue that requires fat, especially in the brain, so their intake of fat and cholesterol should not be greatly restricted.
m If you are looking to decrease the amount of
saturated and trans fats in your diet, it is a good idea to opt for lower-fat substitutes for some of your current high-fat food choices. How do you think this meal compares with the chicken nuggets and French fries meal on p. 176.
vegan A person who eats only plant foods.
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tABLe 5-8 Tips for Avoiding Too Much Fat, Saturated Fat, Cholesterol, and Trans Fat
Eat Less of These Foods
Eat More of These Foods
Grains
• Pasta dishes with cheese or cream sauces
• Croissants
• Pastries
• Doughnuts
• Pie crust
•
•
•
•
•
•
Vegetables
• French fries
• Potato chips
• Vegetables cooked in butter, cheese, or cream sauces
• Fresh, frozen, baked, or steamed vegetables
Fruit
• Fruit pies
• Fresh, frozen, or canned fruits
Milk
•
•
•
•
Whole milk
Ice cream
High-fat cheese
Cheesecake
• Fat-free and reduced-fat milk
• Low-fat frozen desserts
(e.g., yogurt, sherbet, ice milk)
• Reduced-fat/part-skim cheese
Meat and beans
•
•
•
•
Bacon
Sausage
Organ meats (e.g., liver)
Egg yolks
• Fish
• Skinless poultry
• Lean cuts of meat (with fat trimmed away)
• Soy products
• Egg whites/egg substitutes
Oils
• Butter and stick margarine
• Canola oil or olive oil
• Tub or liquid margarine (in small amounts)
To avoid too much saturated fat and choles-
terol, for breakfast eat fewer foods like bacon, sausage, and whole eggs and more foods like whole-grain waffles and fresh fruit.
Whole-grain breads
Whole-grain pasta
Brown rice
Angel food cake
Animal or graham crackers
Air-popped popcorn
In summary, the general consensus among nutrition experts suggests that limitation of saturated fat, cholesterol, and trans fat intake should be the primary focus, and that the diet needs to contain some omega-3 and omega-6 fatty acids (Table 5-8).
Furthermore, if fat intake exceeds 30% of total calories, the extra fat should come from monounsaturated fat.
ConCept CheCk
There is no RDA for fat. We need about 5% of total calorie intake from plant oils to obtain the needed essential fatty acids. Eating fatty fish at least twice a week is also advised to supply omega-3 fatty acids. Many health-related agencies recommend a diet containing no more than 35% of calorie intake as fat, with no more than 7% to 10% of calorie intake as a combination of saturated fat and trans fat for the general public. Cholesterol intake should be limited to 200 to 300 milligrams per day. These practices help maintain a normal LDL value in the blood. The North American diet contains about 33% of calories as fat, with about 13% of calories as saturated fat and about 3% as trans fatty acids. Cholesterol intake varies from about
200 to 400 milligrams per day.
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nutRition and YouR heaLth
Lipids and Cardiovascular Disease
Cardiovascular disease is the major killer of North Americans. Cardiovascular disease typically involves the coronary arteries and, thus, frequently the term coronary heart disease (CHD) or coronary artery disease (CAD) is used. Each year about
500,000 people die of coronary heart disease in the United States, about 60% more than die of cancer. The figure rises to almost 1 million if strokes and other circulatory diseases are included in the global term cardiovascular disease. About 1.5 million people in the United States each year have a heart attack. The overall male-tofemale ratio for cardiovascular disease is about 2:1. Women generally lag about 10 years behind men in developing the disease. Still, it eventually kills more women than any other disease—twice as many as does cancer. And, for each person in
North America who dies of cardiovascular disease, 20 more (over 13 million people) have symptoms of the disease.
m Cardiovascular disease kills more women
than any other disease—twice as many as does cancer. deveLopMent oF
CaRdiovasCuLaR
disease
The symptoms of cardiovascular disease develop over many years and often do not become obvious until old age. Nonetheless, autopsies of young adults under 20 years of age have shown that many of them had atherosclerotic plaque in their arteries (Fig. 5-15). This finding indicates that plaque buildup can begin in childhood and continue throughout life, although it usually goes undetected for some time.
The typical forms of cardiovascular disease—coronary heart disease and strokes—are associated with inadequate blood circulation in the heart and brain related to buildup of this plaque. Blood supplies the heart muscle, brain, and other body organs with oxygen and nutrients.
When blood flow via the coronary arteries surrounding the heart is interrupted, the heart muscle can be damaged. A heart attack, or myocardial infarction, may result (review
Fig. 5-15). This may cause the heart to beat irregularly or to stop. About 25% of people do not survive their first heart attack. If blood flow to parts of the brain is interrupted long enough, part of the brain dies, causing a cerebrovascular accident, or stroke.
A heart attack can strike with the sudden force of a sledgehammer, with pain radiating up the neck or down the arm. It can sneak up at night, masquerading as indigestion, with slight pain or pressure in the chest. Many times, the symptoms are so subtle in women that death occurs before she or the health professional realizes that a heart attack is taking place. If there is any suspicion at all that a heart attack is taking place, the person should first chew an aspirin (325 milligrams) thoroughly and then call 911. Aspirin helps reduce the blood clotting that leads to a heart attack.
Continuous formation and breakdown of blood clots in the blood vessels is a normal process. However, in areas where plaques build up, blood clots are more
plaque A cholesterol-rich substance deposited in the blood vessels; it contains various white blood cells, smooth muscle cells, various proteins, cholesterol and other lipids, and eventually calcium.
myocardial infarction Death of part of the heart muscle. Also termed a heart attack.
cerebrovascular accident (CVA) Death of part of the brain tissue due typically to a blood clot. Also termed a stroke.
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Cell lining
(endothelium)
Muscle layer
c Typical warning signs of a heart attack are:
•
Intense, prolonged chest pain or pressure, sometimes radiating to other parts of the upper body (men and women)
•
Shortness of breath (men and women) •
Sweating (men and women)
•
Nausea and vomiting (especially women) •
Dizziness (especially women)
•
Weakness (men and women)
•
Jaw, neck, and shoulder pain (especially women)
•
Irregular heartbeat (men and women) Vessel opening
Normal artery
Beginning cholesterol plaque
Aorta
Left coronary artery Early injury
Advanced cholesterol plaque
Pulmonary artery Right coronary artery Area of occlusion Significant atherosclerosis Blood clot
Area of muscle damage
Complete
blockage
FiguRe 5-15 c The road to a heart attack. Injury to an artery wall begins the process. This is followed by a progressive buildup of plaque in the artery walls. The heart attack represents the terminal phase of the process. Blockage of the left coronary artery by a blood clot is evident.
The heart muscle served by the portion of the coronary artery beyond the point of blockage lacks oxygen and nutrients and is damaged and may die. This damage can lead to a significant drop in heart function and often total heart failure. m At the first sign of a possible heart at-
tack, the person should first thoroughly chew an aspirin and then call 911.
c Healthy People 2010 has set a goal of reducing death from coronary heart disease by 30%, compared with today’s incidence.
homocysteine An amino acid not used in protein synthesis, but instead arises during metabolism of the amino acid methionine.
Homocysteine is likely toxic to many cells, such as those lining the blood vessels.
likely to remain intact and then lead to a blockage, cutting off or diminishing the supply of blood to the heart (via the coronary arteries) or brain (via the carotid arteries).
More than 95% of heart attacks are caused by such blood clots. Heart attacks generally are caused by total blockage of the coronary arteries due to a blood clot forming in an area of the artery already partially blocked by plaque. Disruption of the plaque may even lead to eventual clot formation.
Atherosclerosis probably first develops to repair damage in a vessel lining. The damage that starts this process can be caused by smoking, diabetes, hyperten-
sion, homocysteine (likely, but not a major factor), LDL, and viral and bacterial infection. Ongoing inflammation in the blood vessel is also suspected to cause blood vessel damage. (A laboratory test for c-reactive protein in the blood is used to assess if inflammation is present.) Atherosclerosis can be seen in arteries throughout the body. The damage develops especially at points where an artery branches into two smaller vessels. A great deal of stress is placed on the vessel walls at these points due to changes in blood flow.
Once blood vessel damage has occurred, the next step in the development
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of atherosclerosis is the progression phase.
This step is characterized by deposition of plaque at the site of initial damage. The rate of plaque buildup during the progression phase is directly related to the amount of LDL in the blood. The form of LDL that contributes to atherosclerosis is oxidized
LDL. This form is preferentially taken up by scavenger cells in the arterial wall.
Nutrients and phytochemicals that have antioxidant properties may reduce LDL oxidation. Fruits and vegetables are particularly rich in these compounds. Eating fruits and vegetables regularly is one positive step we can make to reduce plaque buildup and slow the progression of cardiovascular disease. Some fruits and vegetables particularly helpful in this regard include legumes (beans), nuts, dried plums (prunes), raisins, berries, plums, apples, cherries, oranges, grapes, spinach, broccoli, red bell peppers, potatoes, and onions. Tea and coffee are also sources of antioxidants.
As plaque accumulates, arteries harden, narrow, and lose their elasticity. Affected arteries become further damaged as blood pumps through them and pressure increases. Finally, in the terminal phase, a clot or spasm in a plaque-clogged artery leads to a heart attack.
Factors that typically bring on a heart attack in a person already at risk include dehydration; acute emotional stress (such as firing an employee); strenuous physical activity when not otherwise physically fit (shoveling snow, for example); waking during the night or getting up in the morning (linked to an abrupt increase in stress); and consuming large, high-fat meals (increases blood clotting).
Risk FaCtoRs FoR
CaRdiovasCuLaR
disease
Many of us are free of the risk factors that contribute to rapid development of atherosclerosis. If so, the advice of health experts is to consume a balanced diet, perform regular physical activity, have a complete fasting lipoprotein analysis performed at age 20 or beyond, and reevaluate risk factors every 5 years.
For most people, however, the most likely risk factors are:
• Total blood cholesterol over 200 milligrams per 100 milliliters of blood
(mg/dl; dl is short for deciliter or 100 milliliters). Risk is especially high when total cholesterol is at or over 240 mg/dl and LDL-cholesterol readings are over
130 to 160 mg/dl. (The terms LDLcholesterol and HDL-cholesterol are used when expressing the blood concentration because it is the cholesterol content of these lipoproteins that is measured.) • smoking. Smoking is the main cause of about 20% of cardiovascular disease deaths and generally negates the female advantage of later occurrence of the disease. A combination of smoking and oral contraceptive use increases the risk of cardiovascular disease in women even more. Smoking greatly increases the expression of a person’s genetically linked risk for cardiovascular disease, even if one’s blood lipids are low.
Smoking also makes blood more likely to clot. Even secondhand smoke has been implicated as a risk factor.
• Hypertension. systolic blood pressure over 139 (millimeters of mercury) and diastolic blood pressure over 89 indicate hypertension. Healthy blood pressure values are less than 120 and 80, respectively. (Treatment of hypertension is reviewed in Chapter 9.)
• Diabetes. Diabetes virtually guarantees development of cardiovascular disease, and so puts a person with diabetes in the high-risk group. Insulin increases
oxidize In the most basic sense, the loss of an electron or gain of an oxygen by a chemical substance. This change typically alters the shape and/or function of the substance.
antioxidant Generally a compound that stops the damaging effects of reactive substances seeking an electron (i.e., oxidizing agents). This prevents breakdown (oxidizing) of substances in foods or the body, particularly lipids.
systolic blood pressure The pressure in the arterial blood vessels associated with the pumping of blood from the heart.
diastolic blood pressure The pressure in the arterial blood vessels when the heart is between beats.
m Smoking is one of the four major risk factors
for developing cardiovascular disease.
Making deCisions
Antioxidant Supplements and Cardiovascular Disease
Are large doses of antioxidant vitamin supplements a reliable way to reduce LDL oxidation, and thereby prevent cardiovascular disease? Controversy about such dietary supplementation exists among the experts, as will be detailed in Chapter 8. The American Heart Association does not support use of antioxidant supplements (such as vitamin E) to reduce cardiovascular disease risk.
This is because large-scale studies have shown no decrease in cardiovascular disease risk with use of antioxidant supplements. One study even showed a modest increase for heart failure in people with diabetes or otherwise at high risk for cardiovascular disease after taking antioxidant supplements. However, further trials of antioxidant supplementation ( e.g., 600 IU of vitamin E every other day) for prevention of heart disease in men are ongoing. With regard to women,
600 IU of natural-source vitamin E taken every other day provided no overall benefit for major cardiovascular events (or cancer). More research is warranted, however, as a decrease in sudden cardiac death was seen in a subset of older women in this study. Some experts suggest daily vitamin E supplementation (100 IU to 400 IU) may be helpful for preventing cardiovascular disease, while other experts recommend against the practice. One thing is certain: any supplementation of vitamin E should be taken only under the guidance of a physician, because of interactions with vitamin K and anticlotting medications (and possibly high-dose aspirin use).
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c For more information on cardiovascular disease, see the website of the American
Heart Association at www.americanheart. org or the heart disease section of Healthfinder at www.healthfinder.gov/tours/heart. htm. This is a site created by the U.S. government for consumers. In addition, visit the website www.nhlbi.nih.gov/.
c Healthy People 2010 has set a goal of reducing total blood cholesterol among adults from the average of 206 mg/dl to 199 mg/dl, as well as reducing the percentage of adults with high blood cholesterol from 21% to 17%.
c Cardiovascular Disease Risk Factors
•
Total blood cholesterol >200 mg/dl
•
Smoking
•
Hypertension
•
Diabetes
•
HDL cholesterol < 40 mg/dl
•
Age: Men >45 yr; Women > 55 yr
•
Family history of cardiovascular disease
•
Blood triglycerides > 200 mg/dl
•
Obesity
•
Inactivity
c Two approaches have been shown to cause reversal of atherosclerosis in the body.
One employs a vegan diet and other lifestyle changes that are part of the Dr. Dean Ornish program. The other employs aggressive LDL lowering with medications.
c As noted earlier, aspirin in small doses reduces blood clotting. It is often used under a physician’s guidance to treat people at risk for heart attack or stroke, especially if one has already occurred. About 80 to 160 milligrams per day is needed for such benefits.
Individuals who may especially benefit from aspirin therapy are men over 40, smokers, postmenopausal women, and people with diabetes, hypertension, or a family history of cardiovascular disease.
cholesterol synthesis in the liver, in turn increasing LDL in the bloodstream. This disease negates any female advantage.
Together, the previous four risk factors explain most cases of cardiovascular disease.
Other risk factors to consider:
• HDL-cholesterol under 40 mg/dl, especially when the ratio of total cholesterol to HDL-cholesterol is greater than 4:1
(3.5:1 or less is optimal). Women often have high values for HDL-cholesterol, and therefore it is important for this to be measured in women to establish cardiovascular disease risk. A value of
60 mg/dl or more is especially protective. Exercising for at least 45 minutes four times a week can increase HDL by about 5 mg/dl. Losing excess weight
(especially around the waist) and avoiding smoking and overeating also help maintain or raise HDL, as does moderate alcohol consumption.
• Age. Men over 45 years and women over 55 years.
• Family history of cardiovascular disease, especially before age 50.
• Blood triglycerides 200 mg/dl or greater in the fasting state (less than 100 mg/dl is optimal).
• Obesity (especially fat accumulation in the waist). Typical weight gain seen in adults is a chief contributor to the increase in LDL seen with aging. Obesity also typically leads to insulin resistance, creating a diabetes-like state, and ultimately the disease itself. It also increases overall inflamation throughout the body.
• Inactivity. Exercise conditions the arteries to adapt to physical stress. Regular exercise also improves insulin action in the body. The corresponding reduction in insulin output leads to a reduction in lipoprotein synthesis in the liver. Both regular aerobic exercise and resistance exercise are recommended. A person with existing cardiovascular disease should seek physician approval before starting such a program, as should older adults.
Researchers are trying to unravel and quantify numerous other factors that may be linked to premature cardiovascular disease, such as the connection between inad-
equate intake of vitamin B-6, folate, and vitamin B-12, which can lead to increased homocysteine in the blood. As noted, homocysteine may damage the cells lining the blood vessels, in turn promoting atherosclerosis. Studies show that supplementation with folic acid and B vitamins does not reduce risk of recurrent cardiovascular disease in patients that had previous heart attacks or had existing vascular diseases.
The term risk factor is not equivalent to cause of disease; nevertheless, the more of these risk factors one has, the greater the chances of ultimately developing cardiovascular disease. A good example is the metabolic syndrome (also called Syndrome
X) discussed in Chapter 4. A person with the metabolic syndrome would have abdominal obesity, high blood triglycerides, low HDL-cholesterol, hypertension, poor blood glucose regulation (i.e., high fasting blood glucose), and increased blood clotting. This profile raises the risk for cardiovascular disease considerably. On a positive note, cardiovascular disease is rare in populations who have low LDLcholesterol, normal blood pressure, and do not smoke or have diabetes. By minimizing these risk factors, as well as following the dietary recommendations of the
American Heart Association on page 188 and staying physically active, one will most likely reduce many of the other controllable risk factors listed. In other words, develop and follow a total lifestyle plan.
Medications may also be added to lower blood lipids, as discussed next. Finally, if a person has a family history of cardiovascular disease but the usual risk factors aren’t present, a rarer defect might be the cause. In this case, having a detailed physical examination for other potential causes is advised.
MediCaL inteRventions to LoWeR bLood Lipids
Some people need even more aggressive therapy added to their regimen of a diet and lifestyle overhaul to treat elevated blood lipids. The clearest indication for this more aggressive approach is in people who already have had a heart attack or have cardiovascular disease symptoms or diabetes. 194
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Medications are the cornerstone of this more aggressive therapy. The National
Cholesterol Education Program in the
United States has developed a formula based on age, total blood cholesterol,
HDL-cholesterol, smoking history, and blood pressure to determine who needs such medications. Check out this formula at http://hp2010.nhlbihin.net/atpiii/ calculator.asp?usertype=pub. This formula provides an estimate for having a heart attack in the next 10 years.
Medications work to lower LDL in one of two ways. Some reduce cholesterol synthesis in the liver. Such medicines are known as
“statins” (e.g., atorvistatin [Lipitor]). The cost of treatment with one of these drugs can be from $1,600 or more per year, depending on the dose needed. These medications lead to problems in some people, and so require physician monitoring, especially of liver function. Another group of medications binds bile acids or the cholesterol that is part of bile secreted into the small intestine. This binding leads to their elimination in the feces and so requires the liver to synthesize new bile acids and/or cholesterol. The liver removes
LDL from the blood to do this. Some of these medications taste gritty and therefore are not popular. The current therapeutic goal for people with (or at high risk for) cardiovascular disease (Table 5-9) is to drive LDL down to less than 70 mg/dl.
The statin drug simvastatin (Zocor) has been combined with another drug (ezetimibe) and is marketed as Vytorin, a drug that will treat the two sources of cholesterol, “food and family.” While the statin reduces the cholesterol made by the liver, the ezetimibe helps block the absorption of cholesterol from food.
A third group of drugs can be used to lower blood triglycerides by decreasing the triglyceride production of the liver. These include gemfibrozil (Lopid) and megadoses of the vitamin nicotinic acid. The use of nicotinic acid does result in pesky side effects, however, but these are typically manageable. Other Possible Medical Therapies for Cardiovascular Disease
FDA has approved two margarines that have positive effects on blood cholesterol levels—Benecol and Take Control. These
margarines contain plant stanols/sterols.
The plant stanols/sterols, also called phytosterols, work by reducing cholesterol absorption in the small intestine and lowering its return to the liver. The liver responds by taking up more cholesterol from the blood so it can continue to make bile acids. The studies done on the cholesterollowering effect of these margarines have found that 2 to 5 grams of plant stanols/ sterols per day reduces total blood cholesterol by 8% to 10% and LDL-cholesterol by
9% to 14% (similar to what is seen with some cholesterol-lowering drugs).
Benecol is made from plant stanols extracted from wood pulp. This product is sold as margarine and has been added to salad dressings. Take Control is made from plant sterols isolated from soybeans. The recommended amount for both is about
2 to 3 grams per day as part of at least two meals; this works out to about 1 tablespoon of Benecol or 2 tablespoons of Take
Control per day. Use would cost about
$1.00 per day, as these margarines are more expensive than regular margarines.
In people who have borderline high total blood cholesterol (between 200 and
239 mg/dl), these margarines can be helpful in avoiding future drug therapy. Plant stanols/sterols have been made available in pill form as well. Remember that plant sterols are naturally present in nuts in high concentrations. Wheat germ, sesame seeds, pistachios, and sunflower seeds are some of the richest sources.
The two most common surgical treatments for coronary artery blockage are percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass graft (CABG). PTCA involves the insertion of a balloon catheter into an artery. Once it is advanced to the area of the lesion, the balloon is expanded to crush the lesion.
This method works best when only one vessel is blocked, and it may be held open with metal mesh, called a stent. CABG involves the removal and use of a saphenous vein, a large vein in the leg, or use of a mammary artery. The relocated vein is sewn to the main heart vessel (aorta) and then used to bypass the blocked artery. The procedure can be performed on one or more blockages.
Benecol and Take Control are margarines
that are examples of “functional foods” because they contain added cholesterol-lowering plant stanols/sterols. tABLe 5-9 Sorting Out One’s Goals for Cardiovascular Disease Prevention/Treatment
Risk Class
This Is You If …
Very high
You have cardiovascular disease and other risk factors such as diabetes, obesity, smoking
Below 70
High
You have cardiovascular disease or diabetes or two or more risk factors for cardiovascular disease (e.g., smoking, hypertension)
Below 100
Your LDL (mg/dl) goal
Moderately high You have cardiovascular disease risks and an increased 10-year risk of developing a heart attack
Moderate
You have 2 or more risk factors and a marginal chance of developing cardiovascular disease in the next 10 years.
Low
You have few or no cardiovascular disease risk factors 100–130
Below 130
Below 160
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Case studY
Planning a Heart Healthy Diet
m Are there important food groups missing
from Jackie’s new diet plan?
Jackie is a 21-year-old health-conscious individual majoring in business. She recently learned that a diet high in saturated fat can contribute to high blood cholesterol and that exercise is beneficial for the heart. Jackie now takes a brisk 30-minute walk each morning before going to class, and she has started to cut as much fat out of her diet as she can, replacing it mostly with carbohydrates. A typical day for Jackie now begins with a bowl of Fruity
Pebbles with 1 cup of skim milk and ½ cup of apple juice. For lunch, she might pack a turkey sandwich on white bread with lettuce, tomato, and mustard; a small package of fat-free pretzels; and a handful of reduced-fat vanilla wafers. Dinner could be a large portion of pasta with some olive oil and garlic mixed in, and a small iceberg lettuce salad with lemon juice squeezed over it. Her snacks are usually baked chips, low-fat cookies, fat-free frozen yogurt, or fat-free pretzels. She drinks diet soft drinks throughout the day as her main beverage.
Answer the following questions, and check your response in Appendix A.
1. Has Jackie made the best diet changes with regard to lowering blood cholesterol and maintaining heart health?
2. Is there much fat left in Jackie’s new diet plan? Is it necessary for her to drastically lower her fat intake?
3. What types of fat should Jackie try to consume? Why are these types of fat the most desirable?
4. What types of foods has Jackie used to replace the fat in her diet?
5. What food groups are missing from her new diet plan? How many servings should she be including from these food groups? 6. Is Jackie’s new exercise routine appropriate? suMMaRY
1. Compared with carbohydrates and proteins, lipids are a group of relatively oxygen-poor compounds that do not dissolve in water. Saturated fatty acids contain no carbon-carbon double bonds, monounsaturated fatty acids contain one carbon-carbon double bond, and polyunsaturated fatty acids contain two or more carbon-carbon double bonds in the carbon chain.
2. In omega-3 polyunsaturated fatty acids, the first of the carbon-carbon double bonds is located three carbons from the methyl end of the carbon chain. In omega-6 polyunsaturated fatty acids, the first carbon-carbon double bond counting from the methyl end occurs at the sixth carbon. Both omega-3 and omega-6 fatty acids are essential fatty acids; these must be included in the diet to maintain health.
3. Triglycerides are formed from a glycerol backbone with three fatty acids. Triglycerides rich in long-chain saturated fatty acids tend to be solid at room tem-
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perature, whereas those rich in monounsaturated and polyunsaturated fatty acids are liquid at room temperature.
Triglyceride is the major form of fat in both food and the body. It allows for efficient energy storage, protects certain organs, transports fat-soluble vitamins, and helps insulate the body.
4. Phospholipids are derivatives of triglycerides in which one or two of the fatty acids are replaced by phosphoruscontaining compounds. Phospholipids are important parts of cell membranes, and some act as efficient emulsifiers.
5. Cholesterol forms vital biological compounds, such as hormones, components of cell membranes, and bile acids. Cells in the body make cholesterol whether we eat it or not. It is not a necessary part of an adult’s diet.
6. Body cells can synthesize hormone-like compounds from both omega-3 and omega-6 fatty acids. The compounds produced from omega-3 fatty acids tend to reduce blood clotting, blood
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pressure, and inflammatory responses in the body. Those produced from omega-6 fatty acids tend to increase blood clotting.
7. Foods rich in fat include salad oils, butter, margarine, and mayonnaise.
Nuts, bologna, avocados, and bacon are also high in fat, as are peanut butter and cheddar cheese. Steak and hamburger are moderate in fat content, as is whole milk. Many grain products, and fruits and vegetables in general, are low in fat.
8. Fats and oils have several functions as components of foods. Fats add flavor and texture to foods and provide some satiety after meals. Some phospholipids are used in foods as emulsifiers, which suspend fat in water. When fatty acids break down, food becomes rancid, resulting in a foul odor and unpleasant flavor.
9. Hydrogenation is the process of converting carbon-carbon double bonds into single bonds by adding hydrogen at the point of unsaturation. The
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partial-hydrogenation of fatty acids in vegetable oils changes the oils to semisolid fats and helps in food formulation and reduces rancidity. Hydrogenation also increases trans fatty acid content. High amounts of trans fat in the diet are discouraged, as these increase LDL and reduce HDL.
10. Fat digestion takes place primarily in the small intestine. Lipase enzyme released from the pancreas digests long-chain triglycerides into smaller breakdown products—namely, monoglycerides (glycerol backbones with single fatty acids attached) and fatty acids. The breakdown products are then taken up by the absorptive cells of the small intestine. These products are mostly remade into triglycerides and eventually enter the lymphatic system, in turn passing into the bloodstream.
11. Lipids are carried in the bloodstream by various lipoproteins, which consist of a central triglyceride core encased in a shell of protein, cholesterol, and phospholipid. Chylomicrons are released from intestinal cells and carry lipids arising from dietary intake. Very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) carry lipids both taken up by and synthesized in the liver.
High-density lipoprotein (HDL) picks up cholesterol from cells and facilitates its transport back to the liver.
12. There is currently no RDA for fat for adults. Plant oils should contribute about 5% of total calories to achieve the
Adequate Intakes proposed for essential fatty acids (linoleic acid and alphalinolenic acid). Fatty fish are a rich source of omega-3 fatty acids and should be consumed at least twice a week.
13. Many health agencies and scientific groups suggest a fat intake of no more than 30% to 35% of total calories.
Some health experts advocate an even further reduction to 20% of calorie intake for some people to maintain a normal LDL value, but such a diet requires professional guidance. Medications such as “statins” may be added also to lower LDL. If fat intake exceeds
30% of total calories, the diet should emphasize monounsaturated fat. The typical North American diet contains about 33% of total calories as fat.
14. In the blood, elevated amounts of LDL and low amounts of HDL are strong predictors of the risk for cardiovascular disease. Additional risk factors for the disease are smoking, hypertension, diabetes, obesity, and inactivity.
studY Questions
1. Describe the chemical structures of saturated and polyunsaturated fatty acids and their different effects in both food and the human body.
2. Relate the need for omega-3 fatty acids in the diet to the recommendation to consume fatty fish at least twice a week.
3. Describe the structures, origins, and roles of the four major blood lipoproteins.
4. What are the recommendations from various health-care organizations
regarding fat intake? What does this mean in terms of food choices?
story with respect to cardiovascular disease risk?
5. What are two important attributes of fat in food? How are these different from the general functions of lipids in the human body?
8. List the four main risk factors for the development of cardiovascular disease.
6. Describe the significance of and possible uses for reduced-fat foods.
7. Does the total cholesterol concentration in the bloodstream tell the whole
9. What three lifestyle factors decrease the risk of cardiovascular disease development? 10. When are medications most needed in cardiovascular disease therapy, and how in general do the various classes of medications operate to reduce risk?
CheCk YouR knoWLedge
Answers to the following multiple choice questions are in Appendix A.
1. Margarine usually is made by a process called _____________, in which hydrogen atoms are added to carbon-carbon double bonds in the polyunsaturated fatty acids found in vegetable oils.
a. saturation
b. esterification
c. isomerization
d. hydrogenation
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2. Essential fatty acids that cause a decrease in blood clotting are
a. omega-3.
b. omega-6.
c. omega-9.
d. prostacyclins.
c. an important part of human cell membranes and necessary to make some hormones.
d. All of the above.
4. Which of the following groups of foods would be important sources of saturated fatty acids?
a. olive oil, peanut oil, canola oil
b. palm oil, palm kernel oil, coconut oil
c. safflower oil, corn oil, soybean oil
d. All of the above.
3. Cholesterol is
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5. Lipoproteins are important for
a. transport of fats in the blood and lymphatic system.
b. synthesis of triglycerides.
c. synthesis of adipose tissue.
d. enzyme production.
6. Which of the following foods is the best source of omega-3 fatty acids?
a. fatty fish
b. peanut butter and jelly
c. lard and shortenings
d. beef and other red meats
7. Immediately after a meal, newly digested and absorbed dietary fats appear in the lymph, and then blood, as part of which of the following?
a. LDL
b. HDL
c. chylomicrons
d. cholesterol
8. High blood concentrations of _______ decrease the risk for cardiovascular disease. a. low-density lipoproteins
b. chylomicrons
c. high-density lipoproteins
d. cholesterol
9. Phospholipids such as lecithin are used extensively in food preparation because they
a. provide the agreeable feel of fat melting on the tongue.
b. are excellent emulsifiers.
c. provide important textural features.
d. impart delicate flavors.
10. The main form of lipid found in the food we eat is
a. cholesterol.
b. phospholipids.
c. triglycerides.
d. plant sterols
FuRtheR Readings
1. ADA Reports: Position of the American Dietetic Association: Fat replacers. Journal of the
American Dietetic Association 105:266, 2005.
The majority of fat replacers, when used in moderation by adults, can be safe and useful adjuncts to lowering the fat content of foods and may play a role in decreasing total calorie and fat intake. Still, consumers should not be led to believe that fat and calorie-reduced products can be consumed in unlimited amounts
2. Bønaa KH and others: Homocysteine lowering and cardiovascular events after acute myocardial infarction. The New England Journal of Medicine 354:1578, 2006.
The risk of recurrent cardiovascular disease after acute myocardial infarction was not lowered by treatment with folic acid, vitamin B-12, or vitamin B-6. A trend toward an increased risk suggested a harmful effect of the combined B vitamin treatment. Treatment with these B vitamins is therefore not recommended.
3. Diercks DB and others: The obesity paradox in non–ST-segment elevation acute coronary syndromes. American Heart Journal 152:140,
2006.
This study discovered that although obese individuals are at increased risk of suffering a heart attack, their outcomes are better because they are treated more aggressively. Overweight and obese patients were more likely than normalweight patients to receive medications and to have invasive cardiac procedures.
4. Fisler JS, Warden CH: Dietary fat and genotype: Toward individualized prescriptions for lifestyle changes. American Journal of
Clinical Nutrition 81:1255, 2005.
There is great variation in the effect of dietary fat on blood lipids. At least part of that variation is due to genetics. This article discusses the challenges involved in identifying the interactive
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effects of nutrients and genes in the area of lipids, cholesterol, and coronary artery disease.
5. Fletcher B and others: Managing abnormal blood lipids: A collaborative approach. Circulation 112:3184, 2005.
The treatment of abnormal blood lipids is complex and therefore requires involvement of various types of healthcare professionals, including dietitians. This paper describes a model multidisciplinary case management approach for patients with abnormal blood lipids. The model includes both primary and secondary prevention across the lifespan and incorporates nutritional and exercise intervention.
6. Grundy SM and others: Diagnosis and management of the metabolic syndrome. Circulation 112: 1350, 2005.
The constellation of metabolic risk factors known as metabolic syndrome consists of many factors, including elevated blood triglycerides, low HDL cholesterol concentrations, elevated blood pressure, elevated blood glucose, increased blood clotting, and an inflammatory state in the body. The most important of these underlying risk factors are abdominal obesity and insulin resistance. Other associated conditions include physical inactivity, aging, hormonal imbalance, and genetic or ethnic predisposition.
7. Hansson GK: Inflammation, atherosclerosis, and coronary artery disease. The New England Journal of Medicine 352:1685, 2005.
Excellent review of the role of inflammation in causing cardiovascular disease. Identifying atherosclerosis as an inflammatory disease offers new opportunities for the prevention and treatment of coronary artery disease.
8. He KA, Daviglus ML: A few more thoughts about fish and fish oil. Journal of the American Dietetic Association 105:428, 2005.
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Fish consumption has been shown to have beneficial effects on cardiovascular disease.
Consuming whole fish is thought to have greater benefits and be safer than using fish oil supplements. Fish is not only a rich source of omega-3 fatty acids, but is also a valuable source of protein and trace elements that may also provide protective effects for the cardiovascular system.
Broiled or baked fish is recommended rather than fried fish because frying may decrease the ratio of omega-3 to omega-6 fatty acids and may produce trans fatty acids and oxidized lipid products that may increase cardiovascular disease risk.
9. Kuller LH: Nutrition, lipids, and cardiovascular disease. Nutrition Reviews 64:S15, 2006.
The development of coronary heart disease is discussed as an epidemic due to increased consumption of saturated fat and cholesterol, low intakes of polyunsaturated fat, and obesity. The risk of the disease is increased by hypertension, smoking, and diabetes. The careful monitoring and prevention of this disease beginning in young adults is important but expensive.
10. Lau VWY and others: Plant sterols are efficacious in lowering plasma LDL and non-HDL cholesterol in hypercholesterolemic type 2 diabetic and nondiabetic persons. American
Journal of Clinical Nutrition 81:1351, 2005.
Incorporation of plant sterols into a lowsaturated-fat and low-cholesterol diet for persons at increased risk of CVD mortality could have a positive effect on reducing the mortality rate, including in those people with type 2 diabetes.
11. Lee I and others: Vitamin E in the primary prevention of cardiovascular disease and cancer: The women’s health study: A randomized controlled trial. Journal of the
American Medical Association 294:56, 2005.
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The data from this large trial indicated that
600 IU of natural-source vitamin E taken every other day provided no overall benefit for major cardiovascular events or cancer. These data do not support recommending vitamin E supplementation for cardiovascular disease or cancer prevention among healthy women, but more research is warranted, as a decrease in sudden cardiac death was seen in a subset of older women in this study.
trial. Journal of the American Medical Association 293:1338, 2005.
No significant differences were noted between the vitamin E supplementation group and the control group in the incidence of cancer or deaths related to cancer. There also were no differences in the main composite of cardiovascular outcomes including death or rehospitalization. A significant increased risk in heart failure and related complications occurred in the vitamin E group.
12. Lewis NM and others: The walnut: A nutritional nut case. Today’s Dietitian 6(8):36, 2004.
Compared to other nuts and seeds, walnuts are one of the richest sources of alpha-linolenic acid
(2.6 grams per 1-ounce serving or 14 walnut halves). The DRI for alpha-linolenic acid is
1.6 grams/day for men and 1.1 grams/day for women. In addition, walnuts are a rich source of plant sterols known to inhibit intestinal absorption of cholesterol.
15. Lukaczer D and others: Effect of a low glycemic index diet with soy protein and phytosterols on CVD risk factors in postmenopausal women. Nutrition 22(2):104, 2006.
Women in this 12-week study showed decreases in total cholesterol, LDL cholesterol, and triglycerides after consuming a low-glycemic index diet with a functional food delivering 30 grams of soy protein and 4 grams of phytosterols per day.
13. Lichtenstein AH and others: Diet and lifestyle recommendations revision 2006. A scientific statement from the American Heart
Association Nutrition Committee. Circulation 114:82, 2006.
The American Heart Association presents recommendations designed to reduce the risk of cardiovascular disease risk reduction in the general population. Specific goals are presented and include consuming an overall healthy diet; to aim for a healthy body weight, recommended levels of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides; normal blood pressure; normal blood glucose level; being physically active; and avoiding the use of and exposure to tobacco products. 14. Lonn E and others: Effects of long-term vitamin E supplementation on cardiovascular events and cancer: A randomized controlled
16. Meisinger C and others: Plasma oxidized low-density lipoprotein, a strong predictor for acute coronary heart disease events in apparently healthy, middle-aged men from the general population. Circulation
112:651, 2005.
Elevated concentrations of oxidized low-density lipoprotein are predictive of future coronary heart disease events in apparently healthy men.
Thus, oxidized LDL may represent a promising risk marker for clinical coronary heart disease complications and should be evaluated in further studies. 17. Ohr LM: The (heart) beat goes on. Food Technology 60(6):87, 2006.
Research and ingredient innovations in the food industry related to the heart-health field are described. Ingredients that have the advantage of a heart-health claim include omega-3 fatty acids and plant sterols. Also discussed are
dietary fiber and grains, soy, antioxidants, and nuts. 18. Phillips KM and others: Phytosterol composition of nuts and seeds commonly consumed in the United States. Journal of Agriculture and Food Chemistry 53:9436, 2005.
This study set out to find the nuts and seeds that could provide the most heart-protective benefits.
Of the 27 nuts and seeds analyzed, wheat germ and sesame seeds had the greatest concentration of phytosterols and Brazil nuts and walnuts ranked the lowest. Pistachio and sunflower seeds were the richest sources of phytosterols for products typically consumed as snack foods. 19. Reese MATB: Beyond the headlines: The lowdown on low-fat diets. Today’s Dietitian
8(6):32, 2006.
Results of the 8-year Women’s Health Initiative study showed that a reduction in dietary fat had almost no effect on the incidence of heart attacks and strokes or breast and colon cancer. This article explains some of the study characteristics that most likely contributed to the unexpected results.
Most important to consider is that the subjects were all postmenopausal women who had all been on a high-fat diet. The results suggest that starting a low-fat diet at 50 to 79 years may be too late to see extraordinary benefits. 20. Wang C and others: n-3 fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies:
A systematic review. American Journal of Clinical
Nutrition 84:5, 2006.
This review of previous studies found that consumption of omega-3 fatty acids from fish or fish oil but not alpha-linolenic acid, significantly reduced all-cause mortality, myocardial infarction, cardiac and sudden death, or stroke.
c Check out the Contemporary Nutrition ARIS site
www.mhhe.com/wardlawcont7 for quizzes, flash cards, other activities, and weblinks designed to further help you learn about what you eat and why.
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Rate YouR pLate
i. are You eating a diet that includes Many saturated Fat and Trans Fatty acid sources?
Instructions: In each row of the following list, circle your typical food selection from either column A or B.
Column A
Column B
Bacon and eggs
or
R eady-to-eat whole-grain breakfast cereal
Doughnut or sweet roll
or
W hole-wheat roll, bagel, or bread
Breakfast sausage
or
F ruit
Whole milk
or
R educed-fat, low-fat, or fat-free milk
Cheeseburger
or
T urkey sandwich, no cheese
French fries
or
P lain baked potato with salsa
Ground chuck
or
G round round
Soup with cream base
or
S oup with broth base
Macaroni and cheese
or
M acaroni with marinara sauce
Cream/fruit pie
or
G raham crackers
Ice cream
or
F rozen yogurt, sherbet, or reduced-fat ice cream
Butter or stick margarine
or
V egetable oils or soft margarine in a tub
Interpretation
The foods listed in column A tend to be high in saturated fat, trans fatty acids, cholesterol, and total fat. Those in column B generally are low in these dietary components. If you want to help reduce your risk of cardiovascular disease, choose more foods from column B and fewer from column A.
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ii. Applying the nutrition Facts Label to Your Daily Food Choices
Imagine that you are at the supermarket looking for a quick snack to help you keep your energy up during afternoons. In the snack section, you settle on two choices (see labels a and b). Evaluate the products using the table on the left.
ompare the nutrients in each product by comC pleting this list. For each serving, which product is lower in each of the following? Calories
(a)
Calories from Fat
(a)
(b) (b)
no difference
(a)
(b)
no difference
Saturated Fat
(a)
(b)
no difference
Trans Fat
(a)
(b)
no difference
Cholesterol
(a)
(b)
no difference
Sodium
(a)
(b)
no difference
Total Carbohydrates
(a)
(b)
no difference
Dietary Fiber
(a)
(b)
no difference
Sugars
(a)
(b)
no difference
Protein
(a)
(b)
no difference
(a)
(b)
no difference
Which package has more servings per container? (a) (b) no difference Which of the two brands would you choose? (a) (b) neither What information on the Nutrition Facts labels contributed to your decision?
Nutrition Facts
Serving Size: 2 cookies (38g)
Servings Per Container: about 12
Amount Per Serving
2 bars
Calories 180
Calories from Fat 50
no difference
Total Fat
Iron
Nutrition Facts
Serving Size: 2 bars (42g)
Servings Per Container: 6
Amount Per Serving
% Daily Value*
Total Fat 6g
Saturated Fat 0.5g
Trans fat 0g
Cholesterol 0mg
Sodium 160mg
Total Carbohydrates 29g
Dietary Fiber 2g
Sugars 11g
Protein 4g
Iron
9%
3%
**
0%
7%
10%
8%
6%
Not a significant source of Vitamin A, Vitamin C, and calcium. ** Intake of trans fat should be as low as possible.
* Daily values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs:
** Intake should be as low as possible.
Total Fat
Saturated Fat
Cholesterol
Sodium
Total Carbohydrates
Dietary Fiber
Calories
2,000
2,500
Less than
65g
80g
Less than
20g
25g
Less than
300mg
300mg
Less than
2,400mg 2,400mg
300g
375g
25g
30g
INGREDIENTS: WHOLE GRAIN ROLLED OATS, SUGAR,
CANOLA OIL, CRISP RICE WITH SOY PROTEIN (RICE
FLOUR, SOY PROTEIN CONCENTRATE, SUGAR, MALT,
SALT), HONEY, BROWN SUGAR SYRUP, HIGH
FRUCTOSE CORN SYRUP, SALT, SOY LECITHIN,
BAKING SODA, NATURAL FLAVOR, PEANUT FLOUR,
ALMOND FLOUR, HAZELNUT FLOUR, WALNUT FLOUR,
PECAN FLOUR.
Calories 180
Calories from Fat 70
% Daily Value*
Total Fat 7g
Saturated Fat 2g
Trans fat 2g
Cholesterol 0mg
Sodium 100mg
Total Carbohydrate 26g
Dietary Fiber 1g
Sugars 12g
Protein 2g
Vitamin A 0%
Calcium 0%
•
11%
10%
**
0%
4%
9%
4%
Vitamin C 0%
Iron
2%
•
** Intake of trans fat should be as low as possible. * Daily values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs:
** Intake should be as low as possible.
Total Fat
Saturated Fat
Cholesterol
Sodium
Total Carbohydrates
Calories
2,000
2,500
Less than
65g
80g
Less than
20g
25g
Less than
300mg
300mg
Less than
2,400mg 2,400mg
Dietary Fiber
300g
375g
25g
30g
Calories per gram: • Fat 9 • Carbohydrate 4
• Protein 4
INGREDIENTS: ENRICHED FLOUR (WHEAT FLOUR,
NIACIN, REDUCED IRON, THIAMINE MONONITRATE,
RIBOFLAVIN, FOLIC ACID), SUGAR, VEGETABLE OIL
SHORTENING (PARTIALLY HYDROGENATED SOYBEAN,
COCONUT, COTTONSEED, CORN AND/OR SAFFLOWER
AND/OR CANOLA OIL), CORN SYRUP, HIGH FRUCTOSE
CORN SYRUP, WHEY (A MILK INGREDIENT), CORN
STARCH, SALT, SKIM MILK, LEAVENING (BAKING SODA,
AMMONIUM BICARBONATE), ARTIFICIAL FLAVOR,
SOYBEAN LECITHIN, COLOR (CONTAINING FD&C
YELLOW #5 LAKE).
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ChApter oBJeCtiVeS Chapter 5 is designed to allow you to:
1. List four classes of lipids (fats) and the role of each in nutritional health.
4. Explain how lipids are digested and absorbed. 2. Distinguish between fatty acids and triglycerides. 5. Name the classes of lipoproteins and classify them according to their functions.
3. Differentiate among saturated, monounsaturated, and polyunsaturated fatty acids in terms of structure and food sources.
6. List the function of lipids, including the two essential fatty acids.
7. Discuss the implications of various fats, including omega-3 fatty acids, with respect to cardiovascular disease.
8. Characterize the symptoms of cardiovascular disease and highlight some known risk factors. ChApter outLine
Chapter Objectives
Refresh Your Memory
Broader Roles for Fatty Acids and
Triglycerides in the Body
Case Study: Planning a Heart Healthy
Diet
Lipids: Common Properties
Phospholipids in the Body
Lipids: Main Types
Cholesterol in the Body
Medical Interventions to Lower Blood
Lipids
Fats and Oils in Foods
Recommendations for Fat Intake
Making Lipids Available for Body Use
Nutrition and Your Health: Lipids and
Cardiovascular Disease
Carrying Lipids in the Bloodstream
Summary/Study Questions/Check Your
Knowledge/Further Readings
Rate Your Plate
Essential Functions of Fatty Acids
YOUR
BILL FROM A MEDICAL LABORATORY READS
“BLOOD
LIPID PROFILE—
$95.”
Your doctor informs you that your “triglycerides are too high.” A health-food advertisement suggests using Benecol margarine to lower blood cholesterol.
Advertisers plug foods “lowest in saturated fat.” All of these substances— triglycerides, cholesterol, and saturated fat—are lipids, a collective term referring to fats and oils.
Lipids contain more than twice the calories per gram (on average, 9 kcal) as proteins and carbohydrates (on average, 4 kcal each). Consumption of common saturated fatty acids also contributes to the risk of cardiovascular disease. As the comic in this chapter suggests, for these reasons, some concern about certain lipids is warranted, but lipids also play vital roles both in the body and in foods. Their presence in the diet is essential to good health, and in general, lipids such as those found in vegetable oils should comprise
20% to 35% of an adult’s total calorie intake.
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Let’s look at lipids in detail—their forms, functions, metabolism, and food sources. Chapter 5 will also look at the link between various lipids and the major “killer” disease in North America: cardiovascular disease, which involves the heart, including the coronary arteries (coronary heart disease), as well as other arteries in the body.
Refresh Your Memory
As you begin your study of lipids in Chapter 5, you may want to review:
• Legal definitions for various label descriptors, such as low-fat and fat-free in
Chapter 2
• The concept of energy density in Chapter 2
• The processes of digestion and absorption in Chapter 3
• The metabolic syndrome in Chapter 4
Lipids: Common Properties
Humans need very little fat in their diet to maintain health. In fact, the body’s need for the essential fatty acids can be met by daily consumption of about 2 to 4 tablespoons of plant oil incorporated into foods and consumption of fatty fish such as salmon or tuna at least twice weekly. If fish is not consumed, the essential fatty acids in canola oil, soybean oil, and walnuts contribute much of the same health benefits as those found in fish. Thus, one could follow a purely vegetarian diet containing about 10% of calories from fat and still maintain health. However, as long as saturated fat, cholesterol, and partially-hydrogenated fat (which contains trans fat) is minimized, fat intake can safely be higher than that 10% allotment. The Food and Nutrition Board suggests that fat intake can be as high as 35% of calories consumed for an adult. Some experts sug-
High fat, low fat, no fat—which is best? And why is there such a debate? Would it be easier just to avoid fat altogether? Doesn’t a high-fat diet lead to obesity? To cardiovascular disease? Overall, which are the “best” fats, and why are French fries, doughnuts, stick margarine, and crackers getting such a bad rap? Chapter 5 provides some answers.
© 2001 Batiuk, Inc. Distributed by North American Syndicates, Inc. All rights reserved.
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Chapter 5: Lipids 163
gest that an intake as high as 40% of calories is appropriate. After learning more about lipids—fats, oils, and related compounds—in Chapter 5, you can decide for yourself how much fat you want to consume, as well as how to track your daily intake.
Lipids are a diverse group of chemical compounds. They share one main characteristic: They do not readily dissolve in water. Think of an oil and vinegar salad dressing.
The oil is not soluble in the water-based vinegar; on standing, the two separate into distinct layers, with oil on top and vinegar on the bottom.
triglyceride The major form of lipid in the body and in food. It is composed of three fatty acids bonded to glycerol, an alcohol.
glycerol A three-carbon alcohol used to form triglycerides.
phospholipid Any of a class of fat-related substances that contain phosphorus, fatty acids, and a nitrogen-containing base. The phospholipids are an essential part of every cell.
Lipids: Main Types
The chemical structure of lipids is diverse. Triglycerides are the most common type of lipid found in the body and in foods. Each triglyceride molecule consists of three fatty acids bonded to glycerol. Phospholipids and sterols, including cholesterol, are also classified as lipids, although their structures can be quite different from the structure of triglycerides. All of these lipid compounds are described in Chapter 5.
Food experts, such as chefs, call lipids that are solid at room temperature fats, and lipids that are liquid oils. Most people use the word fat to refer to all lipids because they don’t realize there is a difference. However, lipid is a generic term that includes triglycerides and many other substances. To simplify our discussion, Chapter 5 primarily uses the term fat. When necessary for clarity, the name of a specific lipid, such as cholesterol, will be used. This word use is consistent with the way many people use these terms.
sterol A compound containing a multi-ring
(steroid) structure and a hydroxyl group
(–OH). Cholesterol is a typical example.
cholesterol A waxy lipid found in all body cells. It has a structure containing multiple chemical rings that is found only in foods that contain animal products.
saturated fatty acid A fatty acid containing no carbon-carbon double bonds.
monounsaturated fatty acid A fatty acid containing one carbon-carbon double bond.
polyunsaturated fatty acid A fatty acid containing two or more carbon-carbon double bonds.
Fatty Acids: The Simplest Form of Lipids
In the body and in foods, fatty acids are found in the main form of lipids, triglycerides. A fatty acid is basically a long chain of carbons bonded together and flanked by hydrogens. At one end of the molecule (the alpha end), is an acid group. At the other end (the omega end), is a methyl group (Fig. 5-1).
Fats in foods are not composed of a single type of fatty acid. Rather, each dietary fat, or triglyceride, is a complex mixture of many different fatty acids, the combination of which provides each food its unique taste and smell.
Recall from Chapter 1 that fatty acids can be saturated or unsaturated. Chemically speaking, a carbon atom can form four bonds. Within the carbon chain of a fatty acid, the carbons bond to other carbons and to hydrogens. The carbons that make up the chain of a saturated fatty acid are all connected to each other by single bonds. This allows for the maximum number of hydrogens to be bound. Just as a sponge can be saturated (full) with water, a saturated fatty acid such as stearic acid is saturated with hydrogen (Fig. 5-1a).
As noted earlier, most fats high in saturated fatty acids, such as animal fats, remain solid at room temperature. A good example is the solid fat surrounding a piece of uncooked steak. Chicken fat, semisolid at room temperature, contains less saturated fat than beef fat. However, in some foods, saturated fats are suspended in liquid, such as the butterfat in whole milk, so the solid nature of these fats at room temperature is less apparent.
If the carbon chain of a fatty acid contains a double bond, those carbons in the chain have fewer bonds to share with hydrogen, and the chain is said to be unsaturated.
A fatty acid with only one double bond is monounsaturated (Fig. 5-1b). Canola and olive oils contain a high percentage of monounsaturated fatty acids. Likewise, if two or more of the bonds between the carbons are double bonds, the fatty acid is even less saturated with hydrogens, and so it is polyunsaturated (Fig. 5-1c, d). Corn, soybean, sunflower, and safflower oils are rich in polyunsaturated fatty acids.
Unsaturated fatty acids can exist in two different structural forms, the cis and trans forms. The form of fatty acids known as trans fatty acids was briefly described
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C
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Saturated Fatty Acid (stearic acid)
(Methyl group)
(a)
H
(Acid group)
H
H
H
H
H
H
H
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H
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H
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O
C
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H
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H
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O
H
O
H
O
H
O
H
Monounsaturated Fatty Acid (oleic acid; ω-9)
(b)
(c)
H
H
H
H
H
H
H
H
H
O
C
C
C
C
C
C
C
C
C
H
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H
Polyunsaturated Fatty Acid (alpha-linolenic acid; ω-3)
H
H
H
H
H
H
H
H
H
H
H
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H
H
O
C
C
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H
C
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C
H
H
H
H
H
H
H
Polyunsaturated Fatty Acid (linoleic acid; ω-6)
(d)
H
H
H
H
H
H
H
H
H
O
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
Figure 5-1 c Chemical forms of saturated, monounsaturated, and polyunsaturated fatty acids. Each of the depicted fatty acids contains
18 carbons, but they differ from each other in the number and location of double bonds. The linear shape of saturated fatty acids, as shown in (a), allows them to pack tightly together and so form a solid at room temperature. In contrast, unsaturated fatty acids have “kinks” where double bonds interrupt the carbon chain (see Figure 5-2). Thus, unsaturated fatty acids pack together only loosely, and are usually liquid at room temperature.
in Chapter 1. In their natural form, monounsaturated and polyunsaturated fatty acids usually are in the cis form (Fig. 5-2). By definition, the resulting cis fatty acid has the hydrogens on the same side of the carbon-carbon double bond. During certain types of food processing (discussed later in this chapter), some hydrogens are transferred to opposite sides of the carbon-carbon double bond, creating the trans form, or a trans fatty acid. As seen in Figure 5-2, the cis bond causes the fatty acid backbone to bend, whereas the trans bond allows the backbone to remain straighter.
This makes it similar to the shape of a saturated fatty acid. The Food and Nutrition
Board suggests limiting intake of trans fatty acids in processed foods (also referred to as trans fats) as much as possible. Later you will see why.
You may be surprised to learn that some trans fatty acids, known as conjugated linoleic acid (CLA), occur naturally. CLA is a family of derivatives of the fatty acid linoleic acid. The bacteria that live in the rumens of some animals (cows, sheep, and goats, for example) produce trans fatty acids that eventually appear in foods such as beef, milk, and butter. These naturally occurring trans fats are currently under study for possible health benefits, including prevention of cancer, decreasing body fat, and improvement in insulin levels in diabetics. About 20% of trans fatty acids in our diets come from this source,
Dietary supplements of CLA are available but are highly variable in their quality.
Overall, a fat or an oil is classified as saturated, monounsaturated, or polyunsaturated based on the type of fatty acids present in the greatest concentra-
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Chapter 5: Lipids 165
cis fatty acid A form of an unsaturated fatty acid that has the hydrogens lying on the same side of the carbon-carbon double bond. trans fatty acid A form of an unsaturated
Cis form
(Causes
backbone of molecule to bend) fatty acid, usually a monounsaturated one when found in food, in which the hydrogens on both carbons forming the double bond lie on opposite sides of that bond.
Trans form
long-chain fatty acid A fatty acid that contains 12 or more carbons.
omega-3 (ω-3) fatty acid An unsaturated fatty acid with the first double bond on the third carbon from the methyl end (—CH3).
omega-6 (ω-6) fatty acid An unsaturated fatty acid with the first double bond on the sixth carbon from the methyl end (—CH3).
alpha-linolenic acid An essential omega-3 fatty acid with 18 carbons and three double bonds. Elaidic acid
Oleic acid
linoleic acid An essential omega-6 fatty
FiguRe 5-2 c Cis and trans fatty acids. In the cis form at carbon-carbon double bonds in a
acid with 18 carbons and two double bonds.
fatty acid, the hydrogens (in blue) lie on the same side of the double bond. This causes a “kink” at that point in the fatty acid, typical of unsaturated fatty acids in nature. In contrast, in the trans form at carbon-carbon double bonds in a fatty acid, the hydrogens lie across from each other at the double bond. This causes the fatty acid to exist in a linear form, like a saturated fatty acid.
Cis fatty acids are much more common in foods than trans fatty acids. The latter are primarily found in foods containing partially-hydrogenated fats, notably stick margarine, shortening, and deep-fat fried foods.
essential fatty acids Fatty acids that must
tion (Fig. 5-3). Fats in foods that contain primarily saturated fatty acids are solid at room temperature, especially if the fatty acids have long carbon chains (i.e., a long-chain fatty acid), as opposed to shorter versions. In contrast, fats containing primarily polyunsaturated or monounsaturated fatty acids (long chain or shorter) are usually liquid at room temperature. Almost all fatty acids in the body and in foods are long-chain varieties.
An important characteristic of unsaturated fatty acids is the location of the double bonds. If the first double bond starts three carbons from the methyl (omega) end of the fatty acid, it is an omega-3 (ω-3) fatty acid (review Fig. 5-1c). If the first double bond is located six carbons from the omega end, it is an omega-6 (ω-6) fatty acid (review Fig. 5-1d). Following this scheme, an omega-9 fatty acid has its first double bond starting at the ninth carbon from the methyl end (review Fig. 5-1b). In foods, alphalinolenic acid is the major omega-3 fatty acid; linoleic acid is the major omega-6 fatty acid. These are also the essential fatty acids we need to consume (more on this in the later section on putting lipids to work in the body). Oleic acid is the major omega-9 fatty acid.
Triglycerides
Fats and oils in foods are mostly in the form of triglycerides. The same is true for fats found in body structures. Although some fatty acids are transported in the bloodstream attached to proteins, most fatty acids are formed into triglycerides by cells in the body.
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be supplied by the diet to maintain health.
Currently, only linoleic acid and alphalinolenic acid are classified as essential.
oleic acid An omega-9 fatty acid with
18 carbons and one double bond.
m Plant oils vary in their content of specific
fatty acids. Oils similar in appearance may vary significantly in fatty acid composition. Olive and canola oils are rich in monounsaturated fat; olive oil has been awarded much attention in recent years. Canola oil, however, is a much less expensive choice of monounsaturated fat. Safflower oil is rich in polyunsaturated fat.
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Rich Sources of :
Saturated
Fatty Acids
Monounsaturated
Fatty Acids
Primarily Trans
Fatty Acids
Polyunsaturated
Fatty Acids
Saturated Fatty Acids
Coconut oil
**
Butter
Palm oil
Lard or beef fat
Monounsaturated
Fatty Acids
Olive oil
Canola oil*
Peanut oil
Soybean oil*
Polyunsaturated
Fatty Acids
Safflower oil
Sunflower oil
Corn oil
Trans Fatty Acids
Tub margarine
Stick margarine
Shortening
0
20
40
60
80 100 0
20
40
60
80 100 0
20
40
60
80 100 0
20
40
60
80 100
% of all Fatty Acids in the Product
*Rich source of the omega-3 fatty acid alpha-linolenic acid (7% and 12% of total fatty acid for soybean oil and canola oil, respectively).
**The natural trans fatty acids in butter are not harmful and may even have health-promoting properties, such as preventing certain forms of cancer.
Figure 5-3 c Saturated, monounsaturated, polyunsaturated, and trans fatty acid composition of common fats and oils (expressed as % of all fatty acids in the product).
diglyceride A breakdown product of a triglyceride consisting of two fatty acids bonded to a glycerol backbone.
monoglyceride A breakdown product of a triglyceride consisting of one fatty acid attached to a glycerol backbone.
lecithin A group of compounds that are major components of cell membranes.
As noted before, triglycerides contain a simple three-carbon alcohol, glycerol, which serves as a backbone for the three attached fatty acids (Fig. 5-4a). Removing one fatty acid from a triglyceride forms a diglyceride. Removing two fatty acids from a triglyceride forms a monoglyceride. Later you will see that before most dietary fats are absorbed in the small intestine, the two outer fatty acids are typically removed from the triglyceride. This produces a mixture of fatty acids and monoglycerides, absorbed into the intestinal cells. After absorption, the fatty acids and monoglycerides are mostly re-formed into triglycerides.
Phospholipids
Phospholipids are another class of lipid. Like triglycerides, they are built on a backbone of glycerol. However, at least one fatty acid is replaced with a compound containing phosphorus (and often other elements, such as nitrogen) (Fig. 5-4b). Many types of phospholipids exist in the body, especially in the brain. They form important parts of cell membranes. Lecithin is a common example of a phospholipid. Various forms are found in body cells and they participate in fat digestion in the small intes-
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Chapter 5: Lipids 167
H
H
O
Figure 5-4 c Chemical forms of com-
O
H
C
O
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O
fatty acid
H
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O
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O
fatty acid
H
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O
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O
fatty acid
H
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O
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fatty acid
H
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fatty acid
O
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H
O
P
H
H
Glycerol
O
O
mon lipids: (a) triglyceride, (b) phospholipid
(in this case, lecithin), and (c) sterol (in this case, cholesterol).
Choline
H
H
CH3
C
C
N
H
H
CH3
CH3
Glycerol
Phosphate
Triglyceride
Lecithin—A Phospholipid
(a)
(b)
H
H
H H
H
H
H
C
H
H
C
C
C
H
HC
C
CH3
C
C
HO
H
H
C
C
C
C
H3C
CH3
C
C
H
H
H
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C
C
H
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H
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H
C
CH3
H
C
HC
H
CH3
H H
H
H
H
H
Cholesterol—A Sterol
(c)
tine. The body is able to produce all the phospholipids it needs. Even though lecithin is sold as a dietary supplement and is present as an additive in many foods, phospholipids such as this one are not essential components of the diet.
Sterols
Sterols are the last class of lipids Chapter 5 covers. Their characteristic multi-ringed structure makes them different from the other lipids already discussed (Fig. 5-4c). The most common example of a sterol is cholesterol. This waxy substance doesn’t look like a triglyceride—it doesn’t have a glycerol backbone or any fatty acids. Still, because it doesn’t readily dissolve in water, it is a lipid. Among other functions, cholesterol is used to form certain hormones and bile acids and is incorporated into cell structures.
The body can make all the cholesterol it needs.
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Concept Check
Lipids are a group of compounds that do not dissolve readily in water. Included in this group are fatty acids, triglycerides, phospholipids, and sterols. Fatty acids can be distinguished from one another by the length of the carbon skeleton and the number and position of double bonds along that skeleton. Saturated fatty acids contain no double bonds within their carbon skeleton; that is, they are fully saturated with hydrogens. Monounsaturated fatty acids contain one carbon-carbon double bond, and polyunsaturated fatty acids contain two or more of these bonds. Certain omega-3 and omega-6 fatty acids are essential in the human diet.
Triglycerides are the major form of fat in the body and in foods. These consist of three fatty acids bonded to a glycerol backbone. Phospholipids are similar to triglycerides in structure, but at least one fatty acid is replaced by another compound containing phosphorus. Phospholipids play an important structural role in cell membranes. Sterols, another class of lipids, do not resemble either triglycerides or phospholipids, but instead have a multi-ringed structure.
Cholesterol, one example of a sterol, forms parts of cells, some hormones, and bile acids.
Whereas certain essential fatty acids (as components of triglycerides) are needed in the diet, the body produces all the triglycerides, phospholipids, and cholesterol it needs. Next we will discuss the sources of fat in foods (Fig. 5-5).
MyPyramid: Sources of Fats
Food Group
Sources of Fat
Grams per Serving
Grains
• Crackers
• Pasta dishes with added fat
0 –18
Vegetables
• French fried potatoes 0 –27
Fruits
• Fruit pies
• Avocados
0 –11
Oils
• All
12 –14
Milk
• Whole milk
• Some yogurts
• Many cheeses
• Premium ice cream 0 –10
Meat & Beans
• Marbled meat
• Bacon
• Poultry (skin)
• Deep-fat fried meat • Nuts
7 –17
Figure 5-5 c Sources of fats from MyPyramid. The height of the background color (none, 1/3, 2/3, or completely covered) within each group in the pyramid indicates the average nutrient density for fat in that group. The fruit group and vegetable groups are generally low in fat. In the other groups, both high-fat and low-fat choices are available. Careful reading of food labels can help you choose lower-fat versions of some foods. In general, any type of frying adds significant amounts of fat to a product, as with French fries and fried chicken. With regard to physical activity, fats are a key fuel in prolonged events, such as long-distance cycling, long-distance running, and prolonged slow to brisk walking.
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Chapter 5: Lipids 169
Fats and Oils in Foods
Lipids in the form of triglycerides are abundant in the North American diet. The foods highest in fat (and therefore energy dense) include salad oils and spreads such as butter, margarine, and mayonnaise. All of these foods contain close to
100% of calories as fat. In reduced-fat margarines, water replaces some of the fat.
Whereas regular margarines are 80% fat by weight (11 grams per tablespoon), some reduced-fat margarines are as low as 30% fat by weight (4 grams per tablespoon). When used in recipes, the extra water added to these margarines can cause texture and volume changes in the finished product. Cookbooks can provide guidance for appropriate use of these products by suggesting alterations in recipes to compensate for the increased water content.
Still considering the overall fat content, whole foods highest in fat include nuts, bologna, avocados, and bacon, which have about 80% of calories as fat. Next, peanut butter and cheddar cheese have about 75%. Marbled steak and hamburgers
(ground chuck) have about 60%, and chocolate bars, ice cream, doughnuts, and whole milk have about 50% of calories as fat. Eggs, pumpkin pie, and cupcakes have 35%, as do lean cuts of meat, such as top round (and ground round) and sirloin. Bread contains about 15%. Finally, foods such as cornflakes, sugar, and fat-free milk have essentially no fat. Careful label reading is necessary to determine the true fat content of food—these are only rough guidelines (Fig. 5-6).
m The North American diet contains many high-fat foods—
including typical cookie choices. Portion control with these foods is thus important, especially if one is trying to control calorie intake.
Figure 5-6 c Food sources of fat com-
Food Sources of Fat
Food Item
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Fat
(grams)
Calories from Fat %
% AHA
Recommendation
AHA
Recommendation
70
30%
T-bone steak, 3 ounces
17
66%
24%
Mixed nuts, 1 ounce
16
78%
23%
Canola oil, 1 tablespoon
14
100%
Hamburger with bun,
1 each
12
39%
17%
Margarine, 1 tablespoon
12
100%
17%
Avocado, 1⁄2 cup
11
86%
16%
Cheddar cheese, 1 ounce
10
74%
14%
Whole milk, 1 cup
8
49%
11%
Chicken breast with skin, 3 ounces
7
36%
10%
Whole milk yogurt,
8 ounces
7
28%
10%
Snack crackers, 1 ounce
7
45%
10%
Baked beans, 1⁄2 cup
7
31%
10%
M&M chocolate candies, 1 ounce
6
39%
9%
Flax seeds, 1 tablespoon
3
62%
4%
Fig Newton cookies,
2 each
3
23%
4%
100%
pared to the American Heart Association
(AHA) recommendation of 70 grams per day, or 30% of calories from fat for a
2100 kcal diet.
20%
Key:
Grains
Vegetables
Fruits
Oil
Milk
Meat & Beans
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m Peanuts are a source of lecithins, as are wheat
germ and egg yolks.
emulsifier A compound that can suspend fat in water by isolating individual fat droplets, using a shell of water molecules or other substances to prevent the fat from coalescing.
The type of fat in food is important to consider along with the total amount of fat.
Animal fats are the chief contributors of saturated fatty acids to the North American diet. About 40% to 60% of total fat in dairy and meat products is in the form of saturated fatty acids. In contrast, plant oils contain mostly unsaturated fatty acids, ranging from 73% to 94% of total fat. A moderate to high proportion of total fat (49% to 77%) is supplied by monounsaturated fatty acids in canola oil, olive oil, and peanut oil. Some animal fats are also good sources of monounsaturated fatty acids (30% to 47%) (review Fig. 5-3). Corn, cottonseed, sunflower, soybean, and safflower oils contain mostly polyunsaturated fatty acids (54% to 77%). These plant oils supply the majority of the linoleic and alpha-linolenic acid in the North American food supply.
Wheat germ, peanuts, egg yolk, soy beans, and organ meats are rich sources of phospholipids. Phospholipids such as lecithin, a component of egg yolks, are often added to salad dressing. Lecithin is used as an emulsifier in these and other products because of its ability to keep mixtures of lipids and water from separating (see Fig. 5-7). Emulsifiers are added to salad dressings to keep the vegetable oil suspended in water. Eggs added to cake batters likewise emulsify the fat with the milk.
Cholesterol is found only in animal foods (Table 5-1). An egg yolk contains about 210 milligrams of cholesterol. Eggs are our main dietary source of cholesterol, along with meats and whole milk. Manufacturers who advertise their brand of peanut butter, vegetable shortening, margarines, and vegetable oils as “cholesterol-free” are taking advantage of uninformed consumers—all of these products are naturally cholesterol-free. Some plants contain other sterols similar to cholesterol, but they do not pose the heart health risks associated with cholesterol. In fact, some plant sterols have blood cholesterollowering properties (see the later section on medical interventions to lower blood lipids). Fat Is Hidden in Some Foods
Some fat discussed so far is obvious: butter on bread, mayonnaise in potato salad, and marbling in raw meat. Fat is harder to detect in other foods that also contribute significant amounts of fat to our diets. Foods that contain hidden fat include whole milk, pastries, cookies, cake, cheese, hot dogs, crackers, French fries, and ice cream.
Figure 5-7 c Emulsifiers in action.
Emulsifiers prevent many brands of salad dressings and other condiments from separating into layers of water and fat.
Emulsifiers attract fatty acids inside and have a water-attracting group on the outside. Add them to salad dressing, shake well, and they hold the oil in the dressing away from the water. Emulsification is important in both food production and fat digestion/absorption. Emulsifiers and Agitation in Salad Dressing
Water
Emulsified fat
Oil
Water
Oil droplet attracted to fatty acid core
Water attracted to shell
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Chapter 5: Lipids 171
Table 5-1 Cholesterol Content of Selected Foods in Ascending Order Food
Amount
Skim milk
1 cup
Mayonnaise
Cholesterol in Milligrams
Food
Amount
Cholesterol in Milligrams
4
Oysters, salmon
3 ounces
40
1 tablespoon
10
Clams, halibut, tuna
3 ounces
55
Butter
1 pat
11
Chicken, turkey* (white meat)
3 ounces
70
Lard
1 tablespoon
12
Beef,* pork
3 ounces
75
Cottage cheese
½ cup
15
Lamb, crab
3 ounces
85
Low-fat milk (2%)
1 cup
22
Shrimp, lobster
3 ounces
110
Half-and-half
¼ cup
23
Heart (beef)
3 ounces
165
Hot dog*
1
29
Egg (egg yolk)*†
1
210
Ice cream, ∼10% fat
½ cup
30
Liver (beef)
3 ounces
410
Cheese, cheddar
1 ounce
30
Kidney
3 ounces
540
Whole milk
1 cup
34
Brains
3 ounces
2640
*Leading contributors of cholesterol to the North American diet.
Egg whites are cholesterol-free.
†
When we try to cut down on fat intake, hidden fats need to be considered, along with the more obvious sources.
A place to begin searching for hidden fat is on the Nutrition Facts labels of foods you buy. Some signals from the ingredient list that can alert you to the presence of fat are animal fats, such as bacon, beef, ham, lamb, pork, chicken, and turkey fats; lard; vegetable oils; nuts; dairy fats, such as butter and cream; egg and egg-yolk solids; and partially-hydrogenated shortening or vegetable oil. Conveniently, the label lists ingredients by order of weight in the product. If fat is one of the first ingredients listed, you are probably looking at a high-fat product. Use food labels to learn more about the fat content of the foods you eat (Fig. 5-8).
Figure 5-8 c Reading labels helps lo-
Nutrition Facts
cate hidden fat. Who would think that wieners (hot dogs) can contain about 85% of food calories as fat? Looking at the hot dog does not suggest that almost all of its food calories comes from fat, but the label shows otherwise. Let’s do the math: 13 grams total fat × 9 kcal per gram of fat = 120 kcal from fat; 120 kcal/140 kcal per link = 0.84 or 84% kcal from fat.
Serving Size 1 Link (45g)
Servings Per Container 10
Amount Per Serving
Calories 140 Calories from Fat 120
% Daily Value*
Total Fat 13g
20%
Saturated Fat 5g
23%
Trans Fat 0g
**
Cholesterol 20mg
7%
Sodium 420mg
17%
1%
Total Carbohydrate 2g
Dietary Fiber 0g
0%
Sugars 1g
Protein 5g
Vitamin A 0% • Vitamin C 0%
Calcium 0% • Iron 2%
*Percent Daily Values are based on a 2,000 calorie diet.
**Intake of trans fat should be as low as possible. war43742_ch05_160_201.indd 171
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Definitions for Nutrient Claims About Fat and Cholesterol on Food Labels
Fat
• Fat free: less than 0.5 g of fat per serving
• Saturated fat free: less than 0.5 g per serving, and the level of trans fatty acids does not exceed 0.5 g per serving
• Low fat: 3 g or less per serving and, if the serving is 30 g or less or 2 tablespoons or less, per 50 g of the food. 2% milk can no longer be labeled low fat, as it exceeds 3 g per serving. Reduced fat will be the term used instead.
• Low saturated fat: 1 g or less per serving and not more than 15% of kcal from saturated fatty acids
• Reduced or less fat: at least 25% less per serving than reference food
• Reduced or less saturated fat: at least
25% less per serving than reference food
Cholesterol
• Cholesterol free: less than 2 milligrams
(mg) of cholesterol and 2 g or less of saturated fat per serving
• Low cholesterol: 20 mg or less cholesterol and 2 g or less of saturated fat per serving and, if the serving is 30 g or less or
2 tablespoons or less, per 50 g of the food
• Reduced or less cholesterol: at least 25% less cholesterol and 2 g or less of saturated fat per serving than reference food
The definitions for various fat descriptors on food labels, such as “low-fat,” “fat-free,” and “reduced-fat,” were listed in Table 2-9 in Chapter 2 and are reprinted in the margin here. Recall that “low-fat” indicates, in most cases, that a product contains no more than
3 grams of fat per serving. Products marketed as “fat-free” must have less than one-half of a gram of fat per serving. A claim of “reduced-fat” means the product has at least 25% less fat than is usually found in that type of food. When there is no Nutrition Facts label to inspect, controlling portion size is a good way to control fat intake.
When many North Americans think of a low-fat diet, they include reduced-fat versions of pastries, cookies, and cakes. When health professionals refer to a low-fat diet, they often have a different plan in mind: focusing primarily on fruits, vegetables, and whole-grain breads and cereals. Whether to choose a fat-rich food should depend on how much fat you have eaten or will eat during that particular day. So, if you plan to eat high-fat foods at your evening meal, you could reduce your fat intake at a previous meal to balance overall fat intake for the day.
Fat in Food Provides Some Satiety, Flavor, and Texture
Fat in foods has generally been considered to be the most satiating of all the macronutrients. However, studies show that protein and carbohydrate probably lead to the most satiety (gram for gram). High-fat meals do provide satiety, but primarily because one consumes a lot of calories in the process. A high-fat meal is likely to be a high-calorie meal.
Various fats play important roles in foods, so much ingenuity must go into the production of reduced-fat products to preserve flavor and texture. In some cases, “fat-free” also means tasteless. Fat components in foods provide important textures and carry flavors. If you’ve ever eaten a high-fat yellow cheese or cream cheese, you probably agree that fat melting on the tongue feels good. The fat in reduced-fat and whole milk also gives body, which fat-free milk lacks. The most tender cuts of meat are high in fat, visible as the marbling of meat. In addition, many flavorings dissolve in fat. Heating spices in oil intensifies the flavors of an Indian curry or a Mexican dish, carried to the sensory cells that discriminate taste and smell in the mouth.
Making Decisions
Low-Fat Diets
A person who has been following a typical North American diet will probably need some time to adjust to the taste of a lower-fat diet. Emphasizing flavorful fruits, vegetables, and whole grains will help one to adapt to a low-fat diet. Interestingly, after an adjustment period, higher-fat foods may not be as palatable or may lead to gastrointestinal discomfort. For example, after switching from whole to 1% low-fat milk for a few weeks, whole milk begins to taste more like cream than milk. It is certainly possible to make the change from a higher-fat diet to a lower-fat diet. The benefits of weight control and reduced risk for several chronic diseases make the adjustment worth the effort.
Wise Use of Reduced-Fat Foods Is Important
m Fat is an important component of the flavor
and overall appeal of cheese.
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Manufacturers have introduced reduced-fat versions of numerous food products. The fat content of these alternatives ranges from 0% in fat-free Fig Newtons to about 75% of the original fat content in other products. However, the total calorie content of most fat-reduced products is not substantially lower than that of their conventional versions. Generally, when fat is removed from a product, something must be added— commonly, sugars—in its place. It is difficult to reduce both the fat and sugar contents of a product at the same time and maintain flavor and texture. For this reason, many reduced-fat products (e.g., cakes and cookies) are still energy dense. Use the Nutrition
Facts label to choose the portion size with the desired calories.
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Chapter 5: Lipids 173
Fat Replacement Strategies for Foods
CRitiCaL thinking
Five types of fat replacements are available to food manufacturers. Addition of these substances during manufacturing yields products that, to varying degrees, satisfy consumers’ desire for reduced-fat products that are still tasty.
Water, starch derivatives, and Fibers. The simplest fat replacement is water. The addi-
tion of water yields a product, such as diet margarine, with less fat per serving than the normal product. Starch derivatives that bind water to form a gel are a second type of fat replacement. The resulting gel replaces some of the mouth feel lost by the removal of fat.
Starch derivatives are made from the hulls or bran of various plants, including oats, peas, soybeans, rice, corn, and wheat. Starch derivatives commonly used by food manufacturers include the fiber cellulose, Maltrin, Stellar, Z-trim, and Oatrim. These substances are used in a variety of foods, including luncheon meats, salad dressings, frozen desserts, table spreads, dips, baked goods, and candies. Most starch derivatives contain some calories, but at least half the amount in fat. These starch derivatives cannot be used in fried foods.
Gum fiber extracted from plants can also be used to replace fat. This thickens a product and replaces some of the body that fat provides. Diet salad dressings and fatreduced ice cream have gums added for this reason.
Allison has decided to start eating a lowfat diet. Allison has mentioned to you that all she needs to do is add less butter, oil, or margarine to her foods and she will dramatically lower her fat intake. How can you explain to Allison that she needs to be aware of the hidden fats in her diet as well?
protein-derived Fat Replacements. Both egg and milk proteins can be used as fat
replacements. These proteins are treated to produce microscopic, mistlike protein globules. When these substances replace fat in a food product, they feel like fat in the mouth, although the product does not contain any fatty acids. These proteins are used in milk and other dairy products, baked goods, frostings, salad dressings, and mayonnaise-type products. Such fat replacements yield some calories—but only about 1 to 2 kcal per gram. They have this low-calorie value for two reasons: Proteins contain only 4 kcal per gram, and the products have a high water content. engineered Fats and Related products. The newest form of fat replacement is en-
gineered fat. This type of product is synthesized in the laboratory from various food constituents. Olestra (Olean) is a good example. It is made by chemically bonding fatty acids to sucrose (table sugar). The resulting product cannot be digested by either the human digestive enzymes or bacteria that live in the intestinal tract. Therefore, olestra cannot be absorbed and so provides no calories for the body.
Olestra can replace much of the fat in salad dressing and cakes. It was the first fat replacement that could function like fat in fried foods. Olestra is approved by FDA for use in fried snack foods, such as potato chips.
The major problem associated with the use of olestra is that it binds the fatsoluble vitamins A, D, E, and K, and other fat-soluble phytochemicals such as the carotenoids, thus reducing their absorption. To compensate, food manufacturers add these vitamins to food products containing olestra. Other suspected problems, such as GI tract discomfort, have not been supported by careful research. Thus, warnings about use of olestra and GI tract disturbances, which used to be required on labels for olestra-containing foods, are no longer mandatory.
Food manufacturers are working on other types of engineered fats, which either wholly or partially escape absorption by the body. One example is salatrim, marketed under the name
Benefat, which yields only about 5 kcal per gram. It is composed of some saturated fatty acids poorly absorbed by the body. This product has been used in reduced-fat chocolate.
So far, fat replacements have had little impact on our diets, partly because the approved forms either are not very versatile or have not been used extensively by manufacturers. The public has shown little interest in the use of fat replacers, such as olestra. In addition, fat replacements are not practical to use in the foods that contribute the greatest quantity of fat to our diets—beef, cheese, whole and reduced-fat milk, and pastries.
The main benefit of fat replacements will be in helping us cut some fat and calories from our diets. The reduction in overall calorie intake, however, will probably be small because we tend to compensate for the fewer calories per serving by eating more servings.
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m Fat replacements such as gum fiber are typi-
cally seen in soft serve ice cream.
c Canada has not approved the use of olestra in food products; the United States is the sole country that permits the use of this fat substitute in foods.
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Concept Check
Fat-dense foods—those with more than 60% of total calories as fat—include plant oils, butter, margarine, mayonnaise, nuts, bacon, avocados, peanut butter, cheddar cheese, steak, and hamburger. Of the foods we typically eat, cholesterol is found naturally only in those of animal origin, with eggs being a primary source. Emulsifiers, such as the phospholipids, lecithins, are added to salad dressings and other fat-rich products to keep the vegetable oils and other fats suspended in the water. Hidden fat exists in foods such as whole milk, pastries, cookies, cake, cheese, hot dogs, crackers, French fries, and ice cream. Fat has a variety of roles in foods, including that of contributing to flavor and texture. Fat also provides the pleasurable mouth feel of many of our favorite foods, intensifies the taste of many spices, and tenderizes many popular cuts of meat. Fat free doesn’t mean calorie free; moderation in the use of reduced-fat products is still important.
Fat Rancidity Limits Shelf Life of Foods
BHA, BHT Butylated hydroxyanisole and butylated hydroxytoluene—two common synthetic antioxidants added to foods.
hydrogenation The addition of hydrogen to a carbon-carbon double bond, producing a single carbon-carbon bond with two hydrogens attached to each carbon. Hydrogenation of unsaturated fatty acids in a vegetable oil increases its hardness, so this process is used to convert liquid oils into more solid fats, used in making margarine and shortening. Trans fatty acids are a by-product of hydrogenation of vegetable oils.
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Decomposing oils emit a disagreeable odor and taste sour and stale. Stale potato chips are a good example. The double bonds in unsaturated fatty acids break down, producing rancid by-products. Ultraviolet light, oxygen, and certain procedures can break double bonds and, in turn, destroy the structure of polyunsaturated fatty acids. Saturated fats and trans fats can much more readily resist these effects because they contain fewer carbon-carbon double bonds.
Rancidity is not a major problem for consumers because the odor and taste generally discourage us from eating enough to become sick. However, rancidity is a problem for manufacturers because it reduces a product’s shelf life. To increase shelf life, manufacturers often add partially-hydrogenated plant oils to products. Foods most likely to become rancid are deep-fried foods and foods with a large amount of exposed surface. The fat in fish is also susceptible to rancidity because it is highly polyunsaturated.
Antioxidants such as vitamin E help protect foods against rancidity by guarding against fat breakdown. The vitamin E naturally occuring in plant oils reduces the breakdown of double bonds in fatty acids. When food manufacturers want to prevent rancidity in polyunsaturated fats, they often add the synthetic antioxidants BHA and
BHT or vitamin C to products that contain fat such as salad dressings and cake mixes.
Manufacturers also tightly seal products and use other methods to reduce oxygen levels inside packages.
Hydrogenation of Fatty Acids in Food Production
Increases Trans Fatty Acid Content
As mentioned previously, most fats with long-chain saturated fatty acids are solid at room temperature, and those with unsaturated fatty acids are liquid at room temperature. In some kinds of food production, solid fats work better than liquid oils.
In pie crust, for example, solid fats yield a flaky product, whereas crusts made with liquid oils tend to be greasy and more crumbly. If oils with unsaturated fatty acids are used to replace solid fats, they often must be made more saturated (with hydrogen), as this solidifies the vegetable oils into shortenings and margarines. Hydrogen is added by bubbling hydrogen gas under pressure into liquid vegetable oils in a process called hydrogenation (Fig. 5-9). The fatty acids aren’t fully hydrogenated to the saturated fatty acid form, as this would make the product too hard and brittle.
Partial hydrogenation—leaving some monounsaturated fatty acids—creates a semisolid product.
The process of hydrogenation produces trans fatty acids as was described earlier in this chapter. Most natural monounsaturated and polyunsaturated fatty acids exist in the cis form, causing a bend in the carbon chain, whereas the straighter carbon forms of trans fat more closely resemble saturated fatty acids. This may be
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Chapter 5: Lipids 175
FiguRe 5-9 c How liquid oils become
(a)
solid fats. (a) Unsaturated fatty acids are present in liquid form. (b) Hydrogens are added (hydrogenation), changing some carbon-carbon double bonds to single bonds and producing some trans fatty acids. (c) The partially-hydrogenated product is likely to be used in margarine, shortening, or for deep-fat frying.
Hydrogen source (b)
(c)
Unsaturated vegetable oil (liquid)
Adding hydrogen under pressure
Partially-hydrogenated fat
(semisolid)
the mechanism whereby trans fat increases the risk for heart disease. Studies also indicate that trans fats increase overall inflammation in the body, which is not healthful. Thus, people should limit intake of partially-hydrogenated fat and thus trans fat. This may not be such a concern for the average person, as long as trans fat intake is not excessive and the diet is adequate in polyunsaturated fat. However, because trans fatty acids serve no particular role in maintaining body health, the latest Dietary Guidelines for Americans, the American Heart Association, and the
Food and Nutrition Board each recommend minimal trans fat intake.
Not so long ago, public pressure persuaded manufacturers to eliminate the tropical oils rich in saturated fat (palm, palm olein, and coconut) from food processing. Partially-hydrogenated soybean oil—rich in trans fat—became the major replacement. Currently, trans fat intake in North America is estimated to contribute about 3% to 4% of total calories, amounting to 10 grams per day, on average. Table 5-2 lists typical sources.
FDA is now requiring the trans fat content of foods on food labels (review
Fig 5-8). The food labels in Canada also must list trans fat content. FDA hopes to make consumers more aware of the amounts of trans fat in foods as well as the negative health consequences associated with their excessive consumption. North American companies are already responding to this issue by creating products free of trans fat. For example, Promise, Smart Beat, and some
Fleischmann’s margarines are lower in or free of trans fat (less than 0.5 grams per serving) compared to typical margarines.
This addition of the trans fat listing on labels helps consumers at the supermarket, but when dining out, consumers are “left in the dark” as to which foods contain trans fat. Knowing which foods are low in trans fat when ordering at a restaurant is difficult because information about preparation methods and precise fat composition is rarely available. To minimize trans fat intake, a general guideline is to limit consumption of
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m Tub margarine is much lower in trans fat than
stick margarine or shortenings. Some brands of tub margarines are even free of trans fatty acids.
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tabLe 5-2 Total Fat, Saturated Fat, and Trans Fat Content of Typical Sources of
Trans Fat (in descending order of trans fat) Food Item
Serving
Size
Kcal
Fat (grams)
Saturated
Fat (grams)
Trans Fat (grams)
French fried potatoes (fast-food variety)
Medium size
540
26.9
6.7
7.8
Doughnut
1
330
18.2
4.7
5.0
Cake, pound
1 slice
290
16.4
3.4
4.3
Shortening
1 tbsp
110
13.0
3.4
4.2
Potato chips
Small bag
170
11.2
1.9
3.2
Margarine, stick
1 tbsp
100
11.0
2.1
2.8
Cookies (cream-fi lled)
3
150
6.1
1.2
1.9
Margarine, tub
1 tbsp
60
6.7
1.2
0.6
Butter
1 tbsp
100
10.8
7.2
0.3
Milk, whole
1 cup
140
6.6
4.3
0.2
100
10.8
1.6
0
Mayonnaise (soybean oil) 1 tbsp
The fi ve major sources of trans fat are (in order); cakes, cookies, crackers, pies, and bread; margarine; fried potatoes; potato chips, corn chips, and popcorn; and shortening used in the home.
Source: http://www.cfsan.fda.gov/~dms/qatrans2.html
fried (especially deep-fat fried) food items, any pastries or flaky bread products (such as pie crusts, crackers, croissants, and biscuits), and cookies.
Until all foods are labeled with trans fat content, consumers can also make educated guesses on the trans fat content of foods by examining the list of ingredients on the food label. If partially-hydrogenated vegetable oil is one of the first three ingredients on the label, you can assume there is a significant amount of trans fat in the product.
Limiting trans fat at home is a much easier task. Most importantly, use little or no stick margarine or shortening. Instead, substitute vegetable oils and softer tub margarines (whose labels list vegetable oil or water as the first ingredient). Avoid deep-fat frying any food in shortening. Substitute baking, panfrying, broiling, steaming, grilling, or deep-fat frying in unhydrogenated vegetable oils. Replace nondairy creamers with reduced-fat or fat-free milk, since most nondairy creamers are rich in partiallyhydrogenated vegetable oils. Finally, read the ingredients on food labels, using the previous tips to estimate trans fat content.
ConCept CheCk
m French fries and other fried foods are a com-
mon source of fat and trans fatty acids for many adults. For those who choose to consume these products on a regular basis, a small serving size is recommended, especially if a person has elevated blood lipids.
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Hydrogenation of unsaturated fatty acids is the process of adding hydrogen to carbon-carbon double bonds to produce single bonds. This results in the creation of some trans fatty acids.
Hydrogenation changes vegetable oil to solid fat. It is wise to monitor trans fat intake, as this form of fat increases the risk for heart disease.
The carbon-carbon double bonds in polyunsaturated fatty acids are easily broken, yielding products responsible for rancidity. The presence of antioxidants, such as vitamin E in oils, naturally protects unsaturated fatty acids against oxidative destruction. Manufacturers can use hydrogenated fats and add natural or synthetic antioxidants to reduce the likelihood of rancidity.
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Making Lipids Available for Body Use
It’s no secret that fats and oils make foods more appealing. Their presence in foods adds flavor, lubrication, and texture. What happens to lipids once they are eaten?
Let’s take a closer look at the digestion, absorption, and physiological roles of lipids in the body.
Digestion
In the first phase of fat digestion, the stomach (and salivary glands to some extent) secretes lipase. This enzyme acts primarily on triglycerides that have fatty acids with short chain lengths, such as those found in butterfat. The action of salivary and stomach lipase, however, is usually dwarfed by that of the lipase enzyme released from the pancreas and active in the small intestine. Triglycerides and other lipids found in common vegetable oils and meats have longer chain lengths and are generally not digested until they reach the small intestine (Fig. 5-10).
lipase Fat-digesting enzyme produced by the salivary glands, stomach, and pancreas.
Fat Digestion and Absorption
1
2
2
1
Stomach
Liver
3
3
Pancreas
4
4
Small intestine 5 Anus
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5
Figure 5-10 c A summary of fat digestion and absorption.
Chapter 3 covered general aspects of this process.
Only minor digestion of fat takes place in the stomach through the action of lipase enzymes.
The liver produces bile,stored in the gallbladder and released through the bile duct into the small intestine. Bile aids in fat digestion and absorption by emulsifying lipids in the digestive juices. The pancreas secretes a mixture of enzymes, including lipase, into the small intestine.
The small intestine is the primary site for digestion and absorption of lipids. Once absorbed, long-chain fatty acids are packaged for transport through the lymph and bloodstream. (Shorter-chain fatty acids are absorbed directly into portal circulation.)
Less than 5% of ingested fat is normally excreted in the feces.
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c If the gall bladder is surgically removed
(e.g., in cases of gall stone formation), bile will enter the small intestine directly from the liver.
In the small intestine, triglycerides are broken down by lipase into smaller products, namely monoglycerides (glycerol backbones with a single fatty acid attached) and fatty acids. Under the right circumstances, digestion is rapid and thorough. The
“right” circumstances include the presence of bile from the gallbladder. Bile acids present in the bile act as emulsifiers on the digestive products of lipase action, suspending the monoglycerides and fatty acids in the watery digestive juices. This emulsification improves digestion and absorption because as large fat globules are broken down into smaller ones, the total surface area for lipase action increases.
With regard to phospholipid digestion, certain enzymes from the pancreas and cells in the wall of the small intestine digest phospholipids. The eventual products are glycerol, fatty acids, and remaining parts. With regard to cholesterol digestion, any cholesterol with a fatty acid attached is broken down to free cholesterol and fatty acids by certain enzymes released from the pancreas.
Making Decisions
Bile Acids
Gallbladder
Substances sent from the liver to the small intestine
Liver
as ch d su an d ces
S u b st a n blo o into Enterohepatic
Circulation
b se ile a nt c ba id s r ea ck to t b s o r b e d he li ver Small intestine During meals, bile acids circulate in a path that begins in the liver, goes on to the gallbladder, and then moves to the small intestine. After participating in fat digestion, most bile acids are absorbed and end up back at the liver. Approximately 98% of the bile acids are recycled. Only
1% to 2% ends up in the large intestine to be eliminated in the feces. Using medicines that block some of this reabsorption of bile acids is one way to treat high blood cholesterol. The liver takes cholesterol from the bloodstream to form replacement bile acids. Viscous fiber in the diet can also bind to bile acids to produce the same effect (see a later section on medical interventions related to cardiovascular disease for details).
Absorption
The products of fat digestion in the small intestine are fatty acids and monoglycerides.
These products diffuse into the absorptive cells of the small intestine. About 95% of dietary fat is absorbed in this way. The chain length of fatty acids affects the ultimate fate of fatty acids and monoglycerides after absorption. If the chain length of a fatty acid is less than 12 carbon atoms, it is water soluble and will therefore probably travel as such through the portal vein that connects directly to the liver. If the fatty acid is a more typical long-chain variety, it must be reformed into a triglyceride in the intestinal absorptive cell and eventually enter circulation via the lymphatic system (review
Chapter 3 for an overview of this process).
Concept Check
In the small intestine, a lipase enzyme released from the pancreas digests dietary triglycerides into monoglycerides (glycerol backbones with single fatty acids attached) and fatty acids.
These breakdown products then diffuse into the absorptive cells of the small intestine. Longchain fatty acids are transported through the lymphatic system, whereas fatty acids with shorter carbon chains are absorbed directly into the portal vein that connects directly to the liver. Other lipids are prepared for absorption by different enzymes.
Carrying Lipids in the Bloodstream lipoprotein A compound found in the bloodstream containing a core of lipids with a shell composed of protein, phospholipid, and cholesterol.
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As noted earlier, fat and water don’t mix easily. This incompatibility presents a challenge for the transport of fats through the watery media of the blood and lymph.
Lipoproteins serve as vehicles for transport of lipids from the small intestine and liver to the body tissues (Table 5-3).
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Chapter 5: Lipids 179
Table 5-3 Composition and Roles of the Major Lipoproteins in the Blood
Lipoprotein
Primary Component
Key Role
Chylomicron
Triglyceride Carries dietary fat from the small intestine to cells
VLDL
Triglyceride Carries lipids made and taken up by the liver to cells LDL
Cholesterol Carries cholesterol made by the liver and from other sources to cells
HDL
Protein Contributes to cholesterol removal from cells and, in turn, excretion of it from the body
Lipoproteins are classified into four groups—chylomicrons, VLDL, LDL, and HDL– based on their densities. Lipids are less dense than proteins. Therefore, lipoproteins that contain a large percentage of lipids in comparison to protein are less dense than those depleted of lipids.
Dietary Fats Are Carried by Chylomicrons
As you learned in the previous section, digestion of dietary fats results in a mixture of glycerol, monoglycerides, and fatty acids. Once these products are absorbed by the cells of the small intestine, they are reassembled into triglycerides. Then, the intestinal cells package the triglycerides into chylomicrons, which enter the lymphatic system and eventually the bloodstream (review Fig. 3-5 in Chapter 3 for a depiction of lymphatic circulation). Chylomicrons contain dietary fat and originate only from the intestinal cells. Like the other lipoproteins described in the next section, chylomicrons are composed of large droplets of lipid surrounded by a thin, water-soluble shell of phospholipids, cholesterol, and protein (Fig. 5-11). The water-soluble shell around a chylomicron allows the lipid to float freely in the water-based blood. Some of the proteins present may also help other cells identify the lipoprotein as a chylomicron.
Once a chylomicron enters the bloodstream, the triglycerides in its core are broken down into fatty acids and glycerol by an enzyme called lipoprotein lipase, attached to the inside walls of the blood vessels (Fig. 5-12). As soon as the fatty acids
chylomicron Lipoprotein made of dietary fats surrounded by a shell of cholesterol, phospholipids, and protein. Chylomicrons are formed in the absorptive cells of the small intestine after fat absorption and travel through the lymphatic system to the bloodstream.
lipoprotein lipase An enzyme attached to the cells that form the inner lining of blood vessels; it breaks down triglycerides into free fatty acids and glycerol.
Figure 5-11 c The structure of a lipo-
Protein
protein, in this case an LDL. This structure allows fats to circulate in the water-based bloodstream. Various lipoproteins are found in the bloodstream. The primary component of LDL is cholesterol.
Triglycerides
Phospholipids
Free cholesterol
Cholesterol bound to fatty acids
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Various fats in a meal
Source of Fat
Lipoprotein
Function
The chylomicrons deliver fatty acids to cells.
HDL made by the liver and intestine
Packaged into
VLDL by the liver
Packaged into chylomicrons in the intestine
Lipoprotein
Production
Cholesterol that arises from dying cells or during lipoprotein metabolism
Various lipids collected (or made) by the liver
HDL pick up these lipids and transfer them to other lipoproteins or to the liver.
The VLDL deliver fatty acids to cells
LDL arise from the
VLDL particle as the fatty acids are removed. Ultimate
LDL
Production
LDL deliver primarily cholesterol to cells.*
LDL
Function
* Cholesterol not taken up by body cells can be taken up by scavenger cells in the arteries.
The eventual cholesterol buildup leads to atherosclerosis.
VLDL: Very-Low-Density Lipoprotein
LDL: Low-Density Lipoproteinn
HDL: High-Density Lipoprotein
Figure 5-12 c Lipoprotein production and function. Chylomicrons carry absorbed fat to body cells. VLDL carries fat taken up from the bloodstream by the liver, as well as any fat made by the liver, to body cells. LDL arises from VLDL and carries mostly cholesterol to cells.
HDL arises mostly from the liver and intestine. HDL carries cholesterol from cells to other lipoproteins and to the liver for excretion.
are released to the bloodstream, they are absorbed by cells in the vicinity, while much of the glycerol circulates back to the liver. Muscle cells can immediately use the absorbed fatty acids for fuel. Adipose cells, on the other hand, tend to re-form the fatty acids into triglycerides for storage. After triglycerides have been removed, a chylomicron remnant remains. Chylomicron remnants are removed from circulation by the liver and their components are recycled to make other lipoproteins and bile acids.
Other Lipoproteins Transport Lipids from the Liver to the Body Cells
very-low-density lipoprotein (VLDL)
The lipoprotein created in the liver that carries cholesterol and lipids that have been taken up or newly synthesized by the liver.
low-density lipoprotein (LDL) The lipoprotein in the blood containing primarily cholesterol; elevated LDL is strongly linked to cardiovascular disease risk.
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The liver takes up various lipids from the blood. The liver also is the manufacturing site for lipids and cholesterol. The raw materials for lipid and cholesterol synthesis include free fatty acids taken up from the bloodstream, as well as carbon and hydrogen derived from carbohydrates, protein, and alcohol. The liver then must package these synthesized lipids as lipoproteins for transport in the blood to body tissues.
First in our discussion of lipoproteins made by the liver are very-low-density lipoproteins (VLDL). These particles are composed of cholesterol and triglycerides surrounded by a water-soluble shell. VLDLs are rich in triglycerides and thus are very low in density. Once in the bloodstream, lipoprotein lipase on the inner surface of the blood vessels breaks down the triglyceride in the VLDL into fatty acids and glycerol. Fatty acids and glycerol are released into the bloodstream and taken up by the body cells.
As its triglycerides are released, the VLDL becomes proportionately denser. Much of what eventually remains of the VLDL fraction is then called low-density lipoprotein
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(LDL); this is composed primarily of the remaining cholesterol. The primary function of
LDL is to transport cholesterol to tissues. LDL particles are taken up from the bloodstream by specific receptors on cells, especially liver cells, and are then broken down. The cholesterol and protein components of LDL provide some of the building blocks necessary for cell growth and development, such as synthesis of cell membranes and hormones.
The final group of lipoproteins, high-density lipoproteins (HDL), is a critical and beneficial participant in this process of lipid transport. Its high proportion of protein makes it the densest lipoprotein. The liver and intestine produce most of the HDL in the blood. It roams the bloodstream, picking up cholesterol from dying cells and other sources. HDL donates the cholesterol primarily to other lipoproteins for transport back to the liver to be excreted. Some HDL travels directly back to the liver.
high-density lipoprotein (HDL) The lipoprotein in the blood that picks up cholesterol from dying cells and other sources and transfers it to the other lipoproteins in the bloodstream, as well as directly to the liver; low HDL increases the risk for cardiovascular disease. menopause The cessation of the menstrual cycle in women, usually beginning at about
50 years of age.
scavenger cells Specific form of white
“Good” and “Bad” Cholesterol in the Bloodstream
HDL and LDL are often described as “good” and “bad” cholesterol, respectively. Many studies demonstrate that the amount of HDL in the bloodstream can closely predict the risk for cardiovascular disease. Risk increases with low HDL because little cholesterol is transported back to the liver and excreted. Women tend to have high amounts of HDL, especially before menopause, compared to men. High amounts of HDL slow the development of cardiovascular disease, so any cholesterol carried by HDL can be considered “good” cholesterol.
On the other hand, LDL is sometimes considered “bad” cholesterol. In our discussion of LDL, you learned that LDL is taken up by receptors on various cells. If LDL is not readily cleared from the bloodstream, scavenger cells in the arteries take up the lipoprotein, leading to a buildup of cholesterol in the blood vessels. This buildup, known as atherosclerosis, greatly increases the risk for cardiovascular disease (see the following
“Nutrition and Your Health” section). LDL is only a problem when it is too high in the bloodstream because low amounts are needed as part of routine body functions.
Making Decisions
LDL Cholesterol
The cholesterol in foods is not designated as “good” or “bad.” It is only after cholesterol has been made or processed by the liver that it shows up in the bloodstream as LDL or HDL. Dietary patterns can affect the metabolism of cholesterol, however. Diets low in saturated fat, trans fat, and cholesterol encourage the uptake of LDL by the liver, thereby removing LDL from the bloodstream and decreasing the ability of scavenger cells to form atherosclerotic plaques in the blood vessels. Likewise, diets high in saturated fat, trans fat, and cholesterol reduce the uptake of LDL by the liver, increasing cholesterol in the blood and the risk for cardiovascular disease. What foods in your diet are high in saturated fat, trans fat, or cholesterol?
blood cells that can bury themselves in the artery wall and accumulate LDL. As these cells take up LDL, they contribute to the development of atherosclerosis.
atherosclerosis A buildup of fatty material (plaque) in the arteries, including those surrounding the heart.
See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
c It appears that saturated fatty acids promote an increase in the amount of free cholesterol (not attached to fatty acids) in the liver, whereas unsaturated fatty acids do the opposite. As free cholesterol in the liver increases, it causes the liver to reduce cholesterol uptake from the bloodstream, contributing to elevated LDL in the blood.
(Trans fatty acids are thought to act in the same ways as saturated fatty acids.)
Concept Check
Lipids generally move through the bloodstream as part of lipoproteins. Dietary fats absorbed from the small intestine are packaged and transported as chylomicrons, whereas lipids synthesized in the liver are packaged as very-low-density lipoproteins (VLDL). Lipoprotein lipase removes triglycerides from the interiors of both chylomicrons and VLDL, breaking the triglycerides down into glycerol and fatty acids, which are taken up by tissues for energy needs or storage. What remains after the action of lipoprotein lipase are chylomicron remnants (from chylomicrons), the components of which are recycled by the liver, or low-density lipoproteins (LDL, from VLDL), rich in cholesterol. LDL is picked up by receptors on body cells, especially liver cells. Scavenger cells in the arteries may do the same, speeding the development of atherosclerosis. High-density lipoprotein (HDL), also produced in part by the liver, picks up cholesterol from cells and transports it primarily to other lipoproteins for eventual transport back to the liver. Risk factors for cardiovascular disease include an elevated level of LDL and/or low amounts of HDL in the blood.
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essentiaL FunCtions oF FattY aCids
Omega-3 Fatty Acids in Fish
(grams per 3 ounce serving)
Atlantic salmon
Anchovy
Sardines
Rainbow trout
Coho salmon
Bluefi sh
Striped bass
Tuna, white, canned
Halibut
Catfi sh, channel
The various classes of lipids have diverse functions in the body. All are necessary for health, but, as mentioned earlier, many can be made by the body and therefore are not needed in our diet. Of all the classes of lipids, only certain polyunsaturated fatty acids are essential parts of a diet.
1.8
1.7
1.4
1.0
0.9
0.8
0.8
0.7
0.4
0.2
The Essential Fatty Acids
Recommended omega-3 fatty acid
(alpha-linolenic acid) intake per day:
Men
1.6 grams
Women
1.1 grams
eicosapentaenoic acid (EPA) An omega-3 fatty acid with 20 carbons and five carboncarbon double bonds. It is present in large amounts in fatty fish and is slowly synthesized in the body from alpha-linolenic acid.
docosahexaenoic acid (DHA) An omega-3 fatty acid with 22 carbons and six carbon-carbon double bonds. It is present in large amounts in fatty fish and is slowly synthesized in the body from alpha-linolenic acid. DHA is especially present in the retina and brain.
We must obtain linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) from foods to maintain health, so they are called essential fatty acids (Fig. 5-13). These omega-6 and omega-3 fatty acids form parts of vital body structures, perform important roles in immune system function and vision, help form cell membranes, and produce hormone-like compounds. Omega-6 and omega-3 fatty acids must be obtained through the diet because human cells lack the enzymes needed to produce these fatty acids. Other fatty acids, such as omega-9 fatty acids, can be synthesized in the body, and therefore are not essential components of the diet.
Still, we need to consume only about 5% of our total calories per day from essential fatty acids. That corresponds to about 2 to 4 tablespoons of plant oil each day. We can easily get that much—from mayonnaise, salad dressings, and other foods—without much effort. Regular consumption of vegetables and whole-grain breads and cereals also helps to supply enough essential fatty acids.
Research also suggests that we include a regular intake of the omega-3 fatty acids, eicosapentaenoic acid (ePa) and docosahexaenoic acid (DHa) , which can be made from alpha-linolenic acid. EPA and DHA are naturally high in fatty fish such as salmon, tuna, sardines, anchovies, striped bass, catfish, herring, mackerel, trout, or halibut. Consumption of one or more of these fish at least twice a week is recommended to obtain EPA and DHA. Additional sources of omega-3 fatty acids include canola and soybean oils, walnuts, flax seeds, mussels, crab, and shrimp.
Making deCisions
arachidonic acid An omega-6 fatty acid made from linoleic acid with 20 carbon atoms and 4 carbon-carbon double bonds.
m The American Heart Association recommends
eating fatty fish such as salmon at least twice a week. As a source of omega-3 fatty acids, fish is a heart-healthy alternative to other animal sources of protein, which can be high in saturated fat and cholesterol.
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Mercury in Fish
Consumption of fatty fish at least twice a week is recommended as a good source of the omega-3 fatty acids. Some fish can be a source of mercury, toxic in high amounts, especially swordfish, shark, king mackerel, and tile fish (see Chapter 13). Albacore tuna also is a potential source, while other forms of tuna are much lower in mercury. Those fish low in mercury are salmon, sardines, bluefish, and herring. Shrimp is also low in mercury. For others, varying your choices rather than always eating the same species of fish and limiting overall intake to 12 ounces per week (on average 2 to 3 meals of fish or shellfish per week) is recommended to reduce mercury exposure, especially for pregnant women and children. Existing research indicates that the benefits of fish intake, especially in reducing the risk of cardiovascular disease, outweigh the possible risks of mercury contamination.
The recommendation to consume omega-3 fatty acids stems from the observation that compounds made from omega-3 fatty acids tend to decrease blood clotting and inflammatory processes in the body. The omega-6 fatty acids, notably arachidonic acid made from linoleic acid, generally increase clotting and inflammation, and saturated fatty acids also increase blood clotting.
Some studies show that people who eat fish at least twice a week (total weekly intake: 8 ounces [240 grams]) run lower risks for heart attack than do people who rarely eat fish. In these cases, the omega-3 fatty acids in fish oil are probably acting to reduce blood clotting. As will be covered in detail in the “Nutrition and Your Health”
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FiguRe 5-13 c The essential fatty acid
Polyunsaturated Fatty Acids
(PUFA)
Omega-3 Family
Omega-6 Family
Alpha-linolenic acid
Linoleic acid
Docosahexaenoic acid (DHA)
Eicosapentaenoic acid (EPA)
(EFA) family. All these fatty acids are available from dietary sources; linoleic acid and alpha-linolenic acid must be consumed as body synthesis does not take place. These are the essential fatty acids. The other fatty acids in this figure can be synthesized from the essential fatty acids.
Arachidonic acid section, blood clots are part of the heart attack process. In addition, these omega-3 fatty acids have a favorable effect on heart rhythm. Consequently, the risk of heart attack decreases with the consumption of omega-3 fatty acids from fish, especially for people already at high risk.
We need to remember, however, that blood clotting is a normal body process. Certain groups of people, such as Eskimos in Greenland, eat so much seafood that their normal blood-clotting ability can be impaired. An excess of omega-3 fatty acid intake can allow uncontrolled bleeding and may cause hemorrhagic stroke. However, no increase in risk of stroke has been observed in studies using moderate amounts of omega-3 fatty acids.
Studies also have shown that large amounts of omega-3 fatty acids from fish
(2 to 4 grams per day) can lower blood triglycerides in people with high triglyceride concentrations. In addition, these omega-3 fatty acids are suspected to be helpful in managing the pain of inflammation associated with rheumatoid arthritis by suppressing immune system responses. This may also help with certain behavioral disorders and cases of mild depression.
In some instances, fish oil capsules can be safely substituted for fish consumption if a person does not like fish. Generally, about 1 gram of omega-3 fatty acids (about three capsules) from fish oil per day is recommended, especially for people with evidence of cardiovascular disease. (Freezing fish oil capsules before consumption or using enteric coated capsules will reduce the fishy aftertaste.)
The American Heart Association also recently suggested that fish oil supplements
(providing 2 to 4 grams of omega-3 fatty acids per day) could be employed to treat elevated blood triglycerides, as previously noted. However, fish oil capsules should be limited for individuals who have bleeding disorders, take anticoagulant medications, or anticipate surgery, because they may increase risk of uncontrollable bleeding and hemorrhagic stroke. Thus for fish oil capsules, as well as other dietary supplements, it is important to follow a physician’s recommendations. Remember that fish oil supplements are not regulated by the FDA. The quality of these supplements, therefore, is not standardized, and contaminants naturally present in the fish oil may not have been removed.
In summary, the regular consumption of fatty fish is advised. Consuming whole fish is thought to have greater benefits and be safer than using fish oil supplements. Fish is not only a rich source of omega-3 fatty acids, but is also a valuable source of protein and trace elements that may also provide protective effects for the cardiovascular system. Broiled or baked fish is recommended rather than fried fish because frying may decrease the ratio of omega-3 to omega-6 fatty acids and may produce trans fatty acids and oxidized lipid products that may increase cardiovascular disease risk.
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See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
hemorrhagic stroke Damage to part of the brain resulting from rupture of a blood vessel and subsequent bleeding within or over the internal surface of the brain.
m Walnuts are one of the richest plant sources of
the omega-3 fatty acid, alpha-linolenic acid, and are also a good source of plant sterols.
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CRitiCaL thinking
Making deCisions
Advertisements often claim that fats are bad. Your classmate Mike asks, “If fats are so bad for us, why do we need to have any in our diets?” How would you answer him?
Flax Seeds or Walnuts?
Flax seeds and walnuts are getting attention today because they are rich vegetable sources of the omega-3 alpha-linolenic acid. About 2 tablespoons per day is typically recommended if used as an omega-3 fatty acid source. Flax seeds can be purchased in many natural food stores rather inexpensively. These need to be chewed thoroughly or they will pass through the GI tract undigested. Many people find it easier to grind them in a coffee grinder before eating them. Flax seed oil is also available, but it turns rancid very quickly, especially if not refrigerated. Compared to other nuts and seeds, walnuts are one of the richest sources of alpha-linolenic acid (2.6 grams per 1-ounce serving or 14 walnut halves). The DRI for alphalinolenic acid is 1.6 grams/day for men and 1.1 grams/day for women. In addition, walnuts are a rich source of plant sterols known to inhibit intestinal absorption of cholesterol.
rancid Containing products of decomposed fatty acids that have an unpleasant flavor and odor.
total parenteral nutrition The intravenous feeding of all necessary nutrients, including the most basic forms of protein, carbohydrates, lipids, vitamins, minerals, and electrolytes.
Effects of a Deficiency of Essential Fatty Acids
If humans fail to consume enough essential fatty acids, their skin becomes flaky and itchy, and diarrhea and other symptoms such as infections often are seen. Growth and wound healing may be restricted. These signs of deficiency have been seen in people fed intravenously by total parenteral nutrition containing little or no fat for 2 to 3 weeks, as well as in infants receiving formulas low in fat. However, because our bodies need the equivalent of only about 2 to 4 tablespoons of plant oils a day, even a lowfat diet will provide enough essential fatty acids if it follows a balanced plan such as
MyPyramid and includes a serving of fatty fish at least twice a week.
ConCept CheCk
Because humans can’t make either omega-3 or omega-6 fatty acids, which perform vital functions in the body, they must be obtained from the diet and therefore are called essential fatty acids. Plant oils are generally rich in omega-6 fatty acids. Eating fatty fish at least twice a week is a good way to meet omega-3 fatty acid needs. Fish oil supplementation (about 1 gram per day of the related omega-3 fatty acids) is generally acceptable under a physician’s guidance if a person does not like fish; however, people with certain medical conditions (e.g., taking anticoagulant medications) should be cautious with use of fish oil supplements due to increased risk of hemorrhagic stroke. Essential fatty acid deficiency can occur after 2 to 3 weeks if fat is omitted from total parenteral nutrition solutions, which in turn can lead to skin disorders, diarrhea, and other health problems.
bRoadeR RoLes FoR FattY aCids and tRigLYCeRides in the bodY
Many key functions of fat in the body require the use of fatty acids in the form of triglycerides. Triglycerides are used for energy storage, insulation, and transportation of fat-soluble vitamins.
Providing Energy
m When at rest or during light activity, the body
uses mostly fatty acids for fuel.
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Triglycerides contained in the diet and stored in adipose tissue provide the fatty acids that are the main fuel for muscles while at rest and during light activity. Muscles use carbohydrate for fuel in addition to fatty acids supplied by triglycerides only in endurance exercise, such as long-distance running and cycling, or in short bursts of intense activity, such as a 200-meter run. Other body tissues also use fatty acids for
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energy needs. Overall, about half of the energy used by the entire body at rest and during light activity comes from fatty acids. When considering the whole-body, the use of fatty acids by skeletal and heart muscle is balanced by the use of glucose by the nervous system and red blood cells. Recall from Chapter 4 that cells need a supply of carbohydrate to efficiently process fatty acids for fuel. The details about how we burn fat as a fuel will be discussed in Chapter 7.
Storing Energy for Later Use
We store energy mainly in the form of triglycerides. The body’s ability to store fat is essentially limitless. Its fat storage sites, adipose cells, can increase about 50 times in weight. If the amount of fat to be stored exceeds the ability of the existing cells to expand, the body can form new adipose cells.
An important advantage of using triglycerides to store energy in the body is that they are energy dense. Recall that these yield, on average, 9 kcal per gram, whereas proteins and carbohydrates yield only about 4 kcal per gram. In addition, triglycerides are chemically stable, so they are not likely to react with other cell constituents, making them a safe form for storing energy. Finally, when we store triglycerides in adipose cells, we store little else, especially water. Adipose cells contain about 80% lipid and only 20% water and protein. In contrast, imagine if we were to store energy as muscle tissue, which is about 73% water. Body weight linked to energy storage would increase dramatically. The same would be true if we stored energy primarily as glycogen, as about 3 grams of water are stored for every gram of glycogen.
Nucleus
Cell
membrane
Fat droplet
Adipose cell
Insulating and Protecting the Body
The insulating layer of fat just beneath the skin is made mostly of triglycerides. Fat tissue also surrounds and protects some organs—kidneys, for example—from injury.
We usually don’t notice the important insulating function of fat tissue, because we wear clothes and add more as needed. A layer of insulating fat is important in animals living in cold climates. Polar bears, walruses, and whales all build a thick layer of fat tissue around themselves to insulate against cold-weather environments. The extra fat also provides energy storage for times when food is scarce.
Transporting Fat-Soluble Vitamins
Triglycerides and other fats in food carry fat-soluble vitamins to the small intestine and aid their absorption. People who absorb fat poorly, such as those with the disease cystic fibrosis, are at risk for deficiencies of fat-soluble vitamins, especially vitamin
K. A similar risk comes from taking mineral oil as a laxative at mealtimes. The body cannot digest or absorb mineral oil, so the undigested oil carries the fat-soluble vitamins from the meal into the feces, where they are eliminated. Unabsorbed fatty acids can bind minerals, such as calcium and magnesium, and draw them into the stool for elimination. This can harm mineral status (see Chapter 9). Recall that the main problem with the fat replacer, olestra, is that it can bind the fat-soluble vitamins and reduce their absorption.
Phospholipids in the Body
Many types of phospholipids exist in the body, especially in the brain. They form important parts of cell membranes. Phospholipids are found in body cells, and they participate in fat digestion in the intestine. Recall that the various forms of lecithin
(discussed earlier) are common examples of phospholipids (review Fig. 5-4b).
Cell membranes are composed primarily of phospholipids. A cell membrane looks much like a sea of phospholipids with protein “islands” (Fig. 5-14). The proteins
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Figure 5-14 c Phospholipids are the main components of cell membranes, forming a double or bilayer of lipid.
Carbohydrate chains Protein chain
Membrane protein Globular protein Cholesterol
Phospholipid bilayer form receptors for hormones, function as enzymes, and act as transporters for nutrients. The fatty acids on the phospholipids serve as a source of essential fatty acids for the cell. Some cholesterol is also present in the membrane.
In some foods, phospholipids function as emulsifiers (as covered earlier) allowing fat and water to mix. By breaking fat globules into small droplets, emulsifiers enable a fat to be suspended in water. They act as bridges between the oil and water that in turn lead to the formation of tiny oil droplets surrounded by thin shells of water. In an emulsified solution, such as salad dressing or mayonnaise, millions of tiny oil droplets are separated by shells of water (review Fig. 5-7).
The body’s main emulsifiers are the lecithins and bile acids, produced by the liver and released into the small intestine via the gallbladder during digestion.
Cholesterol in the Body
See Nutrition and Your Health: Lipids and Cardiovascular Disease at the end of Chapter 5.
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Cholesterol plays many vital roles in the body. It forms part of some important hormones, such as estrogen, testosterone, and a form of the active vitamin D hormone.
Cholesterol is also the building block of bile acids, needed for fat digestion. Finally, cholesterol is an essential structural component of cells and the outer layer of the lipoprotein particles that transport lipids in the blood. The cholesterol content of the heart, liver, kidney, and brain is high, reflecting its critical role in these organs.
About two-thirds of the cholesterol circulating through your body is made by body cells; the remaining one-third is consumed in the diet. Each day, our cells produce approximately 875 milligrams of cholesterol. Of the 875 milligrams of cholesterol made by the body, about 400 milligrams are used to make new bile acids to replenish those lost in the feces, and about 50 milligrams are used to make hormones. In addition to all the cholesterol cells make, we consume about 180 to 325 milligrams of cholesterol per day from animal-derived food products, with men consuming the higher amount compared to women. Absorption of cholesterol from food ranges from about 40% to 65%. The effect of blood cholesterol, especially LDL cholesterol, on cardiovascular disease risk will be discussed in the “Nutrition and Your Health” section.
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ConCept CheCk
Triglycerides are the major form of fat in the body. They are used for energy and stored in adipose tissue, they insulate and protect body organs, and they transport fat-soluble vitamins.
Phospholipids are emulsifiers—compounds that can suspend fat in water. Phospholipids also form parts of cell membranes and various compounds in the body. Cholesterol, a sterol, forms part of cell membranes, hormones, and bile acids. If sufficient amounts are not consumed, the body makes what phospholipids and cholesterol it needs.
ReCoMMendations FoR Fat intake
There is no RDA for total fat intake for adults, although there is an Adequate
Intake set for total fat for infants (see Chapter 15). The 2005 Dietary Guidelines for Americans recommend that total fat intake should not exceed 20% to 35% of total calories, which equates to 44 to 78 grams per day for a person who consumes
2000 kcal daily. The most specific recommendations for fat intake come from the
American Heart Association (AHA). Many North Americans are at risk for developing cardiovascular disease, so AHA promotes dietary and lifestyle goals aimed at reducing this risk. The AHA diet and lifestyle goals for cardiovascular disease risk reduction for the general public are presented in Table 5-4. These goals include aiming for an overall healthy eating pattern; appropriate body weight; and a desirable blood cholesterol profile, blood pressure, and blood glucose level. In Table 5-5, a more detailed list of recommendations is provided for those who currently are at high risk or have cardiovascular disease.
To reduce risk for cardiovascular disease, the AHA recommends that no more than 7% of total calories come from saturated fat and no more than 1% from trans fat. These are the primary fatty acids that raise LDL. In addition, cholesterol should amount to a maximum of 300 mg per day. This often happens along with the reduction in saturated fat and trans fat intake. Table 5-6 is an example of a diet that adheres to 20% or 30% of calories as fat. Compare these recommendations to the actual dietary intake patterns of these fats by North Americans: 33% of calories from total fat, about 13% of calories from saturated fat, and 180 to 320 milligrams of cholesterol each day.
Both the National Cholesterol Education Program (NCEP) and the Food and
Nutrition Board are in agreement with the advice of the AHA. One exception in the tabLe 5-4 American Heart Association 2006 Diet and Lifestyle Goals for Cardiovascular Disease Risk Reduction
m Trim the fat from meats before cooking to
help reduce your saturated fat intake. Also, limit use of meat that is highly marbled with fat (seen as streaks of fat).
c One goal of Healthy People 2010 is to increase the proportion of persons age 2 years and older who consume less than 10% of calorie intake from saturated fat.
• Consume an overall healthy diet.
• Aim for a healthy body weight.
• A im for recommended levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides.
• Aim for a normal blood pressure.
• Aim for a normal blood glucose level.
• Be physically active.
• Avoid use of and exposure to tobacco products.
From: Lichtenstein AH et al: Diet and lifestyle recommendations revision 2006. A scientifi c statement from the
American Heart Association Nutrition Committee. Circulation 114:82, 2006.
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c Some advice regarding fat intake from the
2005 Dietary Guidelines:
Table 5-5 American Heart Association 2006 Diet and Lifestyle Recommendations for
Cardiovascular Disease Risk Reduction
• Limit intake of fats and oils high in saturated and/or trans fatty acids, and choose products low in such fats and oils.
• Balance calorie intake and physical activity to achieve or maintain a healthy body weight.
• Consume fats high in polyunsaturated and monounsaturated fatty acids, such as from fish, nuts, and vegetable oils.
• Choose whole-grain, high-fiber foods.
• When selecting and preparing meat, poultry, dry beans, and milk or milk products, make choices that are lean, low-fat, or fat-free. • Limit your intake of saturated fat to less than 7% of energy, trans fat to less than 1% of energy, and cholesterol to less than 300 milligrams per day by
— choosing lean meats and vegetable alternatives;
— selecting fat-free (skim), 1%-fat, and low-fat dairy products; and
— minimizing intake of partially hydrogenated fats.
• Consume a diet rich in vegetables and fruits.
• Consume fish, especially oily fish, at least twice a week.
• Minimize your intake of beverages and foods with added sugars.
• Choose and prepare foods with little or no salt.
• If you consume alcohol, do so in moderation.
• When you eat food prepared outside of the home, follow the AHA Diet and Lifestyle
Recommendations.
From: Lichtenstein AH and others: Diet and lifestyle recommendations revision 2006. A scientific statement from the American Heart Association Nutrition Committee. Circulation 114:82, 2006.
Table 5-6 Daily Menu Examples Containing 2000 kcal and 30% or 20% of Calories as Fat
30% of Calories as Fat
Food
m Whole grains (shredded wheat, whole-wheat
bread, oatmeal cookies, popcorn), fruits (orange juice, apple, banana, raisins), vegetables (carrots, lettuce, tomato), lean meats (roast beef, turkey, chicken), and fat-free milk are the primary components of the low-fat menus in Table 5-6.
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20% of Calories as Fat
Fat
(grams)
Food
Fat
(grams)
Breakfast
Orange juice, 1cup
Shredded wheat, ¾ cup
Toasted bagel
Tub margarine, 3 teaspoons
1% low-fat milk, 1 cup
0.5
0.5
1.1
11.4
2.5
Same
Shredded wheat, 1 cup
Same
Tub margarine, 2 teaspoons
Fat-free milk, 1 cup
Lunch
Whole-wheat bread, 2 slices
Roast beef, 2 ounces
Mayonnaise, 3 teaspoons
Lettuce
Tomato
Oatmeal cookie, 1
2.4
4.9
11.0
—
—
3.3
Same
Light turkey roll, 2 ounces
Mayonnaise, 2 teaspoons
Same
Same
Oatmeal cookie, 2
2.4
0.9
7.3
—
—
6.6
Snack
Apple
—
Same
—
Dinner
Chicken tenders frozen meal
Carrots, ½ cup
Dinner roll, 1
Margarine, 1 teaspoon
Banana
1% low-fat milk, 1 cup
18.0
—
2.0
3.8
0.6
2.5
Fat-free chicken tenders
Same
Same
Same
Same
Fat-free milk, 1 cup
—
—
2.0
3.8
0.6
0.6
Snack
Raisins, 2 teaspoons
Air-popped popcorn, 3 cups
Margarine, 2 teaspoons
—
1.0
7.6
Raisins, ½ cup
Air-popped popcorn, 6 cups
Same
—
2.0
7.6
Totals
73.1
44.3
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Chapter 5: Lipids 189
tabLe 5-7 Food and Nutrition Board Recommendations for Omega-6 and Omega-3
Fatty Acids per Day
Men (grams per day)
Linoleic acid (omega-6)
Women (grams per day)
17
Alpha-linolenic acid (omega-3)
12
1.6
1.1
latest guidelines from the NCEP is that fat intake could be as high as 35% of total calories as long as intakes of saturated fat, cholesterol, and trans fat are minimized.
The Food and Nutrition Board combines the AHA and NCEP recommendations, suggesting that fat provide a range of 20% to 35% of calories. The 2005 Dietary
Guidelines also support this advice. In addition to fat intake, controlling total calorie intake is also significant, as weight control is a vital component of cardiovascular disease prevention.
Regarding essential fatty acids, the Food and Nutrition Board has issued recommendations for both omega-6 and omega-3 fatty acids.
The amounts listed in Table 5-7 work out to about 5% of calorie intake for the total of both essential fatty acids. Infants and children have lower needs (again, see Chapter 15). Consumption of fish at least twice a week is one step toward meeting requirements for essential fatty acids.
The typical North American diet derives about 7% of calories from polyunsaturated fatty acids, and thus meets essential fatty acid needs. An upper limit of 10% of calorie intake as polyunsaturated fatty acids is often recommended, in part because the breakdown
(oxidation) of those present in lipoproteins is linked to increased cholesterol deposition in the arteries (see the “Nutrition and Your
Health” section in this chapter). Depression of immune function is also suspected to be caused by an excessive intake of polyunsaturated fatty acids.
In recent years, the Mediterranean diet has attracted a lot of attention as a result of lower rates of chronic diseases seen in people following such a diet plan. The major sources of fat in the Mediterranean diet include liberal amounts of olive oil compared to a small amount of animal fat (from animal flesh and dairy products). In contrast, major sources of fat in the typical North American diet include animal flesh, whole milk, pastries, cheese, margarine, and mayonnaise. While dietary fat sources definitely play a role in prevention of chronic disease, it is important to remember that other aspects of one’s lifestyle also contribute to disease risk. People who follow a Mediterranean diet also tend to consume moderate alcohol (usually in the form of red wine, which contains many antioxidants), eat plenty of whole grains and few refined carbohydrates, and are also more physically active than typical North Americans.
An alternative plan for reduction of cardiovascular disease is Dr. Dean Ornish’s purely vegetarian (vegan) diet plan. This diet is very low in fat, including only a scant quantity of vegetable oil used in cooking and the small amount of oils present in plant foods. Individuals restricting fat intake to 20% of calories should be monitored by a physician, as the resulting increase in carbohydrate intake can increase blood triglycerides in some people, which is not a healthful change. Over time, however, the initial problem of high blood triglycerides on a low-fat diet may self-correct. Among people following the Ornish plan, blood triglycerides initially increased, but within a year, fell to normal values as long as they emphasized highfiber carbohydrate sources, controlled (or improved) body weight, and followed a regular exercise program.
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c The advice to consume 20% to 35% of calories as fat does not apply to infants and toddlers below the age of 2 years. These youngsters are forming new tissue that requires fat, especially in the brain, so their intake of fat and cholesterol should not be greatly restricted.
m If you are looking to decrease the amount of
saturated and trans fats in your diet, it is a good idea to opt for lower-fat substitutes for some of your current high-fat food choices. How do you think this meal compares with the chicken nuggets and French fries meal on p. 176.
vegan A person who eats only plant foods.
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tABLe 5-8 Tips for Avoiding Too Much Fat, Saturated Fat, Cholesterol, and Trans Fat
Eat Less of These Foods
Eat More of These Foods
Grains
• Pasta dishes with cheese or cream sauces
• Croissants
• Pastries
• Doughnuts
• Pie crust
•
•
•
•
•
•
Vegetables
• French fries
• Potato chips
• Vegetables cooked in butter, cheese, or cream sauces
• Fresh, frozen, baked, or steamed vegetables
Fruit
• Fruit pies
• Fresh, frozen, or canned fruits
Milk
•
•
•
•
Whole milk
Ice cream
High-fat cheese
Cheesecake
• Fat-free and reduced-fat milk
• Low-fat frozen desserts
(e.g., yogurt, sherbet, ice milk)
• Reduced-fat/part-skim cheese
Meat and beans
•
•
•
•
Bacon
Sausage
Organ meats (e.g., liver)
Egg yolks
• Fish
• Skinless poultry
• Lean cuts of meat (with fat trimmed away)
• Soy products
• Egg whites/egg substitutes
Oils
• Butter and stick margarine
• Canola oil or olive oil
• Tub or liquid margarine (in small amounts)
To avoid too much saturated fat and choles-
terol, for breakfast eat fewer foods like bacon, sausage, and whole eggs and more foods like whole-grain waffles and fresh fruit.
Whole-grain breads
Whole-grain pasta
Brown rice
Angel food cake
Animal or graham crackers
Air-popped popcorn
In summary, the general consensus among nutrition experts suggests that limitation of saturated fat, cholesterol, and trans fat intake should be the primary focus, and that the diet needs to contain some omega-3 and omega-6 fatty acids (Table 5-8).
Furthermore, if fat intake exceeds 30% of total calories, the extra fat should come from monounsaturated fat.
ConCept CheCk
There is no RDA for fat. We need about 5% of total calorie intake from plant oils to obtain the needed essential fatty acids. Eating fatty fish at least twice a week is also advised to supply omega-3 fatty acids. Many health-related agencies recommend a diet containing no more than 35% of calorie intake as fat, with no more than 7% to 10% of calorie intake as a combination of saturated fat and trans fat for the general public. Cholesterol intake should be limited to 200 to 300 milligrams per day. These practices help maintain a normal LDL value in the blood. The North American diet contains about 33% of calories as fat, with about 13% of calories as saturated fat and about 3% as trans fatty acids. Cholesterol intake varies from about
200 to 400 milligrams per day.
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nutRition and YouR heaLth
Lipids and Cardiovascular Disease
Cardiovascular disease is the major killer of North Americans. Cardiovascular disease typically involves the coronary arteries and, thus, frequently the term coronary heart disease (CHD) or coronary artery disease (CAD) is used. Each year about
500,000 people die of coronary heart disease in the United States, about 60% more than die of cancer. The figure rises to almost 1 million if strokes and other circulatory diseases are included in the global term cardiovascular disease. About 1.5 million people in the United States each year have a heart attack. The overall male-tofemale ratio for cardiovascular disease is about 2:1. Women generally lag about 10 years behind men in developing the disease. Still, it eventually kills more women than any other disease—twice as many as does cancer. And, for each person in
North America who dies of cardiovascular disease, 20 more (over 13 million people) have symptoms of the disease.
m Cardiovascular disease kills more women
than any other disease—twice as many as does cancer. deveLopMent oF
CaRdiovasCuLaR
disease
The symptoms of cardiovascular disease develop over many years and often do not become obvious until old age. Nonetheless, autopsies of young adults under 20 years of age have shown that many of them had atherosclerotic plaque in their arteries (Fig. 5-15). This finding indicates that plaque buildup can begin in childhood and continue throughout life, although it usually goes undetected for some time.
The typical forms of cardiovascular disease—coronary heart disease and strokes—are associated with inadequate blood circulation in the heart and brain related to buildup of this plaque. Blood supplies the heart muscle, brain, and other body organs with oxygen and nutrients.
When blood flow via the coronary arteries surrounding the heart is interrupted, the heart muscle can be damaged. A heart attack, or myocardial infarction, may result (review
Fig. 5-15). This may cause the heart to beat irregularly or to stop. About 25% of people do not survive their first heart attack. If blood flow to parts of the brain is interrupted long enough, part of the brain dies, causing a cerebrovascular accident, or stroke.
A heart attack can strike with the sudden force of a sledgehammer, with pain radiating up the neck or down the arm. It can sneak up at night, masquerading as indigestion, with slight pain or pressure in the chest. Many times, the symptoms are so subtle in women that death occurs before she or the health professional realizes that a heart attack is taking place. If there is any suspicion at all that a heart attack is taking place, the person should first chew an aspirin (325 milligrams) thoroughly and then call 911. Aspirin helps reduce the blood clotting that leads to a heart attack.
Continuous formation and breakdown of blood clots in the blood vessels is a normal process. However, in areas where plaques build up, blood clots are more
plaque A cholesterol-rich substance deposited in the blood vessels; it contains various white blood cells, smooth muscle cells, various proteins, cholesterol and other lipids, and eventually calcium.
myocardial infarction Death of part of the heart muscle. Also termed a heart attack.
cerebrovascular accident (CVA) Death of part of the brain tissue due typically to a blood clot. Also termed a stroke.
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Cell lining
(endothelium)
Muscle layer
c Typical warning signs of a heart attack are:
•
Intense, prolonged chest pain or pressure, sometimes radiating to other parts of the upper body (men and women)
•
Shortness of breath (men and women) •
Sweating (men and women)
•
Nausea and vomiting (especially women) •
Dizziness (especially women)
•
Weakness (men and women)
•
Jaw, neck, and shoulder pain (especially women)
•
Irregular heartbeat (men and women) Vessel opening
Normal artery
Beginning cholesterol plaque
Aorta
Left coronary artery Early injury
Advanced cholesterol plaque
Pulmonary artery Right coronary artery Area of occlusion Significant atherosclerosis Blood clot
Area of muscle damage
Complete
blockage
FiguRe 5-15 c The road to a heart attack. Injury to an artery wall begins the process. This is followed by a progressive buildup of plaque in the artery walls. The heart attack represents the terminal phase of the process. Blockage of the left coronary artery by a blood clot is evident.
The heart muscle served by the portion of the coronary artery beyond the point of blockage lacks oxygen and nutrients and is damaged and may die. This damage can lead to a significant drop in heart function and often total heart failure. m At the first sign of a possible heart at-
tack, the person should first thoroughly chew an aspirin and then call 911.
c Healthy People 2010 has set a goal of reducing death from coronary heart disease by 30%, compared with today’s incidence.
homocysteine An amino acid not used in protein synthesis, but instead arises during metabolism of the amino acid methionine.
Homocysteine is likely toxic to many cells, such as those lining the blood vessels.
likely to remain intact and then lead to a blockage, cutting off or diminishing the supply of blood to the heart (via the coronary arteries) or brain (via the carotid arteries).
More than 95% of heart attacks are caused by such blood clots. Heart attacks generally are caused by total blockage of the coronary arteries due to a blood clot forming in an area of the artery already partially blocked by plaque. Disruption of the plaque may even lead to eventual clot formation.
Atherosclerosis probably first develops to repair damage in a vessel lining. The damage that starts this process can be caused by smoking, diabetes, hyperten-
sion, homocysteine (likely, but not a major factor), LDL, and viral and bacterial infection. Ongoing inflammation in the blood vessel is also suspected to cause blood vessel damage. (A laboratory test for c-reactive protein in the blood is used to assess if inflammation is present.) Atherosclerosis can be seen in arteries throughout the body. The damage develops especially at points where an artery branches into two smaller vessels. A great deal of stress is placed on the vessel walls at these points due to changes in blood flow.
Once blood vessel damage has occurred, the next step in the development
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of atherosclerosis is the progression phase.
This step is characterized by deposition of plaque at the site of initial damage. The rate of plaque buildup during the progression phase is directly related to the amount of LDL in the blood. The form of LDL that contributes to atherosclerosis is oxidized
LDL. This form is preferentially taken up by scavenger cells in the arterial wall.
Nutrients and phytochemicals that have antioxidant properties may reduce LDL oxidation. Fruits and vegetables are particularly rich in these compounds. Eating fruits and vegetables regularly is one positive step we can make to reduce plaque buildup and slow the progression of cardiovascular disease. Some fruits and vegetables particularly helpful in this regard include legumes (beans), nuts, dried plums (prunes), raisins, berries, plums, apples, cherries, oranges, grapes, spinach, broccoli, red bell peppers, potatoes, and onions. Tea and coffee are also sources of antioxidants.
As plaque accumulates, arteries harden, narrow, and lose their elasticity. Affected arteries become further damaged as blood pumps through them and pressure increases. Finally, in the terminal phase, a clot or spasm in a plaque-clogged artery leads to a heart attack.
Factors that typically bring on a heart attack in a person already at risk include dehydration; acute emotional stress (such as firing an employee); strenuous physical activity when not otherwise physically fit (shoveling snow, for example); waking during the night or getting up in the morning (linked to an abrupt increase in stress); and consuming large, high-fat meals (increases blood clotting).
Risk FaCtoRs FoR
CaRdiovasCuLaR
disease
Many of us are free of the risk factors that contribute to rapid development of atherosclerosis. If so, the advice of health experts is to consume a balanced diet, perform regular physical activity, have a complete fasting lipoprotein analysis performed at age 20 or beyond, and reevaluate risk factors every 5 years.
For most people, however, the most likely risk factors are:
• Total blood cholesterol over 200 milligrams per 100 milliliters of blood
(mg/dl; dl is short for deciliter or 100 milliliters). Risk is especially high when total cholesterol is at or over 240 mg/dl and LDL-cholesterol readings are over
130 to 160 mg/dl. (The terms LDLcholesterol and HDL-cholesterol are used when expressing the blood concentration because it is the cholesterol content of these lipoproteins that is measured.) • smoking. Smoking is the main cause of about 20% of cardiovascular disease deaths and generally negates the female advantage of later occurrence of the disease. A combination of smoking and oral contraceptive use increases the risk of cardiovascular disease in women even more. Smoking greatly increases the expression of a person’s genetically linked risk for cardiovascular disease, even if one’s blood lipids are low.
Smoking also makes blood more likely to clot. Even secondhand smoke has been implicated as a risk factor.
• Hypertension. systolic blood pressure over 139 (millimeters of mercury) and diastolic blood pressure over 89 indicate hypertension. Healthy blood pressure values are less than 120 and 80, respectively. (Treatment of hypertension is reviewed in Chapter 9.)
• Diabetes. Diabetes virtually guarantees development of cardiovascular disease, and so puts a person with diabetes in the high-risk group. Insulin increases
oxidize In the most basic sense, the loss of an electron or gain of an oxygen by a chemical substance. This change typically alters the shape and/or function of the substance.
antioxidant Generally a compound that stops the damaging effects of reactive substances seeking an electron (i.e., oxidizing agents). This prevents breakdown (oxidizing) of substances in foods or the body, particularly lipids.
systolic blood pressure The pressure in the arterial blood vessels associated with the pumping of blood from the heart.
diastolic blood pressure The pressure in the arterial blood vessels when the heart is between beats.
m Smoking is one of the four major risk factors
for developing cardiovascular disease.
Making deCisions
Antioxidant Supplements and Cardiovascular Disease
Are large doses of antioxidant vitamin supplements a reliable way to reduce LDL oxidation, and thereby prevent cardiovascular disease? Controversy about such dietary supplementation exists among the experts, as will be detailed in Chapter 8. The American Heart Association does not support use of antioxidant supplements (such as vitamin E) to reduce cardiovascular disease risk.
This is because large-scale studies have shown no decrease in cardiovascular disease risk with use of antioxidant supplements. One study even showed a modest increase for heart failure in people with diabetes or otherwise at high risk for cardiovascular disease after taking antioxidant supplements. However, further trials of antioxidant supplementation ( e.g., 600 IU of vitamin E every other day) for prevention of heart disease in men are ongoing. With regard to women,
600 IU of natural-source vitamin E taken every other day provided no overall benefit for major cardiovascular events (or cancer). More research is warranted, however, as a decrease in sudden cardiac death was seen in a subset of older women in this study. Some experts suggest daily vitamin E supplementation (100 IU to 400 IU) may be helpful for preventing cardiovascular disease, while other experts recommend against the practice. One thing is certain: any supplementation of vitamin E should be taken only under the guidance of a physician, because of interactions with vitamin K and anticlotting medications (and possibly high-dose aspirin use).
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c For more information on cardiovascular disease, see the website of the American
Heart Association at www.americanheart. org or the heart disease section of Healthfinder at www.healthfinder.gov/tours/heart. htm. This is a site created by the U.S. government for consumers. In addition, visit the website www.nhlbi.nih.gov/.
c Healthy People 2010 has set a goal of reducing total blood cholesterol among adults from the average of 206 mg/dl to 199 mg/dl, as well as reducing the percentage of adults with high blood cholesterol from 21% to 17%.
c Cardiovascular Disease Risk Factors
•
Total blood cholesterol >200 mg/dl
•
Smoking
•
Hypertension
•
Diabetes
•
HDL cholesterol < 40 mg/dl
•
Age: Men >45 yr; Women > 55 yr
•
Family history of cardiovascular disease
•
Blood triglycerides > 200 mg/dl
•
Obesity
•
Inactivity
c Two approaches have been shown to cause reversal of atherosclerosis in the body.
One employs a vegan diet and other lifestyle changes that are part of the Dr. Dean Ornish program. The other employs aggressive LDL lowering with medications.
c As noted earlier, aspirin in small doses reduces blood clotting. It is often used under a physician’s guidance to treat people at risk for heart attack or stroke, especially if one has already occurred. About 80 to 160 milligrams per day is needed for such benefits.
Individuals who may especially benefit from aspirin therapy are men over 40, smokers, postmenopausal women, and people with diabetes, hypertension, or a family history of cardiovascular disease.
cholesterol synthesis in the liver, in turn increasing LDL in the bloodstream. This disease negates any female advantage.
Together, the previous four risk factors explain most cases of cardiovascular disease.
Other risk factors to consider:
• HDL-cholesterol under 40 mg/dl, especially when the ratio of total cholesterol to HDL-cholesterol is greater than 4:1
(3.5:1 or less is optimal). Women often have high values for HDL-cholesterol, and therefore it is important for this to be measured in women to establish cardiovascular disease risk. A value of
60 mg/dl or more is especially protective. Exercising for at least 45 minutes four times a week can increase HDL by about 5 mg/dl. Losing excess weight
(especially around the waist) and avoiding smoking and overeating also help maintain or raise HDL, as does moderate alcohol consumption.
• Age. Men over 45 years and women over 55 years.
• Family history of cardiovascular disease, especially before age 50.
• Blood triglycerides 200 mg/dl or greater in the fasting state (less than 100 mg/dl is optimal).
• Obesity (especially fat accumulation in the waist). Typical weight gain seen in adults is a chief contributor to the increase in LDL seen with aging. Obesity also typically leads to insulin resistance, creating a diabetes-like state, and ultimately the disease itself. It also increases overall inflamation throughout the body.
• Inactivity. Exercise conditions the arteries to adapt to physical stress. Regular exercise also improves insulin action in the body. The corresponding reduction in insulin output leads to a reduction in lipoprotein synthesis in the liver. Both regular aerobic exercise and resistance exercise are recommended. A person with existing cardiovascular disease should seek physician approval before starting such a program, as should older adults.
Researchers are trying to unravel and quantify numerous other factors that may be linked to premature cardiovascular disease, such as the connection between inad-
equate intake of vitamin B-6, folate, and vitamin B-12, which can lead to increased homocysteine in the blood. As noted, homocysteine may damage the cells lining the blood vessels, in turn promoting atherosclerosis. Studies show that supplementation with folic acid and B vitamins does not reduce risk of recurrent cardiovascular disease in patients that had previous heart attacks or had existing vascular diseases.
The term risk factor is not equivalent to cause of disease; nevertheless, the more of these risk factors one has, the greater the chances of ultimately developing cardiovascular disease. A good example is the metabolic syndrome (also called Syndrome
X) discussed in Chapter 4. A person with the metabolic syndrome would have abdominal obesity, high blood triglycerides, low HDL-cholesterol, hypertension, poor blood glucose regulation (i.e., high fasting blood glucose), and increased blood clotting. This profile raises the risk for cardiovascular disease considerably. On a positive note, cardiovascular disease is rare in populations who have low LDLcholesterol, normal blood pressure, and do not smoke or have diabetes. By minimizing these risk factors, as well as following the dietary recommendations of the
American Heart Association on page 188 and staying physically active, one will most likely reduce many of the other controllable risk factors listed. In other words, develop and follow a total lifestyle plan.
Medications may also be added to lower blood lipids, as discussed next. Finally, if a person has a family history of cardiovascular disease but the usual risk factors aren’t present, a rarer defect might be the cause. In this case, having a detailed physical examination for other potential causes is advised.
MediCaL inteRventions to LoWeR bLood Lipids
Some people need even more aggressive therapy added to their regimen of a diet and lifestyle overhaul to treat elevated blood lipids. The clearest indication for this more aggressive approach is in people who already have had a heart attack or have cardiovascular disease symptoms or diabetes. 194
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Medications are the cornerstone of this more aggressive therapy. The National
Cholesterol Education Program in the
United States has developed a formula based on age, total blood cholesterol,
HDL-cholesterol, smoking history, and blood pressure to determine who needs such medications. Check out this formula at http://hp2010.nhlbihin.net/atpiii/ calculator.asp?usertype=pub. This formula provides an estimate for having a heart attack in the next 10 years.
Medications work to lower LDL in one of two ways. Some reduce cholesterol synthesis in the liver. Such medicines are known as
“statins” (e.g., atorvistatin [Lipitor]). The cost of treatment with one of these drugs can be from $1,600 or more per year, depending on the dose needed. These medications lead to problems in some people, and so require physician monitoring, especially of liver function. Another group of medications binds bile acids or the cholesterol that is part of bile secreted into the small intestine. This binding leads to their elimination in the feces and so requires the liver to synthesize new bile acids and/or cholesterol. The liver removes
LDL from the blood to do this. Some of these medications taste gritty and therefore are not popular. The current therapeutic goal for people with (or at high risk for) cardiovascular disease (Table 5-9) is to drive LDL down to less than 70 mg/dl.
The statin drug simvastatin (Zocor) has been combined with another drug (ezetimibe) and is marketed as Vytorin, a drug that will treat the two sources of cholesterol, “food and family.” While the statin reduces the cholesterol made by the liver, the ezetimibe helps block the absorption of cholesterol from food.
A third group of drugs can be used to lower blood triglycerides by decreasing the triglyceride production of the liver. These include gemfibrozil (Lopid) and megadoses of the vitamin nicotinic acid. The use of nicotinic acid does result in pesky side effects, however, but these are typically manageable. Other Possible Medical Therapies for Cardiovascular Disease
FDA has approved two margarines that have positive effects on blood cholesterol levels—Benecol and Take Control. These
margarines contain plant stanols/sterols.
The plant stanols/sterols, also called phytosterols, work by reducing cholesterol absorption in the small intestine and lowering its return to the liver. The liver responds by taking up more cholesterol from the blood so it can continue to make bile acids. The studies done on the cholesterollowering effect of these margarines have found that 2 to 5 grams of plant stanols/ sterols per day reduces total blood cholesterol by 8% to 10% and LDL-cholesterol by
9% to 14% (similar to what is seen with some cholesterol-lowering drugs).
Benecol is made from plant stanols extracted from wood pulp. This product is sold as margarine and has been added to salad dressings. Take Control is made from plant sterols isolated from soybeans. The recommended amount for both is about
2 to 3 grams per day as part of at least two meals; this works out to about 1 tablespoon of Benecol or 2 tablespoons of Take
Control per day. Use would cost about
$1.00 per day, as these margarines are more expensive than regular margarines.
In people who have borderline high total blood cholesterol (between 200 and
239 mg/dl), these margarines can be helpful in avoiding future drug therapy. Plant stanols/sterols have been made available in pill form as well. Remember that plant sterols are naturally present in nuts in high concentrations. Wheat germ, sesame seeds, pistachios, and sunflower seeds are some of the richest sources.
The two most common surgical treatments for coronary artery blockage are percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass graft (CABG). PTCA involves the insertion of a balloon catheter into an artery. Once it is advanced to the area of the lesion, the balloon is expanded to crush the lesion.
This method works best when only one vessel is blocked, and it may be held open with metal mesh, called a stent. CABG involves the removal and use of a saphenous vein, a large vein in the leg, or use of a mammary artery. The relocated vein is sewn to the main heart vessel (aorta) and then used to bypass the blocked artery. The procedure can be performed on one or more blockages.
Benecol and Take Control are margarines
that are examples of “functional foods” because they contain added cholesterol-lowering plant stanols/sterols. tABLe 5-9 Sorting Out One’s Goals for Cardiovascular Disease Prevention/Treatment
Risk Class
This Is You If …
Very high
You have cardiovascular disease and other risk factors such as diabetes, obesity, smoking
Below 70
High
You have cardiovascular disease or diabetes or two or more risk factors for cardiovascular disease (e.g., smoking, hypertension)
Below 100
Your LDL (mg/dl) goal
Moderately high You have cardiovascular disease risks and an increased 10-year risk of developing a heart attack
Moderate
You have 2 or more risk factors and a marginal chance of developing cardiovascular disease in the next 10 years.
Low
You have few or no cardiovascular disease risk factors 100–130
Below 130
Below 160
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Case studY
Planning a Heart Healthy Diet
m Are there important food groups missing
from Jackie’s new diet plan?
Jackie is a 21-year-old health-conscious individual majoring in business. She recently learned that a diet high in saturated fat can contribute to high blood cholesterol and that exercise is beneficial for the heart. Jackie now takes a brisk 30-minute walk each morning before going to class, and she has started to cut as much fat out of her diet as she can, replacing it mostly with carbohydrates. A typical day for Jackie now begins with a bowl of Fruity
Pebbles with 1 cup of skim milk and ½ cup of apple juice. For lunch, she might pack a turkey sandwich on white bread with lettuce, tomato, and mustard; a small package of fat-free pretzels; and a handful of reduced-fat vanilla wafers. Dinner could be a large portion of pasta with some olive oil and garlic mixed in, and a small iceberg lettuce salad with lemon juice squeezed over it. Her snacks are usually baked chips, low-fat cookies, fat-free frozen yogurt, or fat-free pretzels. She drinks diet soft drinks throughout the day as her main beverage.
Answer the following questions, and check your response in Appendix A.
1. Has Jackie made the best diet changes with regard to lowering blood cholesterol and maintaining heart health?
2. Is there much fat left in Jackie’s new diet plan? Is it necessary for her to drastically lower her fat intake?
3. What types of fat should Jackie try to consume? Why are these types of fat the most desirable?
4. What types of foods has Jackie used to replace the fat in her diet?
5. What food groups are missing from her new diet plan? How many servings should she be including from these food groups? 6. Is Jackie’s new exercise routine appropriate? suMMaRY
1. Compared with carbohydrates and proteins, lipids are a group of relatively oxygen-poor compounds that do not dissolve in water. Saturated fatty acids contain no carbon-carbon double bonds, monounsaturated fatty acids contain one carbon-carbon double bond, and polyunsaturated fatty acids contain two or more carbon-carbon double bonds in the carbon chain.
2. In omega-3 polyunsaturated fatty acids, the first of the carbon-carbon double bonds is located three carbons from the methyl end of the carbon chain. In omega-6 polyunsaturated fatty acids, the first carbon-carbon double bond counting from the methyl end occurs at the sixth carbon. Both omega-3 and omega-6 fatty acids are essential fatty acids; these must be included in the diet to maintain health.
3. Triglycerides are formed from a glycerol backbone with three fatty acids. Triglycerides rich in long-chain saturated fatty acids tend to be solid at room tem-
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perature, whereas those rich in monounsaturated and polyunsaturated fatty acids are liquid at room temperature.
Triglyceride is the major form of fat in both food and the body. It allows for efficient energy storage, protects certain organs, transports fat-soluble vitamins, and helps insulate the body.
4. Phospholipids are derivatives of triglycerides in which one or two of the fatty acids are replaced by phosphoruscontaining compounds. Phospholipids are important parts of cell membranes, and some act as efficient emulsifiers.
5. Cholesterol forms vital biological compounds, such as hormones, components of cell membranes, and bile acids. Cells in the body make cholesterol whether we eat it or not. It is not a necessary part of an adult’s diet.
6. Body cells can synthesize hormone-like compounds from both omega-3 and omega-6 fatty acids. The compounds produced from omega-3 fatty acids tend to reduce blood clotting, blood
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pressure, and inflammatory responses in the body. Those produced from omega-6 fatty acids tend to increase blood clotting.
7. Foods rich in fat include salad oils, butter, margarine, and mayonnaise.
Nuts, bologna, avocados, and bacon are also high in fat, as are peanut butter and cheddar cheese. Steak and hamburger are moderate in fat content, as is whole milk. Many grain products, and fruits and vegetables in general, are low in fat.
8. Fats and oils have several functions as components of foods. Fats add flavor and texture to foods and provide some satiety after meals. Some phospholipids are used in foods as emulsifiers, which suspend fat in water. When fatty acids break down, food becomes rancid, resulting in a foul odor and unpleasant flavor.
9. Hydrogenation is the process of converting carbon-carbon double bonds into single bonds by adding hydrogen at the point of unsaturation. The
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partial-hydrogenation of fatty acids in vegetable oils changes the oils to semisolid fats and helps in food formulation and reduces rancidity. Hydrogenation also increases trans fatty acid content. High amounts of trans fat in the diet are discouraged, as these increase LDL and reduce HDL.
10. Fat digestion takes place primarily in the small intestine. Lipase enzyme released from the pancreas digests long-chain triglycerides into smaller breakdown products—namely, monoglycerides (glycerol backbones with single fatty acids attached) and fatty acids. The breakdown products are then taken up by the absorptive cells of the small intestine. These products are mostly remade into triglycerides and eventually enter the lymphatic system, in turn passing into the bloodstream.
11. Lipids are carried in the bloodstream by various lipoproteins, which consist of a central triglyceride core encased in a shell of protein, cholesterol, and phospholipid. Chylomicrons are released from intestinal cells and carry lipids arising from dietary intake. Very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) carry lipids both taken up by and synthesized in the liver.
High-density lipoprotein (HDL) picks up cholesterol from cells and facilitates its transport back to the liver.
12. There is currently no RDA for fat for adults. Plant oils should contribute about 5% of total calories to achieve the
Adequate Intakes proposed for essential fatty acids (linoleic acid and alphalinolenic acid). Fatty fish are a rich source of omega-3 fatty acids and should be consumed at least twice a week.
13. Many health agencies and scientific groups suggest a fat intake of no more than 30% to 35% of total calories.
Some health experts advocate an even further reduction to 20% of calorie intake for some people to maintain a normal LDL value, but such a diet requires professional guidance. Medications such as “statins” may be added also to lower LDL. If fat intake exceeds
30% of total calories, the diet should emphasize monounsaturated fat. The typical North American diet contains about 33% of total calories as fat.
14. In the blood, elevated amounts of LDL and low amounts of HDL are strong predictors of the risk for cardiovascular disease. Additional risk factors for the disease are smoking, hypertension, diabetes, obesity, and inactivity.
studY Questions
1. Describe the chemical structures of saturated and polyunsaturated fatty acids and their different effects in both food and the human body.
2. Relate the need for omega-3 fatty acids in the diet to the recommendation to consume fatty fish at least twice a week.
3. Describe the structures, origins, and roles of the four major blood lipoproteins.
4. What are the recommendations from various health-care organizations
regarding fat intake? What does this mean in terms of food choices?
story with respect to cardiovascular disease risk?
5. What are two important attributes of fat in food? How are these different from the general functions of lipids in the human body?
8. List the four main risk factors for the development of cardiovascular disease.
6. Describe the significance of and possible uses for reduced-fat foods.
7. Does the total cholesterol concentration in the bloodstream tell the whole
9. What three lifestyle factors decrease the risk of cardiovascular disease development? 10. When are medications most needed in cardiovascular disease therapy, and how in general do the various classes of medications operate to reduce risk?
CheCk YouR knoWLedge
Answers to the following multiple choice questions are in Appendix A.
1. Margarine usually is made by a process called _____________, in which hydrogen atoms are added to carbon-carbon double bonds in the polyunsaturated fatty acids found in vegetable oils.
a. saturation
b. esterification
c. isomerization
d. hydrogenation
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2. Essential fatty acids that cause a decrease in blood clotting are
a. omega-3.
b. omega-6.
c. omega-9.
d. prostacyclins.
c. an important part of human cell membranes and necessary to make some hormones.
d. All of the above.
4. Which of the following groups of foods would be important sources of saturated fatty acids?
a. olive oil, peanut oil, canola oil
b. palm oil, palm kernel oil, coconut oil
c. safflower oil, corn oil, soybean oil
d. All of the above.
3. Cholesterol is
a. a dietary essential; the human body cannot synthesize it.
b. found in foods of plant origin. Au: Wardlaw
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198 Contemporary Nutrition www.mhhe.com/wardlawcont7
5. Lipoproteins are important for
a. transport of fats in the blood and lymphatic system.
b. synthesis of triglycerides.
c. synthesis of adipose tissue.
d. enzyme production.
6. Which of the following foods is the best source of omega-3 fatty acids?
a. fatty fish
b. peanut butter and jelly
c. lard and shortenings
d. beef and other red meats
7. Immediately after a meal, newly digested and absorbed dietary fats appear in the lymph, and then blood, as part of which of the following?
a. LDL
b. HDL
c. chylomicrons
d. cholesterol
8. High blood concentrations of _______ decrease the risk for cardiovascular disease. a. low-density lipoproteins
b. chylomicrons
c. high-density lipoproteins
d. cholesterol
9. Phospholipids such as lecithin are used extensively in food preparation because they
a. provide the agreeable feel of fat melting on the tongue.
b. are excellent emulsifiers.
c. provide important textural features.
d. impart delicate flavors.
10. The main form of lipid found in the food we eat is
a. cholesterol.
b. phospholipids.
c. triglycerides.
d. plant sterols
FuRtheR Readings
1. ADA Reports: Position of the American Dietetic Association: Fat replacers. Journal of the
American Dietetic Association 105:266, 2005.
The majority of fat replacers, when used in moderation by adults, can be safe and useful adjuncts to lowering the fat content of foods and may play a role in decreasing total calorie and fat intake. Still, consumers should not be led to believe that fat and calorie-reduced products can be consumed in unlimited amounts
2. Bønaa KH and others: Homocysteine lowering and cardiovascular events after acute myocardial infarction. The New England Journal of Medicine 354:1578, 2006.
The risk of recurrent cardiovascular disease after acute myocardial infarction was not lowered by treatment with folic acid, vitamin B-12, or vitamin B-6. A trend toward an increased risk suggested a harmful effect of the combined B vitamin treatment. Treatment with these B vitamins is therefore not recommended.
3. Diercks DB and others: The obesity paradox in non–ST-segment elevation acute coronary syndromes. American Heart Journal 152:140,
2006.
This study discovered that although obese individuals are at increased risk of suffering a heart attack, their outcomes are better because they are treated more aggressively. Overweight and obese patients were more likely than normalweight patients to receive medications and to have invasive cardiac procedures.
4. Fisler JS, Warden CH: Dietary fat and genotype: Toward individualized prescriptions for lifestyle changes. American Journal of
Clinical Nutrition 81:1255, 2005.
There is great variation in the effect of dietary fat on blood lipids. At least part of that variation is due to genetics. This article discusses the challenges involved in identifying the interactive
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effects of nutrients and genes in the area of lipids, cholesterol, and coronary artery disease.
5. Fletcher B and others: Managing abnormal blood lipids: A collaborative approach. Circulation 112:3184, 2005.
The treatment of abnormal blood lipids is complex and therefore requires involvement of various types of healthcare professionals, including dietitians. This paper describes a model multidisciplinary case management approach for patients with abnormal blood lipids. The model includes both primary and secondary prevention across the lifespan and incorporates nutritional and exercise intervention.
6. Grundy SM and others: Diagnosis and management of the metabolic syndrome. Circulation 112: 1350, 2005.
The constellation of metabolic risk factors known as metabolic syndrome consists of many factors, including elevated blood triglycerides, low HDL cholesterol concentrations, elevated blood pressure, elevated blood glucose, increased blood clotting, and an inflammatory state in the body. The most important of these underlying risk factors are abdominal obesity and insulin resistance. Other associated conditions include physical inactivity, aging, hormonal imbalance, and genetic or ethnic predisposition.
7. Hansson GK: Inflammation, atherosclerosis, and coronary artery disease. The New England Journal of Medicine 352:1685, 2005.
Excellent review of the role of inflammation in causing cardiovascular disease. Identifying atherosclerosis as an inflammatory disease offers new opportunities for the prevention and treatment of coronary artery disease.
8. He KA, Daviglus ML: A few more thoughts about fish and fish oil. Journal of the American Dietetic Association 105:428, 2005.
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Fish consumption has been shown to have beneficial effects on cardiovascular disease.
Consuming whole fish is thought to have greater benefits and be safer than using fish oil supplements. Fish is not only a rich source of omega-3 fatty acids, but is also a valuable source of protein and trace elements that may also provide protective effects for the cardiovascular system.
Broiled or baked fish is recommended rather than fried fish because frying may decrease the ratio of omega-3 to omega-6 fatty acids and may produce trans fatty acids and oxidized lipid products that may increase cardiovascular disease risk.
9. Kuller LH: Nutrition, lipids, and cardiovascular disease. Nutrition Reviews 64:S15, 2006.
The development of coronary heart disease is discussed as an epidemic due to increased consumption of saturated fat and cholesterol, low intakes of polyunsaturated fat, and obesity. The risk of the disease is increased by hypertension, smoking, and diabetes. The careful monitoring and prevention of this disease beginning in young adults is important but expensive.
10. Lau VWY and others: Plant sterols are efficacious in lowering plasma LDL and non-HDL cholesterol in hypercholesterolemic type 2 diabetic and nondiabetic persons. American
Journal of Clinical Nutrition 81:1351, 2005.
Incorporation of plant sterols into a lowsaturated-fat and low-cholesterol diet for persons at increased risk of CVD mortality could have a positive effect on reducing the mortality rate, including in those people with type 2 diabetes.
11. Lee I and others: Vitamin E in the primary prevention of cardiovascular disease and cancer: The women’s health study: A randomized controlled trial. Journal of the
American Medical Association 294:56, 2005.
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The data from this large trial indicated that
600 IU of natural-source vitamin E taken every other day provided no overall benefit for major cardiovascular events or cancer. These data do not support recommending vitamin E supplementation for cardiovascular disease or cancer prevention among healthy women, but more research is warranted, as a decrease in sudden cardiac death was seen in a subset of older women in this study.
trial. Journal of the American Medical Association 293:1338, 2005.
No significant differences were noted between the vitamin E supplementation group and the control group in the incidence of cancer or deaths related to cancer. There also were no differences in the main composite of cardiovascular outcomes including death or rehospitalization. A significant increased risk in heart failure and related complications occurred in the vitamin E group.
12. Lewis NM and others: The walnut: A nutritional nut case. Today’s Dietitian 6(8):36, 2004.
Compared to other nuts and seeds, walnuts are one of the richest sources of alpha-linolenic acid
(2.6 grams per 1-ounce serving or 14 walnut halves). The DRI for alpha-linolenic acid is
1.6 grams/day for men and 1.1 grams/day for women. In addition, walnuts are a rich source of plant sterols known to inhibit intestinal absorption of cholesterol.
15. Lukaczer D and others: Effect of a low glycemic index diet with soy protein and phytosterols on CVD risk factors in postmenopausal women. Nutrition 22(2):104, 2006.
Women in this 12-week study showed decreases in total cholesterol, LDL cholesterol, and triglycerides after consuming a low-glycemic index diet with a functional food delivering 30 grams of soy protein and 4 grams of phytosterols per day.
13. Lichtenstein AH and others: Diet and lifestyle recommendations revision 2006. A scientific statement from the American Heart
Association Nutrition Committee. Circulation 114:82, 2006.
The American Heart Association presents recommendations designed to reduce the risk of cardiovascular disease risk reduction in the general population. Specific goals are presented and include consuming an overall healthy diet; to aim for a healthy body weight, recommended levels of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides; normal blood pressure; normal blood glucose level; being physically active; and avoiding the use of and exposure to tobacco products. 14. Lonn E and others: Effects of long-term vitamin E supplementation on cardiovascular events and cancer: A randomized controlled
16. Meisinger C and others: Plasma oxidized low-density lipoprotein, a strong predictor for acute coronary heart disease events in apparently healthy, middle-aged men from the general population. Circulation
112:651, 2005.
Elevated concentrations of oxidized low-density lipoprotein are predictive of future coronary heart disease events in apparently healthy men.
Thus, oxidized LDL may represent a promising risk marker for clinical coronary heart disease complications and should be evaluated in further studies. 17. Ohr LM: The (heart) beat goes on. Food Technology 60(6):87, 2006.
Research and ingredient innovations in the food industry related to the heart-health field are described. Ingredients that have the advantage of a heart-health claim include omega-3 fatty acids and plant sterols. Also discussed are
dietary fiber and grains, soy, antioxidants, and nuts. 18. Phillips KM and others: Phytosterol composition of nuts and seeds commonly consumed in the United States. Journal of Agriculture and Food Chemistry 53:9436, 2005.
This study set out to find the nuts and seeds that could provide the most heart-protective benefits.
Of the 27 nuts and seeds analyzed, wheat germ and sesame seeds had the greatest concentration of phytosterols and Brazil nuts and walnuts ranked the lowest. Pistachio and sunflower seeds were the richest sources of phytosterols for products typically consumed as snack foods. 19. Reese MATB: Beyond the headlines: The lowdown on low-fat diets. Today’s Dietitian
8(6):32, 2006.
Results of the 8-year Women’s Health Initiative study showed that a reduction in dietary fat had almost no effect on the incidence of heart attacks and strokes or breast and colon cancer. This article explains some of the study characteristics that most likely contributed to the unexpected results.
Most important to consider is that the subjects were all postmenopausal women who had all been on a high-fat diet. The results suggest that starting a low-fat diet at 50 to 79 years may be too late to see extraordinary benefits. 20. Wang C and others: n-3 fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies:
A systematic review. American Journal of Clinical
Nutrition 84:5, 2006.
This review of previous studies found that consumption of omega-3 fatty acids from fish or fish oil but not alpha-linolenic acid, significantly reduced all-cause mortality, myocardial infarction, cardiac and sudden death, or stroke.
c Check out the Contemporary Nutrition ARIS site
www.mhhe.com/wardlawcont7 for quizzes, flash cards, other activities, and weblinks designed to further help you learn about what you eat and why.
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Rate YouR pLate
i. are You eating a diet that includes Many saturated Fat and Trans Fatty acid sources?
Instructions: In each row of the following list, circle your typical food selection from either column A or B.
Column A
Column B
Bacon and eggs
or
R eady-to-eat whole-grain breakfast cereal
Doughnut or sweet roll
or
W hole-wheat roll, bagel, or bread
Breakfast sausage
or
F ruit
Whole milk
or
R educed-fat, low-fat, or fat-free milk
Cheeseburger
or
T urkey sandwich, no cheese
French fries
or
P lain baked potato with salsa
Ground chuck
or
G round round
Soup with cream base
or
S oup with broth base
Macaroni and cheese
or
M acaroni with marinara sauce
Cream/fruit pie
or
G raham crackers
Ice cream
or
F rozen yogurt, sherbet, or reduced-fat ice cream
Butter or stick margarine
or
V egetable oils or soft margarine in a tub
Interpretation
The foods listed in column A tend to be high in saturated fat, trans fatty acids, cholesterol, and total fat. Those in column B generally are low in these dietary components. If you want to help reduce your risk of cardiovascular disease, choose more foods from column B and fewer from column A.
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ii. Applying the nutrition Facts Label to Your Daily Food Choices
Imagine that you are at the supermarket looking for a quick snack to help you keep your energy up during afternoons. In the snack section, you settle on two choices (see labels a and b). Evaluate the products using the table on the left.
ompare the nutrients in each product by comC pleting this list. For each serving, which product is lower in each of the following? Calories
(a)
Calories from Fat
(a)
(b) (b)
no difference
(a)
(b)
no difference
Saturated Fat
(a)
(b)
no difference
Trans Fat
(a)
(b)
no difference
Cholesterol
(a)
(b)
no difference
Sodium
(a)
(b)
no difference
Total Carbohydrates
(a)
(b)
no difference
Dietary Fiber
(a)
(b)
no difference
Sugars
(a)
(b)
no difference
Protein
(a)
(b)
no difference
(a)
(b)
no difference
Which package has more servings per container? (a) (b) no difference Which of the two brands would you choose? (a) (b) neither What information on the Nutrition Facts labels contributed to your decision?
Nutrition Facts
Serving Size: 2 cookies (38g)
Servings Per Container: about 12
Amount Per Serving
2 bars
Calories 180
Calories from Fat 50
no difference
Total Fat
Iron
Nutrition Facts
Serving Size: 2 bars (42g)
Servings Per Container: 6
Amount Per Serving
% Daily Value*
Total Fat 6g
Saturated Fat 0.5g
Trans fat 0g
Cholesterol 0mg
Sodium 160mg
Total Carbohydrates 29g
Dietary Fiber 2g
Sugars 11g
Protein 4g
Iron
9%
3%
**
0%
7%
10%
8%
6%
Not a significant source of Vitamin A, Vitamin C, and calcium. ** Intake of trans fat should be as low as possible.
* Daily values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs:
** Intake should be as low as possible.
Total Fat
Saturated Fat
Cholesterol
Sodium
Total Carbohydrates
Dietary Fiber
Calories
2,000
2,500
Less than
65g
80g
Less than
20g
25g
Less than
300mg
300mg
Less than
2,400mg 2,400mg
300g
375g
25g
30g
INGREDIENTS: WHOLE GRAIN ROLLED OATS, SUGAR,
CANOLA OIL, CRISP RICE WITH SOY PROTEIN (RICE
FLOUR, SOY PROTEIN CONCENTRATE, SUGAR, MALT,
SALT), HONEY, BROWN SUGAR SYRUP, HIGH
FRUCTOSE CORN SYRUP, SALT, SOY LECITHIN,
BAKING SODA, NATURAL FLAVOR, PEANUT FLOUR,
ALMOND FLOUR, HAZELNUT FLOUR, WALNUT FLOUR,
PECAN FLOUR.
Calories 180
Calories from Fat 70
% Daily Value*
Total Fat 7g
Saturated Fat 2g
Trans fat 2g
Cholesterol 0mg
Sodium 100mg
Total Carbohydrate 26g
Dietary Fiber 1g
Sugars 12g
Protein 2g
Vitamin A 0%
Calcium 0%
•
11%
10%
**
0%
4%
9%
4%
Vitamin C 0%
Iron
2%
•
** Intake of trans fat should be as low as possible. * Daily values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs:
** Intake should be as low as possible.
Total Fat
Saturated Fat
Cholesterol
Sodium
Total Carbohydrates
Calories
2,000
2,500
Less than
65g
80g
Less than
20g
25g
Less than
300mg
300mg
Less than
2,400mg 2,400mg
Dietary Fiber
300g
375g
25g
30g
Calories per gram: • Fat 9 • Carbohydrate 4
• Protein 4
INGREDIENTS: ENRICHED FLOUR (WHEAT FLOUR,
NIACIN, REDUCED IRON, THIAMINE MONONITRATE,
RIBOFLAVIN, FOLIC ACID), SUGAR, VEGETABLE OIL
SHORTENING (PARTIALLY HYDROGENATED SOYBEAN,
COCONUT, COTTONSEED, CORN AND/OR SAFFLOWER
AND/OR CANOLA OIL), CORN SYRUP, HIGH FRUCTOSE
CORN SYRUP, WHEY (A MILK INGREDIENT), CORN
STARCH, SALT, SKIM MILK, LEAVENING (BAKING SODA,
AMMONIUM BICARBONATE), ARTIFICIAL FLAVOR,
SOYBEAN LECITHIN, COLOR (CONTAINING FD&C
YELLOW #5 LAKE).
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