Physiology : Eating
• Digestive system – stores and uses energy – Absorption & Fasting – See diagrams • Describe how brain & rest of body controls eating (100%) or role of Hypothalamus
Metabolism – Absorption & Fasting
When we eat we must obtain adequate amounts of carbohydrates, fats, amino acids, vitamins and minerals to construct and maintain our organs, to obtain energy for muscular movement and for keeping our bodies warm. Most of the molecules we eat get ‘burned’ to provide the energy needed. Metabolism consists of two phases; Absorption and Fasting.
Fuel comes from the digestive tract and its presence there is due to eating ….. Digestion Process Once a meal is absorbed it is often empty until the next meal so a reservoir of fuel is needed. In fact we have two; a short term reservoir which stores carbohydrates and a long term reservoir which stores fat.
In the absorption phase …… In the fasting phase ……. See diagrams
When the body is subject to prolonged fasting, the liver converts fatty acids into ketones which provide energy that can be used by the whole body. Energy can also be extracted from muscle proteins which are broken down into amino acids and converted to glucose. Muscles will waste away but the brain is insured of an energy supply until there is nothing left.
Body : (HEAD – STOMACH – DUODENUM)
Scientists have always had the feeling that hunger originated in the stomach however the systematic search for factors that control eating behaviour really only began in the early 20th century.
• Cannon (1912) : persuaded human subjects to swallow a balloon, attached to a recording device that was inflated when it reached the stomach. The subject couldn’t see the inflation however they were asked to press a key when hunger pangs were felt. The results of the experiment showed that stomach contraction and hunger pangs were roughly coincided giving the suggestion that stomach contractions cause the subject to experience hunger.
• HOWEVER, it is known that people who have had their stomach removed, can still experience hunger pangs. Therefore Cannon’s assertion that the stomach is involved is not necessarily true. He did however, place emphasis on the role the body plays in eating behaviour and recognised it as at least as important as the role of the brain itself.
Many stimuli, environmental and physiological, can initiate a meal. Sight, smell and taste of food play an important role in controlling eating behaviour.
• Rats eat more than 3 times their normal amount if different flavours are added to their food so exposure to different tastes stimulates eating.
• Research has shown that taste is also influenced by how hungry we are. Sweet-tasting foods are judged to be pleasant when a subject is hungry and unpleasant by the same subject when they are satiated.
• Taste is also determined in large part by smell and smell influences appetite.
• The act of tasting and swallowing contributes to the feeling of fullness.
However, not all receptors for regulating food intake are in the head.
• Epstein & Teitelbaum (1962) : bypassed the head and delivered a liquid diet directly through a tube into a rat’s stomach every time the rat pressed a lever. The rates regulated their food intake and weight within normal limits even though they couldn’t see, smell or taste their food. The same intragastric procedure was later used on humans with the same.
• Davis & Campbell (1973) : pursued the suggestion that the stomach controls the size of a meal eaten by depriving rates of food for 4 hours and then allowing them to eat their fill of sweet milk. Half the milk ingested was removed from the stomach by suction. Within minutes the rats began to eat again and didn’t stop until they had replaced almost exactly the same amount of milk that had been removed.
• Davis & Campbell (1973) : went on to suggest that receptors in the stomach are sensitive to distension or stretch...
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