Effects of Caffeine on Endurance

What is the Effect of Caffeine Consumption on Endurance among Elite Runners Under the Age of 40 in Developed Countries?

Abstract
Running and other endurance exercises are restricted by various natural causes of fatigue. (Callahan, 2011) Caffeine, a white crystalline drug, is often used by athletes as a stimulant during exercise to minimize the effects of fatigue. The purpose of this literature review is to determine if there is a causal relationship between caffeine consumption and endurance among elite runners under the age of 40 in developed countries. Additionally, the review will analyze similar conditions for caffeine use among untrained athletes and habitual caffeine users. In particular, research by Graham and Spriet (1991) suggests high doses of caffeine intake (9 mg/1 kg of body weight) causes an increase of exercise time until exhaustion. A study by Wiles et al. (1992) provides evidence that a small dose of caffeine (3 mg/1 kg of body weight) also causes enhanced oxygen uptake and running performance in high intensity exercise. In the future, more experiments should be performed to specifically test the effect of caffeine consumption on endurance levels while other factors are controlled. Furthermore, the experiments should utilize more subjects within their studies. This will increase internal validity and make the results capable of being generalized to other runners of a similar status.

What is the Effect of Caffeine Consumption on Endurance among Elite Runners Under the Age of 40 in Developed Countries?
College athletes and other elite runners in distance events including the 10k, half marathon, or full marathon in developed countries are interested in whether caffeine consumption prior to prolonged exercise can affect endurance levels. College athletes commonly use caffeine in various forms as a stimulant. The National Collegiate Athletic Association (NCAA) allows this within limitations. However, when urine concentrations exceed 15 mcg/mL, this privilege is rescinded. It takes the average person eight cups of coffee with 100 mg of caffeine present per cup to reach this concentration. (WebMD, 2009) Interestingly enough, the International Olympic Committee (IOC) had caffeine alongside drugs such as steroids and cocaine as a banned substance. (Anderson) However, according to the World Anti-Doping Code (WADA), in 2012 caffeine was not on this list. (World Anti-Doping Agency, 2011) It is known that VO2 max levels, carbohydrate and fat stores, muscle fibers, the lactate threshold, and endurance training affect endurance (Davies) The purpose of this literature review is to determine if there is a causal relationship between caffeine consumption and endurance among elite runners under the age of 40 in developed countries such as the United States and the United Kingdom. In order to develop an in-depth understanding of this question, research by Graham & Spriet (1991) and Miles et al. (1992) will be critically reviewed.
Caffeine Consumption
Caffeine, the common name for 1, 3, 7-trimethylxanthine, in its purest form is a white crystalline xanthine alkaloid that is considered a stimulant drug. (Kovacs, 2009) It is consumed by ninety percent of North American adults, making it the most widely consumed psychoactive substance. (Kovacs, 2009) The bitter white powder is most often used to provide a distinctive taste in almost all soft drinks. (Kovacs, 2009) Also, caffeine is used to improve alertness, moderate asthma and ADHD, lower blood pressure, and alleviate headaches. Caffeine can be found in caffeine creams for skin treatments and among athletes, as well. (WebMD, 2009) Americans intake caffeine an average of 280 mg/day, considered a moderate amount by medical professionals. (Kovacs, 2009) However, caffeine is considered “likely safe” for adults when used appropriately. (WebMD, 2009) The only time that caffeine consumption is considered excessive or “heavy” is in excess of 6,000 mg/day. (Kovacs, 2009) Humans most often consume this drug through the coffee plant, tea leaves, and kola nut in foods or drinks with these products. These products include coffee, tea, soft drinks, energy drinks, and chocolate. (Kovacs, 2009) Coffee, however, has the highest caffeine content of products containing caffeine with 135 mg for every 8 ounce cup. Energy drinks provide similar concentration levels, roughly 80 mg to 130 mg for an 8 ounce glass and soft drinks contain the next highest caffeine concentration ranging from 25 mg to 72 mg. (Kovacs, 2009) Upon ingestion, caffeine is fully absorbed in the body within 30 to 45 minutes and has lasting effects for up to 3 hours. (Kovacs, 2009) Caffeine stimulates the central nervous system, heart, muscles, and blood pressure control centers in one’s body. (WebMD, 2009) Research has shown that caffeine consumption specifically influences mood, stamina, the cerebral vascular system, and colonic activity. (Kovacs, 2009)
Endurance Among Elite Runners Runners want to increase their endurance, or the ability to continue despite tiredness, by going further. For elite runners that have reached the marathon distance, running faster is the goal. (Burfoot) With proper training and preparation, fatigue can be minimized, therefore enhancing overall endurance. Elite endurance athletes typically have a high VO2 max. (Willmore & Costill, 2005) While VO2 max is generally genetically determined, untrained individuals can increase their VO2 max by up to twenty percent. (Bouchard et al., 1992; Willmore & Costill, 2005) Furthermore, the lactate threshold determines how much of the VO2 max is used. Endurance training can increase lactate accumulation, thus having a positive effect on enhanced endurance performance. (Ready & Quinney, 1982) Next, endurance levels are partly based on what energy stores from a runner’s diet the body is using. Normally, human bodies use fat first, and then carbohydrates to produce energy for exercise. If exercise levels exceed 70 percent of the VO2 max, there is a greater reliance on carbohydrates than on fats. (Brooks & Mercier, 1994) If there are insufficient carbohydrates, exercise intensity and duration is significantly reduced. (Brooks & Mercier, 1994) Type one fibers, compared to type two, are favored in endurance activity because they have a higher mitochondrial count and oxidative enzyme capacity, allowing for energy production to come from aerobic metabolism. (Costill et al., 1976) Lastly, time spent doing endurance-training techniques helps the body become more able to produce ATP (adenosine triphosphate), a substance necessary for energy transportation during metabolism. (Burke, 1977) These adjustments include higher rate of oxygen delivery and oxygen utilization, increased rate of aerobic energy production, utilization of fat fuels, and maintenance of the acid-base balance. (Burke, 1977) If an elite runner maximizes their VO2 max, lactate threshold, fat and carbohydrate energy stores, muscle fibers, and endurance training, they are more likely to have greater endurance abilities during exercise.
Effect of Caffeine Consumption on Endurance among Elite Runners?
Graham and Spriet conducted an experimental study in 1991 to analyze performance and metabolic responses to high caffeine doses prior to prolonged exercise. (Graham & Spriet, 1991) It is important to note that although this study is over twenty years old, it is still relevant because caffeine continues to have the same effects on human bodies. One female and six male volunteers between the ages of 19 and 38 were selected to be the subjects for this experiment. (Graham & Spriet, 1991) Caffeine consumption habits were not considered in the selection of subjects. Instead, the status of “elite” 10k and marathon runners were the major criteria. (Graham & Spriet, 1991) This method of convenience sampling and small number of subjects could threaten the validity of the results because it is possible that the habitual use of caffeine in some runners could be the cause of varying endurance levels instead of the caffeine treatment given.
Each subject reported to the laboratory on two separate occasions prior to the beginning of the experiment to measure the athletes’ VO2 max and exercise at 85 percent of their particular VO2 max. (Graham & Spriet, 1991) For the experiment itself, each subject reported to the laboratory once a week for four weeks at the same time of day. The subjects were randomly assigned to either a placebo group or treatment group that received 9mg/kg of body weight of caffeine in capsule form. (Graham & Spriet, 1991) This amount of caffeine was determined because it maximizes the chance of eliminating fatigue symptoms and enhancing physical performance but also remains under the acceptable limits for competition. (Graham & Spriet, 1991; WebMD, 2009) Both the laboratory consultant and subject were blind to which group they were a participant. (Graham & Spriet, 1991) For the first two trips, subjects would run until exhaustion (i.e. 85 percent of VO2 max) one hour after taking either the placebo or caffeine treatment whereas the last two trips utilized the same procedure but on a bike. (Graham & Spriet, 1991) Diffusion should be considered as a threat to internal validity in this experimental design. There is no part of the experiment that ensures the subjects are not in taking caffeine before they come into the laboratory for assessments. Whether this is intentional or not, additional caffeine intake could skew final results.
Results of the study state that, “the ingestion of 9 mg of caffeine per kilogram of body weight one hour before exercise produced significant increases in exercise time to exhaustion” (Graham & Spriet, 1991, p. 2293) Running time to exhaustion increased from 49.2 minutes in the placebo group to 71.0 minutes during the caffeine trial. (Graham & Spriet, 2009) However, it is important to note some additional limitations of this study. Because eating habits and training practices are likely to vary from runner to runner, this is a threat to the validity of the results because we are unaware if certain training techniques aided in an increase of endurance levels instead of the caffeine treatment. Additionally, readers are not told who is taking measurements or how they are taken, resulting in possible sampling fluctuation. This reduces the reliability and internal validity of the results. Lastly, 9mg/1 kg of body weight is a high amount of caffeine and an unrealistic intake prior to exercise.
One year later, another study was conducted in the United Kingdom by Wiles, Bird, Hopkins, and Riley to measure the effect of caffeinated coffee on running speed, respiratory factors, blood lactate, and perceived exertion during a measured 1500 meter run. (Wiles et al., 1992) To conduct this study, three protocols were used. (Wiles et al., 1992) In all three protocols, subjects were randomly assigned to receive 3 grams of coffee, equivalent to 150 to 200 mg of caffeine, or 3 grams of decaffeinated coffee. This amount of caffeine is equal to what realistically could be ingested prior to a sports performance. (Wiles et al., 1992) Each runner would go through 6 time trials; 3 after ingesting the caffeinated coffee and 3 after having decaffeinated coffee. (Wiles et al., 1992) The random matched pairs design internal validity because the researchers have a control group to compare the treatment results to. However, although these three activities are still considered “high-intensity,” they are not lengthy. The difference in times, while deemed statistically significant, could be a result of the stimulant like nature of caffeine in a shortened running activity instead of actual higher endurance levels that are more likely to be seen in longer exercises.
In protocol one, the 1500-meter run, 18 male middle distance athletes between the ages of 18 and 29 participated. (Wiles et at. 1992) One hour after either receiving the 3 grams of either caffeinated or decaffeinated coffee, their 1500-meter run time was recorded. (Wiles et al. 1992) The results showed a statistically significant difference in the group means for the time. The average mean time to complete the run was 4.2 seconds faster following the intake of caffeinated coffee when compared to the times after receiving decaffeinated coffee. (Wiles et al., 1992) In protocol two, the high intensity 1100-meter run with a 1-minute maximum speed finish, 10 local male athletes of club, county, or national standard participated. (Wiles et at., 1992) After preliminary 1500 meter time trials, the subjects completed a 1100 meter run at a running speed 1 km/hr slower than the 1500 meter pace and finished with a 400 meter finishing burst one hour after being given the caffeinated or decaffeinated beverage. (Wiles et al., 1992) Results from protocol two support those from protocol one. All 10 subjects recorded faster speeds during the final minute of their run and 8 showed higher VO2 values after the intake of caffeinated coffee. (Wiles et al., 1992) After a variance t-test, these results were found to be statistically significant with the mean difference in speed . 6 km/hr. (Wiles et al., 1992) Lastly, the third protocol, the high intensity 1500 meter run at a predetermined speed, was completed by 6 male middle distance athletes of club, county, or national standard between the ages of 17 and 32 who had already completed protocol two earlier in the study. (Wiles et al., 1992) Once again, one hour after ingestion of either the 3 grams of caffeinated coffee or 3 grams of decaffeinated coffee, subjects completed six 1500-meter assessment runs at a speed .5 km/hr slower than the average speed of their fastest recorded 1500-meter run. (Wiles et al., 1992) Results also verified protocol one and two with all six subjects producing higher mean VO2 levels. (Wiles et al., 1992) Overall, these results indicate that smaller and more realistic doses of caffeine can enhance oxygen uptake and running performance in high intensity exercise.
While this study verifies the results of Graham & Spriets’ study in 1991, there are weaknesses to the study that should be addressed in further research. First, the selection of the subjects was never mentioned. To increase validity of the experiment subjects should have been randomly chosen and randomly assigned to each of the three protocols. Also, the researchers do not clarify the difference between club, county, or national standard athletes. These runners could be trained or experienced at different levels based on what standard they are a part of but for the purpose of this study, they are all grouped together as “elite” runners. Lastly, each participant knew which trials were conducted using caffeinated coffee and which were conducted with decaffeinated coffee. Demoralization could be present in this study because the control group knew they were not given the caffeinated coffee before their trial and thus, did not work as hard as they would have had they been given the competitive edge of the treatment group.
Another study by Fisher, McMurray, Berry, Mar, and Forsythe examined the influence of caffeine on exercise performance in six female habitual (>600 mg/day) caffeine users. (Fisher et al., 1986) Subjects completed a placebo trial of a 1-hour run at 75 percent of their VO2 max with normal caffeine intake. After 4 days of abstinence from caffeine, the trial was performed again after receiving a 5 mg/kg of body weight dose of caffeine. (Fisher et al., 1986) After caffeine withdrawal, the caffeine dosage given caused elevated oxygen uptake. This increase was found to be statistically significant. (Fisher et al., 1986) Post withdrawal, caffeine had its greatest effect on the resting free fatty acid levels, closely related to energy expenditures. (Fisher et al., 1986) Because of higher free fatty acid levels, it could be possible that extra energy stores were the result in elevated oxygen uptake instead of the caffeine administered prior to the assessment.
Lastly, a study conducted in 2006 by Malek, Housh, Coburn, and Beck analyzed the effect of a caffeine supplementation among 36 college students who participated in less than 4 hours a week of aerobic activity. (Malek et al., 2006) This study used a randomized, double blind, placebo controlled, parallel design. The subjects were assigned randomly to the supplement group or placebo group. The supplement group ingested 3 tablets of caffeine one hour prior to the exercise session. (Malek et al., 2006) After determination of the VO2 max, subjects ran at 90 percent of this value until voluntary exhaustion. (Malek et al., 2006) The results indicated that there was no difference between the placebo and supplement groups after the intake of caffeine. (Malek et al., 2006) This does not align with other studies’ results. In this study, dietary and other caffeine intake was not controlled. Therefore, it is possible that the results of the study could be because of those factors instead of the caffeine dosage given prior to exercise. Additionally, these participants are all out of shape and are not avid runners. Therefore, it could be a matter of their body condition rather than the caffeine that produced similar results between the supplement and placebo groups.
Conclusion
After an exhaustive review of the literature, the experiments and studies suggest that there is a causal relationship with caffeine consumption and endurance levels among elite runners under the age of 40 in developed countries. A study by Graham and Spriet (1991) proves that high doses of caffeine (9 mg/1 kg body weight) cause significant increases in exercise time to exhaustion. A study by Wiles et al. (1992) provides evidence that a more realistic or small dose of caffeine (3 mg/1 kg of body weight) also causes enhanced oxygen uptake and running performance in high intensity exercise. This study was limited by the duration of the activity. It is unclear whether or not the caffeine causes differences in times throughout the three protocols because of the “boost” caffeine gives humans or if it actually increases endurance levels. In the future, more experiments should be performed to specifically test the effect of caffeine consumption on endurance levels while other factors are controlled. These can include other caffeine intake outside of the laboratory, endurance training techniques, and diet. Furthermore, the experiments should utilize more subjects within their studies. This will increase internal validity and make the results able to be generalized to other runners of similar status. Such information will be beneficial to elite runners, especially as they prepare for major events such as marathons.
References
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References: Anderson, G. D. (1991). Caffeine and Sports. Dynamic Chiropractic, 9(5). Bouchard, C., Dionne, F Brooks, G. A. & Mercier, J. (1994). Balance of Carbohydrate and Lipid Utilization During Exercise: The “Crossover” Concept. Journal of Applied Physiology, 76(6), 2253-2261. Burfoot, A Burke, E. J. (1977). Physiological Effects of Similar Training Programs in Males and Females. Research Quarterly for Exercise and Sport, 48(3), 510-517. Costill, D Davies, P. (2001). Factors Affecting Endurance Performance. Retrieved from http://www.sport-fitness-advisor.com/endurancetraining.html. Fisher, S Graham, T. E. & Spriet, L. L. (1991). Performance and Metabolic Responses to a High Caffeine Dose During Prolonged Exercise. Journal of Applied Physiology, 77(6), 2292-2298. Kovacs, B Malek, M. H., Housh, T. J., Coburn, J. W., & Beck, T. W. (2006). Effects of Eight Weeks of Caffeine Supplementation and Endurance Training on Aerobic Fitness and Body Composition. Journal of Strength and Conditioning Research, 20(4), 751-755. Ready, A WebMD (2009). Find a Vitamin or Supplement: Caffeine. Retrieved from http://www.webmd. com/vitamins-supplements/ingredientmono-979-CAFFEINE. aspx?activeIngredientId=979&activeIngredientName=CAFFEINE. Wiles, J Willmore, J. H. & Costill, D. L. (2005). Human Kinetics. Physiology of Sport and Exercise, 3, 22-24. World Anti-Doping Agency (2011)