Πλεονεκτήματα Καφείνης & Αερόβια Άσκηση
Aerobic Benefits of Caffeine SupplementationTorrey A. Smith, MA, CSCS,*D; NSCA-CPT,*D
Introduction
Caffeine is one of the most highly used substances in the world. In fact, it has been reported that up to 96% of men and nonpregnant women consume caffeine and that approximately 20-30% of Americans consume more than 600 mg of caffeine daily (3). It is a socially accepted drug used by both the general population and athletes and is the only drug that is illegal in some athletics at a certain urine concentration. Caffeine can have positive and negative impacts on performance. How much is enough to gain performance advantages? How much is too much? There are certain risks that go along with supplementation of caffeine and it is only known to work for certain types of events. Who's at risk? What are the benefits and disadvantages to caffeine supplementation? These are some of the questions that will be answered with this article.
Since competition began, athletes, as well as their coaches and trainers, have explored methods to enhance performance and gain an advantage over their competitors. These methods have included employing certain training strategies, diet, the use of ergogenic aids as well as the use of banned substances. Many years ago, it was suggested that caffeine could increase the capacity to perform physical work (9, 10). Since then, numerous other researchers have studied the effects of caffeine associated with various types of exercise (7).
It is well documented in older research studies that caffeine stimulates the mobilization, and may increase oxidation of, free fatty acids (FFA) (8, 12, 15, 20). This oxidation was believed to result in a carbohydrate (CHO) sparing effect which in turn leads to an enhanced capacity for endurance exercise (15). With less CHO and more FFA being utilized for energy, the supposition is that athletes should be able to perform endurance activities for longer durations at a given intensity. There are other proposed mechanisms for how caffeine affects performance as well. Researchers (14, 16, 19, 24) have also looked at the effects of caffeine supplementation resulting from changes in the central nervous system.
This article will discuss how caffeine improves aerobic performance, dietary sources of caffeine, the effect on the human physiology and risks associated with supplementation. The purpose of this article is to educate coaches and athletes about the benefits and risks associated with caffeine supplementation so each will be able to make informed decisions about the use of this substance as an ergogenic aid.
What is Caffeine?
Caffeine (1,3,7-trimethylxanthine) is a group of lipid-soluble compounds found naturally in coffee beans, tea leaves, chocolate, cocoa beans, and kola nuts, known to prolong the stimulating effects of cyclic AMP on the heart and central nervous system (20). Some of the main sources of caffeine include coffee, tea, chocolate, soft drinks, cold remedies, diuretics, pain remedies, stimulants, weight control aids and pain drugs. For example, brewed coffee has approximately 60-150 mg of caffeine per cup, instant coffee has approximately 100 mg per cup, tea has around 20-50 mg per cup and soda contains around 50 mg per 12 ounces (20). To put this into perspective, two and half cups of coffee will contain around 250-400 mg of caffeine; this is equal to around 3-6 mg/kg for an average sized adult. (20). Other sources, such as chocolate can contain up to 15 mg per 3.5 ounces, cold remedies up to 65 mg, diuretics up to 200 mg, pain remedies up to 33 mg, stimulants and weight control aids can contain up to around 200 mg and pain drugs up to 100 mg of caffeine (20). It is important for trainers, coaches and athletes to understand that they may have caffeine from everyday substances in their system prior to supplementing; therefore, the supplement, if not administered in the proper dose, may cause an athlete to be above an allowable level.
The Physiological Effects of Caffeine
Caffeine is quickly absorbed through the intestinal tract and peak concentration of plasma caffeine is reached in about one hour after consumption. Caffeine can be eliminated from the body at a rate where half of it disappears in around 3-6 hours. As a comparison, other stimulants can take up to 10 hours for concentrations to decrease by one half (20). Knowledge of the half-life of caffeine is important because once plasma levels of caffeine decrease beyond a certain threshold, the caffeine will not produce the same effect. Therefore, the timing of intake can be crucial to performance.
Caffeine stimulates the cardiovascular system and the central nervous system. Some of the physiologic responses to caffeine ingestion include: increased urination, increased heart rate, relaxation of smooth muscle, decreased peripheral resistance of blood vessels and increased secretions in the stomach. Some of the proposed exercise-related effects of caffeine include increased secretion of catecholamines (8, 11, 15), improved skeletal muscle contractility (24), and enhanced calcium release from the sarcoplasmic reticulum (17, 20). It has also been suggested that caffeine may improve the ability to achieve maximal muscle activation (16) and may increase force production by enhancing neuromuscular transmission (24, 19). Four factors probably interact to produce caffeine?s facilitating effect on neuromuscular activity: 1) lowered threshold for motor unit recruitment, 2) altered excitation/contraction coupling, 3) facilitated nerve transmission, and 4) increased ion transport within the muscle (19, 20).
Effects of caffeine related to the function of the central nervous system have been reported (3) as well. Caffeine produces analgesic effects on the central nervous system and enhances motoneuronal excitability, thus facilitating motor unit recruitment. The stimulating effects of caffeine do not result from its direct action on the central nervous system. Rather, caffeine provides an indirect nervous system stimulation by blocking another chemical neuromodulator, adenosine, that normally exerts a calming effect on neurons of the brain and spinal cord (20). Increases in mood and alertness and decreases in pain and fatigue have reportedly been linked to caffeine's effect on the central nervous system (3).
Caffeine is a known diuretic, especially in those who do not consume caffeine on a regular basis (3, 20). Diuretics are substances that promote the formation of urine within the kidneys. Water and cranberry juice are considered weak diuretics whereas caffeine (in coffee, tea and soft drinks) and theophylline in tea stimulate increased renal glomerular filtration and inhibit reabsorption of sodium within nephrons, thereby stimulating an increased sodium and water excretion (3).
A summary of the physiological effects after caffeine ingestion can be seen in Table 1.
Table 1. Physiological effects after caffeine ingestion
Physiological Function
Effect
Central Nervous System
ι
Alertness
ι
Mood
κ
Pain
κ
Fatigue
Metabolism
ιθ
O2 consumption
ι
Lipolysis
ι
Glycogen sparing
Endocrine System
ιθ
Catecholamines
ι
ί-endorphin
ι
Cortisol
Skeletal Muscle
ι
Endurance
ιθ
Power
ιθ
Speed
Cardiovascular System
ι
Heart rate
ι
Stroke volume
ιθ
Blood pressure
Respiratory System
ι
Ventilatory rate
Kidneys
ι
Acute urine
production
ιθ
Acute loss of urinary electrolytes
θ
24-hour water balance
θ
24-hour electrolyte balance
*Adapted from (3)
θ refers to "No Change"
Summary of Research on Performance Effects
Caffeine has been studied as an ergogenic aid in a variety of endurance events using a wide range of subjects as well as a wide range of caffeine doses. While some of the studies have shown a significant ergogenic effect for caffeine, reported as an increase in oxygen consumption or an improvement in performance times, other studies have reported no significant effect of caffeine on these variables (5). This article will look at selected data-based articles in the area of caffeine supplementation and performance enhancement to summarize the effect of acute caffeine supplementation on aerobic endurance training.
The purpose of a study conducted by Bell and McLellan (4) was to determine the effect of a second caffeine dose on a repeated exercise bout. Nine male recreational cyclists participated in this study. Each testing session consisted of two rides to exhaustion, one in the morning and again five hours later in the afternoon. The design of the study used a double-blind (a test procedure in which the identity of those receiving the intervention is concealed from both the administrators and the subjects until after the test is completed that is designed to reduce or eliminate bias in the results) and randomized approach. The workload progression for each ride to exhaustion began with five minutes of cycling at 50% VO2max with a pedal frequency self-selected at either 60 or 100 rpm and then increased using the same pedal frequency to 80% VO2max. This level was sustained until exhaustion.
Prior to the exhaustion ride, the caffeine and the placebo were ingested using a volume of Gatorade? equivalent to 5 ml/kg of body weight. The investigators stated the limitation that the Gatorade? may have represented a confounding factor with the ingestion of caffeine (4). It should be noted that the primary function of Gatorade? is to replenish fluid and nutrients lost through perspiration; therefore, the results of the study may be contributed, in whole or in part, to the effect of the Gatorade? and not the acute supplementation of caffeine. Because of this, the researchers did not have true control over the results.
This study found that caffeine ingestion with Gatorade? increased exercise time to exhaustion by 31.3% in comparison to control subjects with Gatorade? alone (placebo). In addition, this study also found that the ergogenic effect during exhaustive exercise at 80% VO2max that follows a 5 ml/kg dose of caffeine is maintained five hours later during a subsequent exercise challenge (4).
Another study (6) using eight male well-trained rowers investigated the effect of varying doses of caffeine on laboratory tests that simulated the physiological demands of competition. To simulate competition, the subjects performed the tests on a Concept II rowing machine. A standard diet of 50 kcal/kg of body mass consisting of 63% carbohydrate, 20% fat and 17% protein was administered during the 24 hours prior to the test and the subjects were asked to abstain from caffeine and alcoholic beverages for 72 hours prior to the test. After a fast of 8-12 hours, the subjects reported to the lab. The subjects ingested 3 ml/kg of water with a capsule of either 6 mg/kg caffeine, 9 mg/kg caffeine or a placebo containing 500 mg of glucose. Subjects then began a submaximal row for four minutes at 60% of peak heart rate followed by one minute of rest followed by six minutes of rowing at 80% peak heart rate (75% VO2max). After the completion of the submaximal row, the subject rested for three minutes prior to beginning their 2000-m time trial.
The results of this study show that ingestion of the lower dose of caffeine (6 mg/kg) resulted in a 1.3% improvement in time to complete the 2000 meters whereas the higher dose resulted in a 1.0% improvement in performance over the placebo (6). While the results of this study are promising, the researchers note that the small sample size and lack of trials (subjects only completed one trial) prevents them from confidently concluding that a substantial graded response to caffeine existed (6).
The major finding of this study is that ingestion of 6-9 mg/kg of caffeine contained within a capsule one hour prior to exercise leads to a significant improvement in 2000-m simulated rowing time performance in well-trained oarsmen (6). Another major finding was that the higher dose of caffeine resulted in urinary caffeine concentrations that sometimes exceeded the legal limit (at the time this study was conducted, the legal limit was 12 ?g/mL) whereas the lower dose elicited a similar performance enhancement without exceeding the legal limit (6). An important goal of research on caffeine supplementation is to identify the potential benefits for endurance training and maintain a concentration level below the allowable limit. As in the first study, the results indicate that there is a significant improvement or benefit from acute caffeine supplementation on aerobic endurance training; however, multiple limitations (use of Gatorade?, small sample sizes and lack of experimental trials) in both studies thus far have left the results somewhat unexplained.
In a recent study (14) comparing nutritionally enriched coffee to commercially available decaffeinated coffee with regard to impact on endurance and anaerobic power performance in physically active college-aged subjects, it was found that the time to exhaustion was significantly higher in the enriched coffee compared with the decaffeinated. The enriched coffee had a higher concentration of caffeine as well as other added ingredients including: chromium polynicotinate, Garcinia cambogia, and Citrus aurantium. These mineral and herbal ingredients often are used in combination to enhance lipolysis and ? -adrenergic stimulation (14). The results of this study indicate that this enhanced coffee beverage appears to enhance time to exhaustion during aerobic exercise (14). However, it is difficult to determine if the effects shown were due to the caffeine supplementation, the other herbal ingredients included in the beverage or the combination of the two.
A study (7) with a purpose of clarifying the effects of caffeine on submaximal endurance time in males and females found that while there was a tendency for the submaximal exercise times to be enhanced for both sexes with caffeine ingestion, the increases were not significant. This finding is in conflict with other sources reviewed. In this study, the investigators used 15 active females and 13 active males. Prior to testing, subjects were randomly assigned into two groups using a standard double blind design. Each subject underwent VO2max testing and two submaximal exercises to exhaustion. The experimental rides to exhaustion were completed at an approximate workload of 70-75% VO2max. Exhaustion was defined as the inability to maintain at least 60 rpm for a 15 second period. The caffeine group in this study was administered 300 mg of caffeine in 250 ml of decaffeinated coffee. The decaffeinated group was administered 250 ml of decaffeinated coffee. The treatments were administered one hour prior to the test.
The results of this study for the VO2max tests indicate no significant differences in maximal heart rates, respiratory exchange ratio, RPE and VO2max relative to body weight (7). The researchers speculated that caffeine may enhance performance only when it is sufficient to produce an appropriate interaction between free fatty acid mobilization and a corresponding rise in glycerol levels (7).
A major difference between this study and the two previous studies that did find a significant difference due to caffeine supplementation, is that this study used the same quantity of caffeine for each subject, regardless of body weight whereas the two previous studies used a treatment of a specified amount of caffeine per kg of body weight. Not accounting for the total body weight could be an explanation of the study showing no significance of the measured values using caffeine.
Another study reviewed (5) attempted to determine if caffeine affected any one of several physiological parameters (VO2, heart rate, blood pressure, pulmonary ventilation, frequency of breaths, tidal volume and RPE) during submaximal exercise. Investigators used 10 college students, which consisted of five male and five female cross country runners involved with their postseason training regimen. The subjects were asked to refrain from physical activity for 24 hours prior to each testing session. All subjects underwent the VO2max test to determine a baseline value. Subjects were then randomly assigned to either complete the caffeine run first or the placebo run first. After a one week rest period, each subject completed the first of two 30 minute runs on a treadmill at 70% VO2max. After a one week rest, the subjects returned to the lab to complete their second 30 minute run on the treadmill at the same 70% VO2max. Utilizing previous research, the investigators chose 7 mg/kg of body weight of caffeine as the dosage. The placebo in this study was Vitamin C.
The subjects in this study were asked to abstain from caffeine for four days prior to testing to decrease the effect of caffeine tolerance and to fast for three hours prior to arriving in the lab. This short three hours could have resulted in the subjects having residual caffeine from their food intake in their system and a higher blood CHO level prior to arriving at the lab; ultimately, affecting the outcome of the study. Upon arrival, each subject was administered their treatment with 12 ounces of water, waited one hour and then were tested.
The results of this study found no significant differences in the cardiovascular responses. However, tidal volume and alveolar ventilation were significantly increased for the caffeine group. Additionally, the investigators found a statistically significant decrease in RPE within the caffeine group (placebo = 11.2 + 1.7; caffeine = 10.8 + 1.5). Although, it is pointed out here that the Borg scale is represented by whole numbers; therefore, when rounding the placebo value and caffeine value, both will be an 11 on the Borg scale. The work was perceived to be less difficult, while there was no increase in aerobic efficiency. Those who perceive they are doing less work are more likely to voluntarily increase their work load (i.e., they think they are not working hard enough).
In conclusion, these researchers stated that it is unlikely that caffeine provided an improvement in energy expenditure during moderate exercise since the VO2, heart rate, and blood pressure responses were unchanged (5). Therefore, by ingesting caffeine the subjects did not burn less energy. However, this study did find a significant increase in some of the respiratory responses, possibly mediated by caffeine's ability to produce bronchodilation. Therefore, breathing efficiency was improved with the drop in RPE.
In another study (18) examining the effects of daily administration of a supplement that contained caffeine in conjunction with eight weeks of aerobic training on VO2peak, time to running exhaustion at 90% VO2peak, body weight, and body composition, it was found that there were equivalent training-induced increases in VO2peak and time to running exhaustion for the supplement and placebo groups, but no changes in body weight, percent body fat, fat weight, or fat-free weight for either group. These findings suggest that chronic use of the caffeine-containing supplement, in conjunction with aerobic training, may provide no ergogenic effects and no benefit for altering body weight or body composition (18).
The purpose of another study (1) was to determine the effect of two levels of caffeine supplementation on the metabolic and cardiorespiratory responses to treadmill walking in women. Twenty women of average fitness were randomly assigned to a group (3 mg/kg, 6 mg/kg, placebo group). Each subject then completed three trials of moderate steady-state treadmill walking at 3.5 mph. The researchers found no significant differences between the placebo trials and the 3 mg/kg dose trials. However, a 6 mg/kg dose of caffeine significantly increased the respiratory exchange ratio during exercise (1), indicating decreased fat utilization rather than increased as suggested by others (19, 20). Further, results indicate that no significant difference was found in RPE and the researchers state that trainers should not recommend caffeine as a method to decrease perception of effort during exercise (1). However, it should be noted that it is difficult to find significance with small groups.
The benefits achieved from supplementing caffeine for aerobic performance will vary by individual. The amount of performance improvement from caffeine ingestion may depend on multiple factors: age, weight, metabolism, amount of caffeine, timing of intake, caffeine sensitivity, hydration status, training level and type of training. Research indicates that the typical adult athlete may see performance improvement through an increase in alertness, glycogen sparing, endurance, power, speed, heart rate and stroke volume and a decrease in fatigue (refer to Table 1 for a complete listing of effects). However, the often stated claim on glycogen sparing is somewhat controversial (19, 20). While this article reviews aerobic benefits of caffeine supplementation, it is important to understand how other systems in the body are affected by the use of caffeine.
The research reviewed in this article shows conflicting results regarding the effects of caffeine on aerobic performance. Different amounts of caffeine supplementation given to different subjects using a variety of methods have led to these inconclusive results. The important thing for coaches and athletes to know is that caffeine, or any supplement, should be used with caution. Some supplements are effective for some people, while others may not work. It appears that proper dosage and timing of caffeine ingestion are important factors for the effectiveness of this supplement.
NCAA Limits
It is human nature for an athlete to strive for improvement. One method athletes have utilized for years is caffeine supplementation to improve aerobic endurance performance. According to the National Collegiate Athletic Association (NCAA), caffeine is a legal supplement up to a urine caffeine concentration level of 15 ?g/mL (21). Subjects with a caffeine intake that exceeds this concentration level will be disqualified. Even though the NCAA level is set at 15 ?g/mL, Graham and Spriet have suggested that caffeine may improve performance when the urinary concentration of caffeine is as low as 10 ?g/mL (13). The International Olympic Committee (IOC), in January 2005, removed caffeine from their banned substance list; however, it continues to monitor caffeine consumption in competitive athletes (25).
Cautions
Caffeine is a drug with adverse side effects in some people. Just as the performance effect of caffeine is highly variable between people, the adverse effects are probably also highly variable in people. The adverse effects of caffeine are well known and include anxiety, jitters, inability to focus, gastrointestinal discomfort, insomnia, irritability, and, with higher doses, the risk of heart arrhythmias and mild hallucinations (19, 20, 22). Typically, most individuals will start to see signs of these adverse effects when they supplement with around 9 mg/kg of body weight. This is not dangerous for most people. However, for those who are highly sensitive to caffeine, it can become dangerous. In caffeine sensitive individuals, the caffeine has an exacerbated effect to the heart rate, blood pressure and neurological functions. It is estimated that 150 mg of caffeine per kg body mass is considered to be a lethal dose in most humans; although, lethal doses have been reported with as little as 35 mg/kg in small children (20). Caffeine is a psychoactive substance and, if consumed on a daily basis, can lead to a number of caffeine use disorders, including caffeine intoxication, withdrawal and dependence, and caffeine-induced anxiety and sleep disorders (22).
Habitual caffeine consumption is another concern for athletes who are supplementing with caffeine. Caffeine metabolism is not increased by use, but the effects of caffeine may be altered by habitual use (22). Therefore, with habitual use, the caffeine supplementation may become less likely to improve performance. Additionally, if the athlete supplements with caffeine and already has a higher level of caffeine in their system, they could potentially be tested with a higher-than-allowed by NCAA amount of urinary caffeine concentration.
Athletes who rely on a pre-competition meal or high-CHO diet should take caution before supplementing aerobic training with caffeine. Some research indicates that a high-CHO diet and a pre-race meal negates the expected increase in plasma free fatty acids after caffeine ingestion during two hours of exercise at 75% VO2max (23). The outcome of this study suggests that a pre-race meal of high-CHO should not be consumed if the athlete is supplementing with caffeine. The caffeine supplement may be more effective without consumption of the high-CHO meal. It should be noted here that this is only one study and it is important to understand the implications of ingesting less carbohydrates prior to competition. More research in this area needs to be conducted before a consensus can be drawn on whether or not athletes should consume a pre-race meal or ingest a caffeine supplement. Will the less carbohydrate in the body be outweighed by ingesting caffeine? Only research will tell, and there is not enough available to make that determination at this time.
Caffeine is often thought to act as a diuretic leading to dehydration during exercise. This unnecessary pre-exercise loss of fluid could negatively affect thermal balance and exercise performance in hot environments (20). However, as indicated by Spriet (22), no changes in core temperature, sweat loss, or plasma volume were reported during exercise after caffeine ingestion. Additionally, there is no evidence to suggest that moderate caffeine intake (< 456 mg) induces chronic dehydration or negatively affects exercise performance, temperature regulation, and circulatory strain in a hot environment (3). There is conflicting evidence regarding the diuretic effect of caffeine and its effects on thermal balance. This is another area that needs future research.
Recommendations
Caffeine supplementation is generally tested using urinary analysis either before or after a competition. It is noted that only around 0.5-3.0% of orally ingested caffeine actually reaches the urine (22). Factors that can affect the urinary concentration of caffeine include body weight, gender, hydration levels and the amount of time that has elapsed between consumption and testing (22). The half-life of caffeine is approximately 3-6 hours. This means that in 3-6 hours, half the caffeine that was ingested will be out of the body. One of the major problems with testing athletes after a competition is that an illegal level of caffeine may have been present at the beginning of the event; however, by the time the athlete is tested, the levels may have dropped back to within an allowable range. The timing issue is a topic that sports governing bodies need to standardize.
It is possible to consume large amounts of caffeine before reaching the legal limit. For example, a 154-pound (70-kg) person could drink about six regular-sized cups of coffee (about 9 mg caffeine/kg body mass) about one hour before exercise and then exercise for about one hour and a subsequent urine sample would only approach (not exceed) the urinary caffeine limit (22). However, ingesting this amount of coffee prior to an endurance event may lead to some adverse effects. To minimize the negative outcomes, athletes are advised to stay within the recommended dosage to still improve performance. A recommendation for the optimal dose is around 3-6 mg/kg (22) in capsule form and taken about one hour prior to an endurance event.
Summary
Caffeine supplementation for performance enhancement has existed for many years. The allowable limits have changed over time and are continually being monitored. The current regulation is only through the NCAA at 15 ?g/mL (21), as the IOC removed caffeine from their banned substances list in 2005 (25). Athletes will continue to do what they can to improve performance.
The coach?s responsibility with caffeine supplementation is to, above all, keep the athlete safe, healthy and within their sports governing body?s allowable limits. Athletes tend to have the mindset to win and achieve success, perhaps at a high cost. Therefore, it will often be up to the coach to monitor the health of an athlete who is supplementing aerobic endurance training with caffeine.
This article?s intent was to educate both the athlete and the coach about the benefits, side effects, allowable limits and performance effects of caffeine supplementation. With the effects of caffeine supplementation being inconclusive in the research, it comes down to each coach and athlete making the decision to supplement or not with caffeine. Some studies have shown and increase, some have not. The risks involved with caffeine supplementation may be high in some athletes. Therefore, the decision needs to be made with as much information as possible about the potential benefits and risks associated with caffeine use.
References
1. Ahrens, J.N., S.H. Crixell, L.K. Lloyd, and J.L. Walker. The physiological effects of caffeine in women during treadmill walking. J. Strength Cond. Res. 21:164-168. 2007.
2. Antonio, J. Caffeine: The forgotten ergogenic aid. Strength Cond. J. 26:50-51. 2004.
3. Armstrong, L.E., D.J. Casa, C.M. Maresh, and M.S. Ganio. Caffeine, fluid-electrolyte balance, temperature regulation, and exercise-heat tolerance. Exerc. Sport Sci Rev. 35:135-140. 2007.
4. Bell, D.G., & T.M. McLellan. Effect of repeated caffeine ingestion on repeated exhaustive exercise endurance. Med. Sci. Sports Exerc. 35:1348-1354. 2003.
5. Birnbaum, L.J., & J.D. Herbst. Physiologic effects of caffeine on cross-country runners. J. Strength Cond. Res. 18:463-465. 2004.
6. Bruce, C.R., Anderson, M.E., Fraser, S.F., Stepto, N.K., Klein, R., Hopkins, W.G., & J.A. Hawley. Enhancement of 2000-m rowing performance after caffeine ingestion. Med. Sci. Sports Exerc. 32:1958-1963. 2000.
7. Butts, N.K., & D. Crowell. Effect of caffeine ingestion on cardiorespiratory endurance in men and women. Research Quarterly for Exercise and Sport , 56:301-305. 1985.
8. Costill, D.L., G.P. Dalsky, and W.J. Fink. Effects of caffeine ingestion on
metabolism and exercise performance. Med. Sci. Sports Exerc ., 10:155-158. 1978.
9. Foltz, E., A. Ivy, & C. Barborka. The use of double work periods in the study of fatigue and the influence of caffeine on recovery. American Journal of Physiology , 136:79-86. 1942.
10. Foltz, E., A. Ivy, & C. Barborka. The influence of amphetamine (Benzedrine) sulfate, D-desoxyephedrine hydrochloride (Pervitan), and caffeine upon work output and recovery when rapidly exhausting work is done by trained subjects. Journal of Laboratory and Clinical Medicine , 28:603-606. 1943.
11. Graham, T.E. Caffeine and exercise: Metabolism, endurance and performance. Sports Med . 31:786-807. 2001.
12. Graham, T.E., & L.L. Spriet. Caffeine and exercise performance. Gatorade
Sports Science Institute: Sports Science Exchange , 9:60-64. 1996.
13. Graham, T.E. and L.L. Spriet. Performance and metabolic responses to a high caffeine dose during prolonged exercise. J. Appl. Physiol . 71:2292-2298. 1991.
14. Hoffman, J.R., J. Kang, N.A. Ratamess, P.F. Jennings, G.T. Mangine, and A.D. Faigenbaum. Effect of nutritionally enriched coffee consumption on aerobic and anaerobic exercise performance. J. Strength Cond. Res. 21:456-459. 2007.
15. Ivy, J.L., D.L. Costill, W.J. Fink, and R.W. Lower. Influence of caffeine and carbohydrate feedings on endurance performance. Med. Sci. Sports Exerc ., 11:6-11. 1978.
16. Kalmar, J.M., and E. Cafarelli. Effects of caffeine on neuromuscular function. J. Appl. Physiol . 87:801-808. 1999.
17. Lopes, J.M., M. Aubier, J. Jardim, J.V. Aranda, and P.T. Macklem. Effect of caffeine on skeletal muscle function before and after fatigue. J. Appl. Physiol . 54:1303-1305. 1983.
18. Malek, M.H., T.J. Housh, J.W. Coburn, T.W. Beck, R.J. Schmidt, D.J. Housh, and G.O. Johnson. Effects of eight weeks of caffeine supplementation and endurance training on aerobic fitness and body composition. J. Strength Cond. Res . 20:751-755. 2006.
19. McArdle, W.D., F.I. Katch, and V.L. Katch. Exercise physiology: energy, nutrition, and human performance (fifth edition). Baltimore, MD: Lippincott Williams & Wilkins. 2001. p. 564-568.
20. McArdle, W.D., F.I. Katch, and V.L. Katch. Sports and exercise nutrition . Baltimore, MD: Lippincott Williams & Wilkins. 1999. p. 321-326.
21. National Collegiate Athletic Association. NCAA Banned-Drug Classes 2004-2005. Available at: http://www1.ncaa.org/membership/ed_o...ug_classes.pdf.
22. Spriet, L.L. Caffeine. In: Performance-Enhancing Substances in Sport and Exercise . M.S. Bahrke and C.E. Yesalis, eds. Champaing, IL: Human Kinetics, 2002. p. 267-278.
23. Weir, J., T.D. Noakes, K. Myburgh, and B. Adams. A high carbohydrate diet negates the metabolic effect of caffeine during exercise. Med. Sci. Sports Exerc ., 19:100-105. 1987.
24. Williams, J.H. Caffeine, neuromuscular function and high-intensity exercise performance. J. Sports Med. Phys. Fitness 31:481-489. 1991.
25. World Anti-Doping Agency. The 2005 Monitoring Program. Available at: http://www.wada.ama.org/rtecontent/document/Monitoring_Program_2005.pdf
