March 2009 Issue

Back to the Grind: The Return of Caffeine as an Ergogenic Aid
By Ellen Coleman, MA, MPH, RD, CSSD
Today’s Dietitian
Vol. 11 No. 3 P. 8

Suggested CDR Learning Codes: 2000, 2070, 2110, 4000, 4060; Level 2

Caffeine is a socially acceptable stimulant that is used in various forms worldwide. In addition to helping people get started each day, caffeine has been an integral part of many athletes’ diets. Most believe it helps their performance, and research now supports this belief.

Caffeine is a naturally occurring methylxanthine that stimulates the release and activity of the catecholamine epinephrine.1 The major dietary sources of caffeine (eg, tea, coffee, chocolate, cola drinks) typically provide 30 to 100 milligrams of caffeine per serving.1 Some over-the-counter medications (eg, NoDoz, Vivarin) contain 100 to 200 milligrams of caffeine per tablet. Caffeine is also found in the herbs guarana, yerba mate, and kola nut.

Substantial research suggests that caffeine enhances endurance performance.1,2 It also provides a small but worthwhile improvement during short-term, intense aerobic exercise lasting four to eight minutes and prolonged, high-intensity aerobic exercise lasting 20 to 60 minutes. The stimulant’s effect on strength/power and sprints lasting less than 90 seconds is unclear.1,2 Caffeine will not turn a couch potato into a triathlete, as it does not improve exercise capacity in untrained subjects, regardless of dose.1

The World Anti-Doping Agency (WADA) removed caffeine from its list of prohibited substances in January 2004. Athletes who compete in sports compliant with the WADA code can consume caffeine as part of their everyday diet or specifically as an ergogenic aid without fear of sanctions. A moderate amount of caffeine (up to 6 milligrams of caffeine per kilogram of body weight) does not raise urinary caffeine levels above the National Collegiate Athletic Association’s doping threshold of 15 micrograms per milliliter.

Recent research has challenged two widely held beliefs about the use of caffeine as an ergogenic aid: first, that caffeine enhances endurance performance by increasing the utilization of fat and sparing muscle glycogen, and second, that caffeine has a diuretic effect and increases the risk of dehydration. The latter belief has caused concern among athletes, since proper hydration is essential.

Mechanism of Action
Caffeine has several effects on skeletal muscle, involving calcium handling, sodium-potassium pump activity, elevation of cyclic adenosine monophosphate (AMP), and direct action on enzymes such as glycogen phosphorylase. Caffeine’s effects on epinephrine and cyclic AMP may increase lipolysis in adipose and muscle tissue, thereby raising plasma free fatty acid concentrations and increasing the availability of intramuscular triglyceride.1

Three potential mechanisms have been proposed for the performance-enhancing effects of caffeine.1,2 In the classic, or “metabolic,” theory, caffeine may increase fat utilization and decrease glycogen utilization. Caffeine mobilizes free fatty acids from adipose and/or intramuscular triglyceride by increasing circulating epinephrine levels. The increased availability of free fatty acids increases fat oxidation and spares muscle glycogen, thereby enhancing endurance performance.1,2

Second, caffeine may increase the force of muscle contractions by positively influencing calcium kinetics and the sodium-potassium pump activity within the exercising muscles.1,2

Third, as a central nervous system stimulant, caffeine increases alertness and decreases the perception of effort. Caffeine may reduce the perception of effort by lowering the neuron activation threshold, making it easier to recruit the muscles for exercise.1,2

A meta-analysis by Doherty and Smith found that caffeine reduced ratings of perceived exertion (RPE) by 5.6% and improved exercise performance by 11.2% compared with placebo.3 Regression analysis also revealed that the RPE obtained during exercise accounted for approximately 29% of the variance in the improvement in performance. Thus, reducing RPE may partly explain how caffeine improves exercise performance.3

Determining caffeine’s method(s) of action is complicated because caffeine crosses the membranes of all tissues in the body, including the blood-brain barrier.2 Since caffeine enters both the central nervous system and peripheral tissues (skeletal muscle, adipose tissue, liver), it is impossible to separate caffeine’s central effects from its peripheral effects.2 Different mechanisms may also be responsible for performance improvement in different exercise situations.2

Caffeine supplementation is difficult to investigate due to the complexity of isolating the different effects of caffeine and the large variability in individuals’ response to caffeine.1

Although caffeine’s mechanism of action is unknown, it is highly unlikely that caffeine improves endurance by increasing fat oxidation and sparing muscle glycogen utilization—the metabolic theory.1,2 It is far more likely that a decreased perception of effort and/or direct effects on the muscle are responsible for the improvements in performance observed with caffeine supplementation.1

Early Research
The interest in caffeine as an endurance aid and the metabolic hypothesis were initiated by research from David Costill’s laboratory about 30 years ago.1,2 The first study reported that consuming 5 milligrams of caffeine per kilogram one hour before cycling at 80% of VO2max improved endurance performance by 19%—90 minutes compared with 75 minutes. A second study found that consuming 250 milligrams of caffeine was associated with a 20% increase in the amount of work that could be performed in two hours.

These two studies suggest that the utilization of fat for energy increased by about 30% in the caffeine trials. The third study from Costill’s laboratory found that consuming 5 milligrams of caffeine per kilogram reduced muscle glycogen usage by 42% and increased muscle triglyceride usage by 150% during 30 minutes of cycling at 70% of VO2max.1,2

Later Research
Several well-controlled studies in the 1990s found that varying caffeine doses (3 to 13 milligrams per kilogram) improved endurance performance by 20% to 50% in elite and recreationally trained athletes during running or cycling at 80% to 90% of VO2max.2 Side effects of caffeine consumption (eg, dizziness, headache, insomnia, gastrointestinal distress) were rare with doses at or below 6 milligrams per kilogram but prevalent at higher doses of 9 and 13 milligrams per kilogram.2

Caffeine generally produced a twofold increase in plasma epinephrine (at rest and during exercise) and in plasma free fatty acids at rest. However, the elevation in free fatty acids lasted for only the first 15 to 20 minutes of exercise. Muscle glycogen usage was also reduced following caffeine consumption, but this “glycogen-sparing” effect was limited to the initial 15 minutes of exercise at 80% of VO2max. At the lowest dose, caffeine improved performance without an increase in plasma epinephrine or free fatty acids.2

Caffeine Dose and Timing
Caffeine is rapidly absorbed, reaching peak concentrations in the blood within one hour of ingestion. A commonly recommended dose to improve performance is 3 to 6 milligrams of caffeine per kilogram, consumed one hour before exercise.2 However, recent evidence suggests that the beneficial effects of caffeine occur at low levels of intake—1 to 3 milligrams of caffeine per kilogram—when caffeine is consumed before and/or during exercise.4 There is also little evidence of a dose-response relationship to caffeine, as the benefits in exercise performance do not increase with higher doses of caffeine.1

Cox and colleagues found a similar 3% improvement in time trial performance with the following:

• six doses of 1 milligram of caffeine per kilogram spread throughout two hours of submaximal cycling prior to the time trial;

• 6 milligrams of caffeine per kilogram consumed one hour prior to the cycling bout; or

• 1.5 milligrams of caffeine per kilogram (from Coca-Cola) consumed over the last one third of the exercise protocol.4

This study found that consuming 6 milligrams per kilogram of caffeine improved performance independent of the timing of intake.4 Also, an intake of only 1.5 milligrams of caffeine per kilogram consumed toward the end of exercise enhanced time trial performance to the same degree as an intake of 6 milligrams per kilogram consumed before or during exercise.5 Although it is impossible to prove, it is thought that the subjects may have become more sensitive to small amounts of caffeine as they became fatigued.1

Kovacs and associates evaluated the effect on performance during a one-hour cycling time trial of different dosages of caffeine (2.1, 3.2, and 4.5 milligrams per kilogram) added to a 7% carbohydrate-electrolyte drink.6 All three caffeine doses improved performance compared with the carbohydrate-electrolyte drink alone. The improvement in performance was the same with caffeine doses of 3.2 milligrams per kilogram and 4.5 milligrams per kilogram and greater than with 2.1 milligrams per kilogram.6 This study suggests that once the threshold dose of caffeine was reached, there was no further performance benefit from a higher amount of caffeine.1

Jenkins and associates found that low doses of caffeine improved high-intensity aerobic cycling performance.5 The subjects consumed 1, 2, or 3 milligrams of caffeine per kilogram one hour prior to a 15-minute performance ride. Compared with placebo, the caffeine dose of 2 milligrams per kilogram improved performance by 4%, and the 3-milligrams-per-kilogram dose improved performance by 3%. The ergogenic effects of caffeine varied considerably in magnitude among individual cyclists. The authors recommended further research to explain the considerable variability of caffeine’s ergogenic properties between individuals.5

Ideally, the optimal caffeine dose is the amount that elicits the greatest performance benefit for the minimum level of risk or side effect. Further research is necessary to help identify the smallest caffeine dose that produces a meaningful improvement in performance. Additional study is also needed to investigate the potential for strategically timing caffeine intake.1

Caffeine and Dehydration
Athletes and active people are commonly advised to avoid caffeinated beverages before, during, and after exercise. The rationale is that the diuretic effect of caffeine increases urinary water losses and contributes to dehydration, thereby harming athletic performance and/or health. This has become so firmly entrenched in the common wisdom that many athletes believe it must be based on substantial research. But recent research and literature reviews (Armstrong LE. Caffeine, body fluid-electrolyte balance, and exercise performance. Int J Sport Nutr Exerc Metab. 2002;12(2):189-206) refute this notion.1,7 There is also virtually no evidence in the scientific literature that caffeine intake impairs fluid status.1,7

Armstrong and colleagues evaluated hydration status and urine losses in subjects who were first habituated to a daily caffeine intake of 3 milligrams per kilogram for six days.10 The subjects then consumed either 0 milligrams per kilogram, 3 milligrams per kilogram (average dose was 226 milligrams), or 6 milligrams per kilogram (average dose was 452 milligrams) of caffeine for five days. There were no differences in body weight, urine losses, or serum osmolality. These findings counter the widely held belief that caffeine acts chronically as a diuretic.7 

Caffeine is unlikely to elevate urine output or cause dehydration if consumed in moderation.1,7 The effect of caffeine on diuresis is overstated and may be minimal in those who regularly consume caffeine.1,7 Furthermore, caffeine-containing drinks such as tea, coffee, and cola provide a significant source of fluid in many athletes’ everyday diets.1

Burke and associates noted that any small fluid loss due to caffeine-containing beverages can be easily offset by an athlete’s fluid intake during meals and social gatherings. Furthermore, if the athlete is abruptly advised to remove caffeinated beverages from the diet or postexercise meal and does not replace these beverages with an equal volume of other fluids, he or she could become dehydrated.1

Caffeine and Carbohydrate
Consuming carbohydrate alone and caffeine alone during endurance exercise have both been shown to delay the onset of fatigue and increase exercise capacity. The benefits of carbohydrate ingestion are attributed to the maintenance of blood glucose levels and high rates of carbohydrate oxidation in the latter stages of exercise, when muscle and liver glycogen stores are low. Kovacs and associates found that the addition of moderate amounts of caffeine (3.2 and 4.5 milligrams per kilogram) to a 7% carbohydrate-electrolyte drink (0.5 grams of carbohydrate per minute) significantly improved one-hour cycling time trial performance compared with a lower dose of caffeine (2.1 milligrams per kilogram) and the carbohydrate-electrolyte drink.6

The addition of caffeine to carbohydrate may enhance performance by increasing intestinal carbohydrate absorption and exogenous carbohydrate oxidation. Increasing exogenous carbohydrate oxidation reduces the reliance on endogenous carbohydrate sources (muscle and liver glycogen). It appears that exogenous carbohydrate oxidation is primarily limited by intestinal carbohydrate absorption.

In a 2000 Journal of Applied Physiology article, Van Nieuwenhoven and colleagues found that adding a small amount of caffeine (1.4 milligrams of caffeine per kilogram) to a carbohydrate-electrolyte drink (0.5 grams glucose per minute) produced 23% greater intestinal glucose absorption compared with the carbohydrate-electrolyte drink during 90 minutes of cycling at 70% of maximum power output.

Yeo and colleagues from the University of Birmingham in the United Kingdom evaluated the effect of caffeine on exogenous carbohydrate oxidation.8 The subjects consumed either a 5.8% glucose solution (0.8 grams glucose per minute), glucose with caffeine (0.8 grams glucose per minute and 10 milligrams of caffeine per kilogram), or water during two hours of cycling at 64% of VO2max. The average exogenous carbohydrate oxidation during the final 30 minutes of exercise was 26% higher for glucose with caffeine compared with glucose. The combination of caffeine and glucose produced a significantly higher total rate of carbohydrate oxidation (2.47 grams per minute) compared with glucose (1.84) and water (1.12), possibly due to an enhanced intestinal absorption of glucose.8

Hulstin and Jeukendrup, also from the University of Birmingham, conducted a follow-up study to evaluate the effect of caffeine on exogenous carbohydrate oxidation and determine whether the combined ingestion of caffeine and carbohydrate enhanced cycling performance compared with carbohydrate alone.9 The subjects consumed either a 6.4% glucose solution (0.71 grams per minute), glucose with caffeine (0.71 grams per minute and 5.3 milligrams of caffeine per kilogram), or a placebo during 105 minutes of steady-state cycling at 62% of VO2max. This was followed by a time trial lasting about 45 minutes, during which the subjects drank water. The combination of glucose and caffeine enhanced time trial performance by 4.6% compared with glucose and by 9% compared with placebo. However, caffeine did not influence exogenous carbohydrate oxidation during exercise.9

Hulstin and Jeukendrup noted that the disparate findings from Yeo and colleagues may be explained by the use of a lower caffeine dose and/or individual differences between subjects in the two studies. It is also difficult to determine the most effective caffeine dose given the variability of experimental protocols used in research studies.9 Nevertheless, the findings of Hulstin and Jeukendrup and other studies demonstrate that the combined ingestion of caffeine and carbohydrate enhances endurance performance compared with carbohydrate alone.4,6,9 Further research is required to elucidate the mechanism for the additional performance effect of caffeine.9

Source of Caffeine
Studies utilizing coffee as the source of caffeine have found that it can either enhance performance or fail to have a noticeable effect.1 Coffee is not an ideal source of caffeine due to the large variability in caffeine content depending on brand, variety, method of preparation, and serving size.1 Furthermore, other chemicals within coffee (eg, derivates of the chlogogenic acids) may counteract the ergogenic effect of caffeine.1

Cox and colleagues investigated the effect of a cola drink on endurance cycling performance to replicate athletes’ common practice of replacing carbohydrate-electrolyte drinks with “de-fizzed” cola drinks during the latter stages of an endurance race.4 The authors found that consuming Coca-Cola (1.5 milligrams of caffeine per kilogram) toward the end of the protocol improved time trial performance by 3% and that most of this effect could be explained by Coke’s modest caffeine content.4

Hogervorst and associates examined the effects of ingesting a caffeinated sports bar (PowerBar with ActiCaf, which contains 100 milligrams of caffeine and 45 grams of carbohydrate) before and during cycling exercise on physical and cognitive performance.10 The subjects cycled for 2.5 hours at 60% of VO2max, followed by a time-to-exhaustion trial at 75% of VO2max. They consumed the caffeinated sports bar, a noncaffeinated sports bar (PowerBar, with 45 grams of carbohydrate), or placebo before exercise and after 55 and 115 minutes of exercise. The caffeinated sports bar improved time to exhaustion by 27% compared with the noncaffeinated sports bar and by 84% compared with the placebo. Caffeine also improved complex cognitive ability during and after exercise.10

The majority of research studies have used pure caffeine rather than caffeinated drinks (eg, colas, energy drinks) or products such as caffeinated gels and sports bars. Further research is required on the effectiveness of caffeinated products (energy drinks, caffeinated beverages, and caffeinated gels and sports bars) that athletes commonly use.1 Athletes who wish to try caffeine should experiment with pure caffeine during training.

Other Considerations
Most studies of caffeine and performance have been conducted in laboratories. Studies investigating performance in elite athletes during real-life sporting events should be undertaken to help provide specific recommendations for caffeine supplementation protocols. In addition to enhancing competitive performance, caffeine is also likely to be a helpful training aid by allowing the athlete to engage in harder, more consistent workouts.1

The effects of caffeine vary between individuals. Some athletes do not respond and others experience adverse effects such as tremors, increased heart rate, headaches, and disrupted sleep. These side effects may both directly and indirectly impair performance.

For example, excessive caffeine intake can cause disrupted sleep and therefore interfere with the ability to recover between training sessions and multiday competitions. This side effect is more common at caffeine doses exceeding 6 to 9 milligrams per kilogram. Often, athletes will increase their intake of caffeine to offset the fatigue they experience due to disrupted sleep patterns, thus perpetuating and worsening the problem. The potential for adverse effects at high caffeine doses emphasizes the importance of finding the lowest effective dose of caffeine that can be used to achieve a performance enhancement.1

Athletes’ caffeine supplementation practices are often improvised and haphazard. Many athletes are unaware of current research demonstrating that beneficial effects of caffeine occur at low intake levels. They’re also uninformed about the potential for side effects or negative outcomes from excess caffeine use. Dietitians who counsel athletes should inquire about their supplement use and educate them about the potential benefits and risks of popular sports supplements, including caffeine. Athletes who wish to use caffeine should experiment with pure caffeine in training before and during exercise to determine the dose that elicits the greatest benefits and least adverse effects.1

Summary
Caffeine enhances endurance performance and provides a small but worthwhile improvement during short-term, intense aerobic exercise lasting four to eight minutes and prolonged, high-intensity aerobic exercise lasting 20 to 60 minutes. Although caffeine’s mechanism of action is unknown, it is highly unlikely that caffeine improves endurance by increasing fat oxidation and sparing muscle glycogen utilization.

Recent evidence suggests that the beneficial effects of caffeine occur at low intake levels—1 to 3 milligrams of caffeine per kilogram—when caffeine is consumed before and/or during exercise. There is also little evidence of a dose-response relationship to caffeine. Caffeine is unlikely to elevate urine output or cause dehydration if consumed in moderation.

Coffee is not an ideal vehicle for caffeine supplementation for athletes due to the variability of caffeine content and the possible presence of chemicals that may impair exercise performance. Athletes who want to use caffeine should experiment with pure caffeine in training before and during exercise to determine the dose that elicits the greatest benefits and least adverse effects.

— Ellen Coleman, MA, MPH, RD, CSSD, is a nutrition consultant at The SPORT Clinic in Riverside, Calif.

References
1. Burke L, Cort M, Cox G, et al. Supplements and sports foods. In: Burke L, Deakin V (eds). Clinical Sports Nutrition, 3rd ed. Australia: McGraw-Hill; 2006.

2. Spriet L. Caffeine. In: Bahrke MS, Yesalis CE (eds). Performance-Enhancing Substances in Sport and Exercise. Champaign, Ill.: Human Kinetics; 2002.

3. Doherty M, Smith PM. Effects of caffeine ingestion on rating of perceived exertion during and after exercise: A meta-analysis. Scand J Med Sci Sports. 2005;15(2):69-78.

4. Cox GR, Desbrow B, Montgomery PG, et al. Effect of different protocols of caffeine intake on metabolism and endurance performance. J Appl Physiol. 2002;93(3):990-999.

5. Jenkins NT, Trilk JL, Singhal A, O’Connor J, Cureton RS. Ergogenic effects of low doses of caffeine on cycling performance. Int J Sports Nutr Exerc Metab. 2008;18(3):328-342.

6. Kovacs EM, Stegen JHCH, Brouns F. Effect of caffeinated drinks on substrate metabolism, caffeine excretion, and performance. J Appl Physiol. 1998;85(2):709-715.

7. Armstrong LE, Pumerantz AC, Roti MW, et al. Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. Int J Sport Nutr Exerc Metab. 2005;15(3):252-265.

8. Yeo SE, Jentjens RL, Wallis GA, Jeukendrup AE. Caffeine increases exogenous carbohydrate oxidation during exercise. J Appl Physiol. 2005;99(3):844-850.

9. Hulston CJ, Jeukendrup AE. Substrate metabolism and exercise performance with caffeine and carbohydrate intake. Med Sci Sports Exerc. 2008;40(12):2096-2104.

10. Hogervorst E, Bandelow S, Schmitt J, et al. Caffeine improves physical and cognitive performance during exhaustive exercise. Med Sci Sports Exerc. 2008;40(10):1841-1851.

Learning Objectives
After completing this continuing education exercise, the student will be able to:

1. Explain why two common beliefs about caffeine and exercise performance are probably incorrect.

2. Explain why the addition of caffeine to carbohydrate may enhance performance.

3. Cite two reasons why coffee is not an ideal source of caffeine for athletes seeking to enhance performance.

4. Discuss the concept of timing in relation to caffeine intake and performance.

5. Discuss the dose-response concept in relation to caffeine intake and performance and note the preferred dosage to enhance short-duration and endurance exercise.

6. Explain why the risk of dehydration from caffeine use is probably overstated.

Examination
1. Caffeine does not appear to enhance:
a. endurance exercise.
b. short-term, intense aerobic exercise lasting four to eight minutes.
c. sprints lasting less than 90 seconds.
d. prolonged, high-intensity aerobic exercise lasting 20 to 60 minutes.
e. None of the above

2. Of the three mechanisms proposed for the performance-enhancing effects of caffeine, which is the least likely explanation?
a. Decreases perception of effort
b. Increases fat oxidation and spares muscle glycogen utilization
c. Increases force of muscle contractions
d. Increases plasma epinephrine
e. None of the above

3. The study by Cox et al suggests that the beneficial effects of caffeine occur at:
a. 9 to 12 milligrams per kilogram.
b. 6 to 9 milligrams per kilogram.
c. 3 to 6 milligrams per kilogram.
d. 1 to 3 milligrams per kilogram.
e. None of the above

4. Which of the following statements about caffeine is false?
a. Caffeine-containing drinks can provide a significant source of fluid in many athletes’ everyday diets.
b. The diuretic effect is minimal in those consuming caffeine regularly.
c. It increases urinary water losses and contributes to dehydration.
d. There is no evidence in the scientific literature that caffeine intake impairs fluid status.
e. None of the above

5. Ideally, the optimal dose of caffeine is the amount that elicits the greatest performance benefit for the minimum level of risk or side effect.
a. True
b. False

6. The addition of caffeine to carbohydrate may enhance performance by:
a. increasing intestinal carbohydrate absorption.
b. increasing endogenous carbohydrate oxidation.
c. increasing exogenous carbohydrate oxidation.
d. a and c
e. a and b

7. The caffeine content in coffee can vary depending on:
a. brand.
b. variety.
c. method of preparation.
d. serving size.
e. All of the above

8. The Doherty and Smith study suggests that 29% of the variance in the improvement in performance could be explained by caffeine’s effect on:
a. the force of muscle contractions.
b. fat oxidation and muscle glycogen utilization.
c. rating of perceived exertion.
d. serum epinephrine levels.
e. None of the above

9. Determining caffeine’s method(s) of action is complicated because:
a. caffeine enters the central nervous system.
b. caffeine enters peripheral tissues.
c. different mechanisms may also be responsible in different exercise situations.
d. a, b, and c
e. a and b

10. Which of the following statements about coffee is true?
a. It can enhance performance or fail to have an effect.
b. Other chemicals within coffee may counteract the ergogenic effect of caffeine.
c. The caffeine content of coffee is quite variable.
d. None of the above
e. All of the above