Coffee is considered the most popular beverage in the world, and many believe that the caffeine in the drink is what makes it most appealing. The amount of caffeine in coffee varies according to the roasting and brewing technique. Caffeine is only one of many chemicals in coffee; however, it is caffeine that is most recognized by the general public. It can also be found in a variety of other beverages and is frequently an ingredient in food products and medications (cola and other soft drinks, chocolate bars, and medications such as NoDoz and Vivarin). It is well known that coffee and other caffeinecontaining beverages and foods have worldwide social acceptance. Additionally, caffeine is a component of many OTC energy-enhancement products.
Internet Resource Box
A simple and quick reference system to determine the caffeine content in foods and drugs is found at http://www.cspinet.org/new/cafchart.htm.
For many years, caffeine has been documented as having an ergogenic benefit for endurance athletes.4,6,9,13,20,21 The enhancement effect of caffeine on endurance performance is being studied even today, and different theories exist as to why it might be advantageous to the athlete. Spriet outlines the different research lines being followed on the physiology of how caffeine may have an ergogenic effect on the athlete.30 It is important to keep an open mind and realize that future research on the enhancement effects of caffeine may take us in a totally new direction.
Many suggest that the research that inspired this curiosity about the ergogenic effects of caffeine dates back more than 20 years, when a group of researchers was exploring the effect of caffeine on endurance performance.6,9,20 Early studies were the basis for the long-held hypothesis that caffeine increases fat metabolism. Caffeine, through one or more metabolic processes, increases the free fatty acid (FFA) concentration in the blood, providing an available energy source in the bloodstream, which spares muscle glycogen during exercise. This glycogen sparing in the muscle was thought to be the reason for the test subject's ability to increase cycling endurance by as much as 20 percent.
After these reports, other researchers began to investigate the reason or reasons why caffeine might be ergogenic in nature, and specifically how it affected fat metabolism in the body. Consequently, many research reports have been published in the last 20 years investigating the mechanism that caffeine uses to produce an increase in athletic performance. However, even the most recent published research does not explain the specific mechanism by which caffeine has an ergogenic effect on athletic performance.4,7,19,32,36 As mentioned, the ability of an athlete to maintain a high level of performance through the use of caffeine supplements has been verified by numerous authors. Figure 12–1 reveals that endurance performance is improved in cycling and treadmill efforts by athletes working at about 80 to 85 percent of their maximum oxygen consumed during each minute of near-maximal exercise (VO2max).
Mean performance time for subjects running and cycling to exhaustion (caffeine and placebo). Reprinted with permission from L.L. Spriet, 1995, Caffeine and Performance, International Journal of Sport Nutrition, 5: S88.
The following section reviews the various ergogenic mechanisms of caffeine that have been postulated. The reader will have the opportunity to consider the results of the major research published on caffeine as a performance enhancement product. Readers particularly interested in this topic should review articles published by Nehling (184 references),26 Spriet (56 references),30 and Hawley (160 references)19 for excellent reviews of the literature and discussions of related research.
Caffeine belongs to the xanthine class of drugs. The xanthines, as one of their actions in the body, act as a CNS stimulant. Caffeine is 99 percent bioavailable, and plasma concentrations peak within 15 to 45 minutes after oral ingestion. Its half-life has been reported to be 3.0 to 7.5 hours in nonexercising adults. The xanthines (e.g., theophylline) are sometimes prescribed by a physician to reduce the incidence of exercise-induced bronchospasm. However, caffeine is not the drug of choice for control of exerciseinduced bronchospasm.
There are three different theories that attempt to explain why caffeine has an ergogenic effect on endurance activity in the athlete. All three theories have subtheories, which make it difficult to discern why caffeine exhibits an ergogenic effect on endurance performance.
The metabolic theory postulates increases in free fatty acids and catecholamines, which decreases the use of muscle glycogen. One of the effects of caffeine ingestion is an increase in circulating epinephrine levels, which can lead to an increase in free fatty acids (FFAs) in the bloodstream. This could be an argument for enhanced endurance capability in the athlete. However, it is also documented that exercise in general increases circulating epinephrine levels.
The neurologic theory suggests that caffeine activity affects the perception of effort at the central nervous system level and reduces the athlete's ability to perceive exhaustion. The muscular theory postulates that caffeine has a direct effect on the skeletal muscle via cellular calcium activity during muscle contraction. The general principles of these three theories are presented in the following paragraphs with a brief overview of the history and research supporting each theory.
A common finding in the earlier endurance-related studies found that caffeine enhances fat oxidation during exercise.5,6,9 In the metabolic theory, the mechanism of action for caffeine to improve endurance performance is by enhancing fat oxidation in the body. This results in an increase in FFAs in the blood. These FFAs are circulating and can be broken down for use as energy by the muscles. When this happens, the enzymes that metabolize carbohydrates are also inhibited.30
Because of this increase in FFA mobilization throughout the body, available muscle glycogen stores are spared during exercise. More total energy can be derived from a gram of fat than from a gram of carbohydrate; therefore, during aerobic exercise, the body prefers to gain its energy source from fat stores.24 It is also known that fat can be stored in the body in much greater quantities than carbohydrates. If more fats are used for energy during aerobic exercise, it is hypothesized that: (1) overall energy production for the body is greatly increased, and (2) muscle glycogen stores are spared so that the system is able to utilize these fuel sources at a later time. When fats are used as an energy source for the body, a greater amount of oxygen is needed to liberate the fat from its storage site. Therefore there is less oxygen for use by the cells, which alters the utilization of the available oxygen in the body during exercise. This is one reason why the metabolic theory is limited in applicability to aerobic exercise (i.e., to times when oxygen is plentiful in the body during exercise).
The metabolic theory has not been as widely accepted recently as it had been in the 1990s as a result of follow-up studies by other researchers who were not able to specifically identify the ergogenic effect of FFAs on the endurance performance of athletes. Specifically, subsequent research projects were not able to positively discern the muscle glycogen-sparing effect of increased FFAs in the bloodstream.4,14,21,36 The process of muscle glycogen sparing in endurance performance activities has been a basic question for many researchers since the initial metabolic theory information was published. Other researchers have two fundamental questions regarding the effects of caffeine on athletic performance. First, is there an increase in FFAs in the bloodstream with caffeine ingestion? Second, if muscle glycogen is spared in this process, what effect does that have on endurance performance by athletes? The common thought is that aerobic activity is fueled by FFAs and glycogen is not a primary fuel source in endurance activity. If this is true, what does sparing of glycogen do for the system in an endurance activity? Many of the studies published in the late 1990s and after indicate the possibility of a CNS link to the ergogenic effect of caffeine on endurance performance.
Much of the recently published research suggests that caffeine supplementation has an effect on the central nervous system. Specifically, it masks the effects of fatigue.3,4,21 Different researchers have speculated on how this is done. Bruce et al. suggest that the masking of fatigue occurs in the CNS via alterations in neurotransmitter function or possibly by overruling fatigue signals to the brain during exercise, thus not providing the athlete's brain with the correct information regarding the true level of fatigue.4 Figure 12–2 demonstrates exercise to exhaustion of well-trained athletes who were given three different dosages of caffeine. The athletes ran at approximately 85 percent of VO2max and there was no significant difference in the time to exhaustion for any of the doses of caffeine administered.
Exercise to exhaustion in well-trained athletes with 0 caffeine in their system and 3, 6, or 9 mg/kg of caffeine administered before exercise. The 3, 6, and 9 mg/kg doses are significantly different from the 0 dose, but not significantly different from each other. Reprinted with permission from Graham, TE & Spriet, LL, Metabolic catecholamines and exercise performance responses to various doses of caffeine. J Appl Physiol 78(3):867–874, 1995.
It could also be reasoned that caffeine binds to adenosine receptors in the brain and other parts of the CNS. Normally, when adenosine binds to adenosine receptors, the result is drowsiness and a concomitant dilation of blood vessels (this dilation allows increased oxygen delivery to the brain during sleep). Caffeine looks like adenosine to a nerve cell and will attach to the adenosine receptor, but it has the opposite affect of adenosine in the CNS. Caffeine, in essence, fools the CNS and does not allow the system to become drowsy. Therefore the system remains stimulated and functions at a higher level of activity for a much longer period of time when, in fact, the muscles are tired and ready to slow down.26 Figure 12–3 shows a typical perceived exertion chart that is used when someone is exercising to exhaustion. Charts like this are a way for exercising athletes to let the examiners know how tired they are getting. Perceived exertion may not match actual time to exhaustion when stimulatory supplements are being used.
Example of a perceived exertion chart.
During the late 1990s, a number of authors suggested that caffeine has a direct effect at the cell level in producing its ergogenic benefit.4,21,32 Recently, Tarnopolsky published a study that concluded that there was a direct link between the ergogenic effect of caffeine and the ability of the muscle to perform endurance activity.33 The subjects in this study were able to increase the contractibility of their muscles with as little as 6 mg/kg of caffeine ingestion. In this study of skeletal muscle force, it was concluded that the endurance effect of the caffeine is via a potentiation of an increase in the calcium release from the sarcoplasmic reticulum. This increase in calcium enhances the ability of the muscle to contract over longer periods of time, thus providing an increase in endurance performance during aerobic activity.
Implications for Activity
Caffeine is a common substance that can easily be obtained by the athlete. Exactly how caffeine works to enhance the athlete's level of performance is not clear at this time. If an athlete is looking to increase his or her exercise endurance, caffeine might be helpful if consumed properly. It is important for athletes to understand that the use of caffeine can hinder their fine motor skills, which may be detrimental to their performance if they need to perform fine motor tasks. Like any drug, caffeine produces adverse effects, and consuming too much caffeine or obtaining it from an unfamiliar source can cause an athlete to feel ill. As a result, the athlete's performance may actually be decreased.
Endurance vs. High-Intensity Exercise
Originally, the majority of the research on the ergogenic effect of caffeine was directed at increasing performance during endurance activities (e.g., activities lasting an hour or more). Trice and Haymes concluded in 1995 that caffeine can have a glycogen-sparing effect during high-intensity moderate-length (30 minutes) stationary cycling.34 They reported a 29 percent increase in stationary cycling time to exhaustion. The 29 percent increase was attributed to an increase in plasma FFAs, indicating an increase in fat mobilization. In 1996 Jackman et al. published a study that indicated caffeine can be ergogenic in intense activity (lasting approximately 5 minutes).21 Jackman et al. did not indicate that the caffeine had a glycogen-sparing effect, but they did suggest that there may be a CNS or muscular connection to the increased performance after caffeine ingestion.21 Similarly, Doherty determined similar findings regarding the effect of caffeine on brief, intense activity. The author suggested that the improvement in short-term, high-intensity treadmill running appeared to be from caffeine either having some type of influence on the CNS or acting directly on the muscle cell.7 Shortly thereafter, other studies were published that contradicted these results, challenging the general theory that caffeine provides an ergogenic effect during short-duration exercise.2,17 Essentially, the most recent evidence suggests that caffeine does not provide any ergogenic assistance to the athlete during brief, high-intensity activities.28,35 Figure 12–4 demonstrates the results of a study in which anaerobic capacity was measured over three different bouts of exercise on a stationary bicycle with caffeine and creatine ingestion by trained athletes.
Anaerobic capacity measured for three different bouts of exercise on a stationary bicycle with caffeine and creatine ingestion by trained athletes.
Habitual Caffeine User vs. Nonuser
The effect of caffeine in the habitual user was questioned early in the search for the most efficient use of caffeine as an ergogenic aide. Tarnopolsky outlined much of the early research and questioned the physiologic responses to exercise of the habitual caffeine-consuming athlete.33 Earlier results indicted that habitual caffeine users did not experience a metabolic or neuromuscular benefit during exercise following the consumption of caffeine.33 Nor were any detrimental effects from the caffeine ingestion noticed in the performance of the subjects. However, in his most recent work on the subject, published in 2000, Tarnopolsky concludes that there is an increase in muscle contraction in both habitual users and nonusers of caffeine. It now appears that caffeine ingestion can be ergogenic to both the regular user and the nonuser of caffeine. This is definitely an area that will be researched in much more detail in the future. Figure 12–5 demonstrates that cyclists in endurance-to-exhaustion tests show a similar result to that in runners.
Endurance performances. Cyclists' performance to exhaustion with three different dosages of caffeine. The 0 represents no caffeine in the cyclists and the 5, 9, and 13 are doses of caffeine in mg/kg. The caffeine doses are significantly different from the no-caffeine group, but there is no significant difference between the caffeine doses. Adapted with permission from Pasman et al: The effect of different doses of caffeine on endurance performance time. Int J Sports Med 16, 1995.
Athletes using different sources of caffeine seem to have various results in their ability to maintain exercise endurance. Capsules appear to be slightly more effective than beverages in producing this change.14 The reason why capsules appear to be more effective is also unknown. See Figure 12–6 for a comparison of types of caffeine ingestion and time to exhaustion during exercise.
Types of caffeine ingestion and time to exhaustion during exercise. Healthy males exercised to exhaustion on a treadmill after ingesting the equivalent of 4.45 mg/kg caffeine in tablet form (CAF), decaffeinated coffee (DC), decaffeinated coffee with caffeine added (DC+C), regular coffee (RC), or a placebo (PL). Caffeine tablet ingestion produced a longer time to exhaustion than the other treatments. Reprinted with permission from Graham, Hibbert, et al.: Coffee, caffeine, and metabolism. J Appl Physiol 85:833, 1998.
Athletes need to understand that some of the supplements they can use might contain both caffeine and ephedra.
What to Tell the Athlete
The majority of published research in the past 20 years supports the premise that caffeine can have an ergogenic effect on endurance performance in athletes. From a research standpoint, there appears to be a combination of events that may lead to the ergogenic effect of caffeine on the athlete. Some tips for the athletic trainer when educating the athlete about these drugs include:
High levels of caffeine can be detected in the urine.
Caffeine is a "restricted" substance according to the NCAA and the USOC.
The level of caffeine in the system needed to result in disqualification would require an average athlete to consume six regular-size cups of drip or percolated coffee about 1 hour before competition.
Lower doses of caffeine, such as 5 mg/kg, appear to provide an ergogenic effect to the endurance athlete.27 To translate this amount into ounces, an athlete weighing 70 kg (154 lb) could consume approximately 20 oz of brewed coffee (135 mg/8 oz) to gain an ergogenic effect.
It appears that caffeine capsules are more effective than coffee or other highly caffeinated beverages.14
Quite possibly, more is not better in the case of caffeine as a supplement.
Scenario from the Field
A 21-year-old amateur bicycle racer decided to use caffeine as an ergogenic aid before a 50-km road race. The racer thought that if he drank a large amount of coffee, it would provide a hydration base and the caffeine necessary for an ergogenic effect. He consumed five large cups of coffee starting 1 hour before and finishing the last cup just before the race started. He did not eat very much before the race, anticipating that the caffeine would be more effective on an empty stomach. After the race, he explained that during the race he felt that the caffeine was providing an ergogenic benefit, but he did not feel well enough to capitalize on the advantages the caffeine was supposed to provide. The cyclist suffered from severe stomach pains, and his bladder was extremely active during the race (caffeine has a diuretic effect). This amateur racer had a slower time than in his previous races and was nauseated when he finished.
Does the type of caffeine that an athlete consumes make a difference? (Some familiar types include coffee, soda pop, chocolate bars, No-Doz, Vivarin, and diet pills.)
By taking ephedra, will an athlete lose weight or get an increase in energy?
What are the associated factors one must watch carefully when taking ephedra that can lead to a dangerous, possibly fatal outcome?
What are the newest kinds of stimulants being used by athletes today?
Stimulant drugs provoke a stimulating action on the central nervous system (CNS) through a number of different actions.
Drug manufacturers sometimes combine ephedra and caffeine in OTC products because both are stimulatory in nature.
The ma huang plant is harvested and processed into two types of stimulants: ephedra and pseudoephedrine.
Ephedrine is argued to be an effective CNS stimulant, but in scientific studies it does not improve athletic performance.
The production of herbal supplements does not have to conform to the regulations of the FDA, so the final product may vary in concentration of the included drug.
Numerous athletes in the 2002 Winter Olympics tested positive for banned drugs after consuming approved energy drinks containing supplements.
There are three main theories as to why caffeine has an ergogenic effect on athletic performance: metabolic, neurologic, and muscular.
Caffeine appears to be more ergogenic for the endurance athlete.
Athletes using caffeine as a performance enhancer do not have to abstain from caffeine at other times for it to be ergogenic.
Certain urine levels of caffeine in athletes can present a problem because caffeine is categorized as a restricted substance by the National Collegiate Athletic Association and the United States Olympic Committee.