OVERVIEW OF CAFFEINE: ITS USE, FUNCTION, AND MECHANISM OF ACTION
Caffeine is a unique substance in that it can be found in common foods, drugs, or dietary supplements. It occurs naturally in the leaves, nuts, and seeds of many plants, or it can be synthesized in the laboratory and used as an ingredient in various products. In addition, there has been an increase in the advertising and popularity of beverages and other products that contain caffeine. Whether natural or synthetic, the active ingredient is methylxanthine, which is a diuretic agent that acts as a cardiac and central nervous system stimulant (19). Caffeine is often used by competitive athletes or active individuals in hopes of enhancing training, gaining a competitive edge, or in some cases increasing metabolism and/or promoting fat loss. Fitness professionals will likely encounter many clients who are using caffeine for daily pleasure or in attempts to improve performance or body composition. It is, therefore, important to understand the physiological effects of caffeine and the efficacy of using the substance as a performance enhancer or metabolism booster.
Caffeine's mechanism of action is thought to be related to the compound adenosine, which is present in all cells in the body. Adenosine aids the body in falling asleep by blunting communication between nerve cells and by dilating the blood vessels to permit increased oxygen flow. Caffeine and adenosine compete for the same receptors in the brain, therefore, reduced adenosine binding caused by increased caffeine availability results in more alertness (19). From a performance perspective, caffeine was once thought to exert an ergogenic effect by enhancing free fatty acid (FFA) release and use by active skeletal muscle, thus, sparing glycogen. More recent data suggest that although FFA release increases with caffeine ingestion, FFA oxidation is not increased nor is glycogen spared. The ergogenic benefit may be related to caffeine's role as a central nervous system stimulant. In endurance activities, caffeine may cause a heightened sense of awareness and decreased perception of effort, whereas in resistance training or short power activities, it may influence motor unit recruitment during skeletal muscle contraction (21).
CAFFEINE AS A BANNED SUBSTANCE
Currently, the National Collegiate Athletic Association has established limits for caffeine use that include disqualification with urinary levels greater than or equal to 15 μg/mL (15). Drinking six to eight cups of coffee a few hours before testing may result in disqualification. Caffeine was removed from the World Anti-Doping Agency list of banned substances and is now on the 2009 Monitoring Program, which monitors substances that are not on the prohibited list to detect patterns of misuse in sport (27). Similarly, the International Olympic Committee moved it from its Prohibit List to Monitoring Programme (13). For both of these agencies, caffeine ingestion from normal diets and beverages should not affect eligibility for competition.
FORMS, DOSES, AND TIMING OF CAFFEINE
Caffeine contents in foods and beverages vary considerably. The Table provides information on caffeine content of some common beverages and products. The major dietary sources are soft drinks, tea, and coffee, which contain approximately 30 to 250 mg per serving (21). Caffeine also is widely available in a capsulated form, in specialty energy drinks, and as an addition to popular sports nutrition products such as sports drinks, gels, and jelly beans/gummy products. Many of these products, especially those marketed toward athletes, will list the caffeine content on the package. On the other hand, because it is currently not required that caffeine content be listed on a food label, many products keep this a mystery to consumers. Limited research studies suggest that caffeine in the form of coffee actually blunts any potential ergogenic effects, but these results are not consistent (21). More research is needed to examine the effects of the various forms of caffeine on performance and to determine if one form of caffeine is superior to another at improving performance.
One of the challenges in reviewing and interpreting the research data is that some studies use absolute doses (milligrams), whereas other studies use relative doses (milligrams per kilogram body weight). The relative doses reported in the literature range from 1 to 13 mg/kg body weight, with a commonly studied dose of 5 to 6 mg/kg body weight (8,11,14,17,20,26). In studies evaluating activities lasting at least an hour, intakes as small as 1 to 2 mg/kg body weight have been shown to improve performance. Although many athletes may believe that "if some is good, more is better," most of the research data examining dose-response effects support a ceiling somewhere between 5 and 6 mg/kg body weight, where no further increase in performance is reported with higher doses (17). From a practitioner's standpoint, it seems prudent to assist an individual in identifying the smallest amount of caffeine that produces a performance benefit without side effects.
The stimulatory effects of caffeine usually peak between 30 and 75 minutes after ingestion. The half-life of caffeine in the body is usually between 4 and 5 hours with modest intakes, but may be longer with doses higher than 300 mg. The body's physiological response to caffeine ingestion also depends on normal use. Some individuals may be very sensitive to caffeine, whereas others are habitual users and have decreased sensitivity or increased tolerance. Some undesirable symptoms include dizziness, headache, jitteriness, insomnia, elevated heart rate, elevated blood pressure, and gastrointestinal distress, which can be experienced with low doses in new users or higher doses in habitual users (usually >500 mg). Generally, individuals who do not ingest caffeine regularly can see physiological and performance effects at doses of 1 to 5 mg/kg, whereas chronic users may need more. Remember, some individuals may be extremely sensitive to caffeine and should start with very low doses on a practice or training day, not an important event day. It may take a few days to a week for a nonuser to begin building a tolerance. In addition, it is not prudent for chronic users to ingest more than 9 mg/kg body weight, and the lowest dose possible to see a performance benefit should be encouraged (21).
Although caffeine use may indeed result in a performance benefit, it also is possible to experience withdrawal symptoms, which can appear anytime between 24 hours and 7 days of nonconsumption. Athletes intending on improving performance with caffeine need to be aware of the potential for withdrawal symptoms that will likely have an adverse effect on performance. If someone wishes to purposefully discontinue caffeine use in an attempt to gain better performance gains as an acute user, a minimum of 1 week is recommended, and gradual tapering of caffeine intake is preferred over abrupt discontinuation (21).
CAFFEINE AND PERFORMANCE
From a small pool of studies, it seems that caffeine ingestion does not enhance performance in endurance activities lasting less than 20 minutes. On the other hand, caffeine has been shown to enhance performance in longer exercise bouts by increasing work output and time to exhaustion in studies examining swimming, running, and cycling (7,9,16,25). The caffeine doses in most of these studies ranged from 2 to 6 mg/kg body weight. Research in anaerobic activities is equivocal. Some studies report improved sprint cycle performance, running speeds, and peak power outputs, with caffeine use, whereas others do not (4,11). Similarly, two studies reported that doses of 6 mg/kg body weight improved sprint speed, power, and accuracy in male team sports (20). More research is needed to determine if caffeine is indeed ergogenic during these types of activities, which specific activities and intensities caffeine may enhance, and the doses that may produce the desired effects.
Some studies examining the effects of caffeine on resistance training performance report increased maximal strength, number of repetitions, total volume of exercise, and reduction in pain and force loss (1,12,14,26). One study did report increased heart rate and blood pressure with caffeine ingestion before resistance training (2). Although data on cardiovascular function are limited, it may be wise to monitor at-risk individualswho use caffeine and participate in a resistance training program.
CAFFEINE AND METABOLISM
Much of the research on caffeine and metabolism has come from studies examining the use of tea or tea extracts, and many of these products have been marketed as metabolism boosters or fat burners. Green tea contains antioxidants called catechins. The most abundant one is epigallocatechin gallate, commonly known as EGCG (22). Several in vitro studies in both animal and human tissue and a handful of animal studies provide plausible evidence that green tea catechins and caffeine work synergistically to enhance thermogenesis. Although studies in humans do exist, the data are less consistent. Most of the researchers reporting on the effects of tea on metabolism are really evaluating some combination of caffeine from green, black, or oolong tea, and the active ingredient EGCG. Some studies measured and reported increases in fat oxidation with ingestion of caffeine and EGCG, whereas other studies actually measured and reported increases in energy expenditure. The studies that did find a positive effect on energy expenditure reported increases ranging from 12 kcal to 175 kcal per day (6,22). Although the upper range ofincreased energy expenditure sounds promising, there have been a limited number of studies published in this area and more data are needed before definitive conclusions and practical applications can be made. Furthermore, the quantities of tea or caffeine/EGCG products needed to enhance metabolism are not established.
CAFFEINE AND BODY WEIGHT/BODY FAT
Many studies examining the effect of caffeine on body weight are difficult to interpret because caffeine is usually combined with another substance (i.e., ephedrine). Recently, there have been studies published examining the effects of tea or the active ingredients EGCG and caffeine on body weight and body fat. Those studies that found positive effects reported increased weight loss, increased fat loss, and decreased waist circumference. Body weight losses ranged from 4.5% to 7% of total body weight (22,24). Although the studies that report positive effects of caffeine use on body weight are scarce, even fewer studies have examined the role of caffeine in weight maintenance and those few have reported conflicting results. Similar to the concept of acute use versus habitual use of caffeine on performance, more data are needed to evaluate if there is a difference in acute versus chronic use on body weight and body composition.
CAFFEINE AND HYDRATION
It is commonly believed that caffeine is a diuretic that will lead to hypohydration (commonly referred to as dehydration). A few studies specifically examining this topic reported that moderate caffeine use seems to have no negative effects on fluid and electrolyte balance. The American College of Sports Medicine Position Stand on Exercise and Fluid Replacement states that "caffeine consumption will not markedly alter urine output or hydration status" (Evidence Category A) (18). As a practitioner, one can share this evidence with your clients and encourage them to ingest adequate fluids with or without caffeine use.
There has been an increase in the availability and advertising of energy drinks during the past few years. Many of these are marketed toward individuals who are fatigued. Although lack of sleep is a common cause of fatigue in many individuals, including athletes, there are other potential causes of fatigue in athletes and active individuals. Examples include insufficient energy intake, insufficient carbohydrate intake, hypohydration, iron-deficiency anemia, other anemias, vitamin/mineral deficiencies, and overtraining. Although individual products vary, many energy drinks contain 28 to 35 g carbohydrate, 110 to 140 kcal, and 75 to 80 mg of caffeine per 8-oz serving. Most cans or bottles have more than one serving per can. Caution should be used with any product that does not provide the caffeine content, as too much caffeine may cause overstimulation that can be detrimental to performance. It also is important to educate athletes on the various causes of fatigue. Altering the perception of fatigue with high levels of caffeine will not make up for other causes of fatigue that may be contributing to poor performance.
CAFFEINE AND AGE
Years ago, many health professionals believed that caffeine may have an adverse effect on cardiac health, particularly in those with existing risk factors or disease. Recent reviews reported that moderate caffeine use in adults does not increase risk of myocardial infarction, and there are not conclusive data supporting adverse effects of caffeine on the cardiovascular system (19,21). Although there are several studies examining caffeine use and effects in adults, data in children are limited. The few studies that have been published in this area report that caffeine does not seem to have an effect on energy expenditure in children, but increased blood pressure and decreased heart rates were observed (23). Because children seem to be consuming large quantities of caffeinated soft drinks and energy drinks, fitness professionals working with children should be aware of the potential physiological effects.
COULD PERCEPTION AFFECT PERFORMANCE?
The placebo effect is a common phenomenon where an outcome can change based on the belief that one has received a treatment. In the case of caffeine, if one believes that caffeine enhances performance, and an athlete believes he or she has ingested the substance, performance may improve. One study reported a dose-response placebo effect (5). Another reported that individuals who were chronic users of higher doses of caffeine (>200 mg/day) perceived more positive effects of caffeine compared with individuals who use less than 200 mg/day (3). A recent study on cycling trials was conducted in individuals who believed caffeine enhanced performance. The effectiveness of caffeine was further confirmed during these trials where caffeine ingestion improved performance, even though subjects were told they had not ingested it. There was a negative performance effect when subjects were correctly informed that they were not given any caffeine before the cycling trial (10). More studies examining this topic are needed before any practical applications can be made.
SUMMARY AND PRACTICAL APPLICATIONS
Caffeine can be found in various foods, beverages, and other products. It is often used in hopes of improving performance or increasing metabolism. Caffeine probably exerts its effects through stimulation of the central nervous system. It may cause an increased sense of awareness, decreased perception of effort, or may play a role in cellular level skeletal muscle contraction. These proposed physiological mechanisms may be the explanation for the studies that do report improvements in endurance performance, anaerobic performance, or resistance training activities (21). Studies examining caffeine and exercise performance used doses ranging from 1 to 13 mg/kg body weight, with a commonly studied dose of 5 to 6 mg/kg. Although moderate caffeine use seems to be safe, intakes greater than 500 mg/day or 9 to 13 mg/kg body weight per day may result in undesirable side effects.
Caffeine affects the body differently for acute and habitual users. Acute users tend to be more sensitive to caffeine, whereas habitual users have built a tolerance. Individuals wishing to try caffeine as an ergogenic aid should do so during practice or training, not the day of an important event and should start at lower doses of 1 to 2 mg/kg body weight. These lower doses have been shown to improve performance in some individuals. Habitual users who wish to discontinue caffeine use and then reintroduce the compound to improve sensitivity should do so a minimum of 1 week before the event day and gradual reduction of intake is preferred to abrupt discontinuation (21).
Caffeine peaks between 30 and 75 minutes after ingestion and has a half-life of 4 to 5 hours. Therefore, caffeine ingestion should be timed to coincide with the particular activity. During events lasting several hours, redosing may be beneficial for a continued desirable effect (21).
By itself, caffeine does not seem to have a positive effect on thermogenesis as it does when combined with EGCG. Some studies show increased fat oxidation, energy expenditure, and weight loss with use of green tea beverages or substances. Most of the increases in daily energy expenditure are modest and, therefore, consumption of these products should not be used as a substitute for overall energy balance through proper food intake and exercise. More data are needed to determine recommended amounts of consumption and effects on metabolism and body composition.
Although caffeine may be a performance-enhancing substance, more research is needed to answer some practical questions. Most research studies are conducted in a laboratory setting and the application to a field setting is unknown. Because of the variability of the research data, it is difficult to make absolute conclusions for the optimal form, dose, or timing of caffeine to maximize performance. Fitness professionals should encourage their clients to identify and consume the smallest amount of caffeine that will produce the desired performance benefit with minimal side effects. Minimal doses also will help diminish withdrawal symptoms upon cessation.
CONDENSED VERSION AND BOTTOM LINE
There has been an increased availability of products containing caffeine. Many individuals consume caffeine in attempts to improve performance or boost metabolism. Some research data support the use of caffeine as an ergogenic aid. For maximum safety, the fitness professional should encourage using the smallest effective dose. Although caffeine, when combined with epigallocatechin gallate, may increase energy expenditure, the values are small and, therefore, caffeine intake may play a very small role in weight management.
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