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Myth buster: caffeine does not exhibit a diuretic effect during exercise performance.

Research on the physiological effects of caffeine as it relates to athletic performance is extensive. While caffeine has been well established to enhance endurance performance (1-7), its impact on short-term high intensity exercise is much more controversial (8-13)' Nonetheless, researchers continue to explore caffeine's effect on performance in different environments and under different exercise conditions. Recently, during the Games of the XXIX Olympiad in Beijing, China, athletes experienced harsh environmental conditions such as poor air quality, high ambient temperatures and high humidity. Furthermore, there are various parts of the world where athletes live in such environments and thus are exposed to high temperatures and humidity during regular training and competition.

Multiple mechanisms have been proposed to explain the effects of caffeine supplementation on exercise performance. One of caffeine's primary sites of action is the central nervous system (CNS). Caffeine can easily cross the blood brain barrier as well as cellular membranes of all tissues in the body (14,15). For this reason it is difficult to determine in which physiological system caffeine has the greatest impact. While the exact mechanism related to caffeine's ergogenic effect on exercise is unknown, it has been shown to alter intramuscular pH, muscle force production, central fatigue and metabolic function (16-20). Thus, it would appear that the ergogenic impact of caffeine on exercise performance is due to its effect on both central and peripheral systems. Further, it is likely that a combination of mechanisms (e.g. adenosine receptor antagonist in addition to caffeine's effect on neuromuscular function) may be operating at the same time.

One of the pharmacological actions of acute caffeine administration is an increase in urine output from the kidneys (21). It is well established that large losses of body water impose significant stress on the circulatory system and can significantly decrease exercise performance. Research has suggested that a 1-2 per cent decrease in body weight adversely alters normal heart rate and core temperature responses to exercise. A 3 per cent weight decrease results in significant decrements of cardiovascular endurance performance and aerobic function (V[O.sub.2max]). Strength and power reduction occur when body weight loss reaches 5 per cent. Further, without adequate fluid replacement, the risk of heat illness increases as dehydration progresses (22). Athletes who train and/ or compete in environments with high heat and humidity will experience higher sweat rates and thus greater water loss. It is possible then, that if caffeine exacerbates water loss, athletes who ingest this substance while exposed to such an environment will be at an even greater risk for heat related illnesses.

Because of its diuretic potential, nutrition experts, exercise physiologists and clinicians often advise athletes to abstain from ingesting caffeine or caffeinated beverages prior to exercise (23,24). Investigations examining the potential diuretic effect of caffeine and its subsequent impact on exercise performance are lacking, however, it has been suggested that caffeine reduces heat tolerance during exercise in a hot environment through the following three physiological mechanisms: (i) the diuretic effect of caffeine may augment the decreases that already occur with plasma volume and stroke volume; (ii) since it has been established that caffeine stimulates the sympathetic nervous system, this stimulation may further increase sweat rate; and (iii) the metabolic rate of an individual is increased following caffeine ingestion. This may increase heat storage and raise body temperature. Thus, it is plausible, that these effects of caffeine ingestion reduce heat tolerance by exacerbating dehydration and increasing body temperature (25,26).

Since caffeine is the most widely consumed drug in the world (27,28) and is one of the most commonly consumed substances by athletes in attempt to enhance performance (29), it is pertinent to (i) investigate the effect of caffeine on exercise performance in environmental conditions where water losses are inherently greater, and (ii) examine settings in which the ingestion of caffeine could be detrimental due to the environmental condition itself.

The following points should be considered when evaluating the literature examining the effects of caffeine-induced dieresis: (i) Are the results reported based on resting data? If so, it is inappropriate to expect these findings to be relevant to exercise performance; (ii) Are the subjects exercising in a thermoneutral environment, or under hot and/or humid conditions.? It is possible that the diuretic effects of caffeine may be different in these two environments. While there does appear to be an argument for caffeine-induced diuresis at rest, research does not support any significant effect of caffeine on sweat loss and thus fluid balance during exercise that would adversely affect performance. Thus, restricting dietary intake of caffeine is not scientifically and physiologically supported (26,30).

In this issue, Ping and colleagues (31) investigated the effect of acute caffeine consumption on cardiorespiratory responses of heat acclimated recreational male runners in a hot and humid environment. In a well designed study, they reported a significant increase in run time to exhaustion at 70 per cent V[O.sub.2max] following acute caffeine ingestion compared to the placebo trial. In addition, heart rate, core body temperature, oxygen uptake and Rating of Perceived Exertion (RPE) were not significantly different throughout the exercise bout during the caffeine trial compared to placebo. Since literature examining the effects of caffeine in a hot and humid environment is lacking, the findings of Ping and colleagues add to the literature in this area. Prior to the current investigation, only six studies have examined the impact of environmental temperature on caffeine effects on heat tolerance (32-37). All six investigations, including the present one, concur that caffeine intake, when compared to a control condition, does not increase heat storage during exercise, nor does it adversely affect performance in a hot and/or humid environment (26).

In summary, given the sparse data in this area, future investigations should include non-heat acclimated subjects, since athletes from more temperate conditions are often required to compete in hot and humid environments. The impact on habitual caffeine users may also render different results since the pharmacokinetics of habitual users is different compared to caffeine naive subjects. Furthermore, the initial hydration status of the individual may be important when addressing the potential caffeine-induced diuresis and the impact this may have on heat tolerance and subsequent exercise performance.

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Felicia Greer

Department of Kinesiology

California State University, Fresno

5275 N. Campus Drive M/S SG 28

Fresno, California 93740-8018, USA

fgreer@csufresno.edu
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Title Annotation:Commentary
Author:Greer, Felicia
Publication:Indian Journal of Medical Research
Geographic Code:9INDI
Date:Jul 1, 2010
Words:2016
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