Caffeine is the most widely used behaviorally active substance on earth. In the United States, 89%-96% of men and nonpregnant women (age, >18 yr) consume caffeine; their mean ± SD daily intake of 238 ± 61 mg·d−1 derives from coffee (71%), tea (16%), and soft drinks (12%) (13). Approximately 20%-30% of Americans consume more than 600 mg of caffeine daily (22).
Because of its widespread use as an ergogenic aid and its ubiquitous availability, caffeine is no longer on the banned substance list of the International Olympic Committee. The general consensus of research findings indicates that caffeine improves continuous exercise time to exhaustion. This effect increases as the duration of the event exceeds 30 min, but during incremental exercise protocols (8-22 min) or sprints (<90 s), caffeine does not enhance performance (28). A few discordant findings (i.e., involving intense exercise lasting 5 min and maximal muscular power) suggest that different caffeine-induced effects may be at work in different types of exercise. The exact mechanism of this ergogenic effect has not been identified (28), but plasma epinephrine rises after caffeine intake independent of plasma norepinephrine that may or may not rise. Future research may determine that caffeine alters intramuscular pH, muscle force production, central fatigue, or tissue glycogen concentration. However, it is evident that caffeine has wide-ranging physiological effects on the sympathetic nervous, muscular, endocrine, cardiovascular, pulmonary, and renal systems (7).
The widespread belief that caffeine exerts a diuretic effect prompts segments of the medical, exercise physiology, and nutrition communities to recommend that caffeine not be consumed before or during exercise (28). This produces a theoretical question in the minds of athletes and exercise enthusiasts. If caffeine is consumed to improve performance or training, will it induce dehydration that counteracts its ergogenic properties?
Although scientific data are extremely limited, it might be inferred that caffeine reduces heat tolerance during exercise in a hot environment, via three physiological mechanisms (2). First, the diuretic effect of caffeine may exaggerate the declines that occur with plasma volume and stroke volume. Second, caffeine stimulates the sympathetic nervous system, and it may increase sweat rate. Third, caffeine increases resting metabolic rate in physically trained and sedentary individuals; this may increase heat storage and internal body temperature. Theoretically, these effects reduce heat tolerance (i.e., the exercise time to fatigue or exhaustion) by exacerbating dehydration and increasing body temperature.
This article will do the following: a) summarize studies that have evaluated the influences of caffeine intake on body water homeostasis, electrolyte balance, and thermoregulation - all of which affect exercise performance; and b) ascertain whether abstaining from or reducing dietary intake of caffeine is scientifically and physiologically supported. We hypothesize, contrary to popular beliefs, that caffeine consumption, in moderation, does not induce water and electrolyte imbalances or hyperthermia.
Diuretics are substances that promote the formation of urine within the kidneys. Water, cranberry juice, and alcohol are weak diuretics. Caffeine (in coffee, tea, and soft drinks) and theophylline in tea stimulate increased renal glomerular filtration and inhibit reabsorption of sodium (Na+) within nephrons, thereby stimulating an increased Na+ and water excretion. The diuretic effect of caffeine at rest has been recognized since 1928 (10). Although this landmark study has influenced practitioner beliefs for decades, it involved only three subjects, and the observations spanned only a few hours.
When physicians, physiologists, and dietitians recommend that caffeinated beverages not be consumed before or during exercise (16,28), they rarely cite research findings to support their assumptions. Opposing this viewpoint, other professionals view caffeine as a mild diuretic that poses no harm to health or exercise performance (7,9,26). In fact, three previously published review articles (2,20,21) concluded that caffeinated beverages and water affect body water balance similarly during exercise.
Table 1 considers the diuretic effect of caffeine versus a control fluid (i.e., water or placebo) as published in 15 different research studies; seven involved exercise. Specifically, this table considers the amount of caffeine and the volume of fluid consumed; all investigations are ranked on the basis of caffeine dose. All studies in which caffeine doses were less than 226 mg (see rows 16-23 in Table 1) reported no statistical difference. As shown in column 3, a significantly greater within-study diuresis occurred in 6 of 23 total experiments when caffeine (vs control) was consumed at doses ranging from 240 (25) to 642 mg (22). These doses are equivalent to approximately 1.5 to 4.3 cups of brewed coffee (150 mg of caffeine/150 mL) and approximately 5 to 13 cans of cola soft drink (50 mg of caffeine/360 mL). No difference was reported in 17 experiments (Table 1), when up to 553 mg of caffeine was consumed. Furthermore, there seems to be no relationship between the volume of fluid consumed (column 4) and the appearance of a significant diuresis because of caffeine (column 3).
The duration of observations (i.e., 1 h to 11 d) is important when considering column 1 because 73% of these 15 studies involved acute measurements (1 h to 6 h). Brief observation periods may result in considerably different conclusions than longer periods (16 h to 11 d) because human water balance is challenged by diuresis several times each day. Acute diuresis per se must be distinguished from chronic depletion of body water. That is, diuresis subsequent to caffeine consumption (column 3) does not necessarily mean that 24-h or weekly total body water turnover results in dehydration or hypohydration; neuroendocrine mechanisms regulate fluid volume successfully as indicated by plasma osmolality across a wide range of 24-h fluid intakes (i.e., 1.7-7.9 L·d−1 (23)) and urine volumes. Indeed, the only investigations in Table 1 to observe chronic effects of caffeine consumption (3,12) found no indication of dehydration in numerous indices of hydration status. Thus, although some experiments reported that caffeine caused diuresis exceeding that of water, a) no evidence of chronic dehydration exists in the scientific literature, and b) restricting dietary intake of caffeine is not scientifically and physiologically supported. This is compounded by the lack of a universally accepted "gold standard" that serves to assess hydration state.
Figure 1 provides another method to analyze the influence of caffeine on body water balance. If the ingested volume of a fluid exceeds the resulting urine volume, the difference represents the volume that is retained by the body during the observation period. This figure depicts water retention as a positive value and net fluid loss as a negative value. These values differ considerably among studies because of differences of experimental protocols and fluid volumes ingested. When caffeinated beverages (gray bars) are compared with control fluids (e.g., water or placebo (black bars)), the mean values for fluid balance (fluid intake-urine volume) are not statistically different (mean ± SD: caffeine, 994 ± 1839 mL; placebo, 1068 ± 1707 mL); if the 72-h study by Fiala et al. (12) is removed from Figure 1, the values remain statistically similar (P > 0.05), but the means and SD are smaller (caffeine, 511 ± 625 mL; placebo, 624 ± 618 mL). We interpret this to mean that caffeinated beverages, in general, contribute to euhydration similarly to water.
For each experiment, the reader should note that the duration of observations is labeled in hours (h) or days (d) and that observations of exercise (E) and rehydration (R) are annotated at the end of each bar. Thus, only four studies involved observations greater than 4 h, three studies observed fluid turnover during exercise, and three studies observed the effects of rehydration after exercise. These underpowered categories do not allow conclusions to be drawn about specific time, exercise, or rehydration matters.
The reader also should note that the hydration state, and the amount of fluid consumed during the previous 12 h, of test subjects was generally not controlled in these studies. This suggests that the negative values in Figure 1 (i.e., representing a net fluid loss) result from overhydration before experiments began, because even placebo trials resulted in a net fluid loss.
Table 2 compares the urinary excretion of sodium (Na+) and potassium (K+) while consuming either caffeine or a control (i.e., one fluid or total diet); eleven distinct experimental treatments (8 studies) are depicted. A large range of Na+ and K+ losses occurred (columns 2 and 3); this range likely stems from the varied electrolyte contents of the fluid consumed by the subjects (column 1).
A greater (P < 0.05) acute natriuresis (column 2) was reported after caffeine intake in 7 of 9 acute experiments (duration of 2-4 h). This is consistent with the physiological action of caffeine, inhibiting reabsorption of sodium (Na+) at renal nephrons. However, the lone chronic study (6 or 11 d of controlled caffeine intake) reported no statistical differences between 0-, 226-, and 452-mg conditions in 24-h pooled urine collections (3). As with acute urinary water loss, Na+ excretion may increase acutely when caffeine is consumed, but daily Na+ balance is positive; this indicates that human Na+ regulatory mechanisms successfully defend whole-body exchangeable Na+.
Regarding K+ excretion (Table 2), the data in column 3 suggest that kaluresis is similar in virtually all water and caffeine trials. One investigation (3) controlled dietary caffeine intake at 452 mg·d−1 for six consecutive days. The 19 subjects exhibited a mean 24-h K+ excretion (column 3) that was within 2% of the mean level of 20 subjects who consumed 0 mg of caffeine.
The adequate intake value published by the National Academy of Sciences of the United States is 204 mEq Na+ per day, for male and female subjects older than 14 y (23). Thus, an adequate diet provides ample sodium to cover the urinary losses reported in Table 2 (125 or 140 mEq·d−1). Also, the 24-h K+ losses after caffeine intake (column 3) are similar to the adequate intake value for K+ (23). Thus, the available research on Na+ and K+ excretion does not support abstaining from or reducing dietary intake of caffeine.
Interestingly, the studies conducted by González-Alonso et al. (14) and Brouns et al. (5) indicate that electrolyte losses are acutely reduced when caffeine is consumed in a fluid that contains electrolytes and carbohydrates during a 2-h postexercise recovery period. Although not significantly different, a trend existed in 3 of 4 experiments (Table 2, columns 2 and 3) for the total urinary Na+ and K+ losses to be greater during water trials. This possibility deserves further research and may involve reduced glomerular filtration during exercise.
TEMPERATURE REGULATION AND EXERCISE-HEAT TOLERANCE
Although few studies have evaluated the influence of environmental temperature on caffeine effects or on exercise-heat tolerance, Table 3 presents such data from studies conducted in mild-to-hot environments. Regarding temperature regulation, rectal and skin temperatures (columns 3 and 4) represent heat storage in central and peripheral tissues. Metabolic rate and sweat rate (column 5) relate to heat production and heat loss, respectively. Heart rate reflects circulatory strain during heat exposure, and exercise duration is a classic index of heat tolerance.
The six studies in Table 3 present a unified message: caffeine intake (vs control) exerts little or no influence on human thermal balance, circulatory strain, and exercise time to exhaustion. Thus, restricting dietary intake of caffeine is not scientifically and physiologically supported. This conclusion includes running, cycling, and prolonged walking in mild-to-hot environments. Despite these results, most of these studies provide low statistical power because of small sample size; some did not use subjects as their own controls. Additional studies should examine exercise in a hot environment.
PHYSIOLOGICAL AND PSYCHOLOGICAL FUNCTION
Table 4 presents a summary of the effects that caffeine imposes on the human body. Clearly, caffeine has multiple effects, but this does not mean that caffeine, when used in moderation, induces dehydration, electrolyte depletion, or hyperthermia. We conclude, based on the information above, that the popular beliefs about caffeine in regard to fluid balance, electrolyte balance, and exercise-heat tolerance are incorrect. Simply stated, the physiological process that oppose dehydration (i.e., arginine vasopressin effects on water retention and aldosterone effects on sodium balance) are sufficient to handle the effects of a mild diuretic that is consumed in moderation. Because dehydration is absent, the body's ability to regulate temperature (i.e., via sweating and increased skin blood flow) is not impaired, even in a hot environment. In turn, because hyperthermia is not greater when caffeine is consumed (vs a water placebo), exercise performance is enhanced without increased physiological strain.
SUMMARY AND FUTURE RESEARCH
A variety of investigations have been reviewed spanning more than 75 yr. The evidence indicates that consuming a moderate level of caffeine results in a mild increase of urine production. Although this diuresis may (240-642 mg of caffeine) or may not (<240 mg) be significantly greater than a control fluid (0 mg of caffeine), 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. Caffeinated fluids contribute to the daily human water requirement in a manner that is similar to pure water. It is possible that urinary Na+, but less likely that urinary K+, excretion is increased somewhat by caffeine consumption; however, an affluent Western diet provides Na+ and K+ in amounts that exceed these losses. Furthermore, little or no evidence supports the contention that caffeine increases heat storage during exercise or that caffeine affects exercise performance in a hot environment negatively. Finally, little is known about the efficacy of caffeine administration regarding the following: a) large doses of caffeine (>600 mg) that are consumed at one time and b) differences between modes of caffeine delivery (i.e., via capsules, tablets, coffee, tea, soft drinks, sport drinks, and solid food). These are worthy of future research.
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