Abstract and Introduction
Dietitians, exercise physiologists, athletic trainers, and other sports medicine personnel commonly recommend that exercising adults and athletes refrain from caffeine use because it is a diuretic, and it may exacerbate dehydration and hyperthermia. This review, contrary to popular beliefs, proposes that caffeine consumption does not result in the following: (a) water-electrolyte imbalances or hyperthermia and (b) reduced exercise-heat tolerance.
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%). Approximately 20%-30% of Americans consume more than 600 mg of caffeine daily.
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. 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, 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.
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. 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. 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.
Exerc Sport Sci Rev. 2007;35(3):135-140. © 2007 American College of Sports Medicine
Cite this: Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance - Medscape - Jul 01, 2007.