Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise

This study investigated the effects of caffeine supplementation on thermoregulation and body fluid balance during prolonged exercise in a thermoneutral environment (25°C, 50% RH). Seven trained male subjects exercised on a treadmill at an intensity of 70-75% of maximal oxygen consumption to self-determined exhaustion. Subjects exercised once after caffeine and once after placebo ingestion, given in a double-blind crossover design. Five milligrams per kilogram body weight of caffeine followed by 2.5 mg · kg^-1 of caffeine were given 2 and 0.5 h before exercise, respectively. Rectal temperature was recorded and venous blood samples were withdrawn every 15 min. Water loss and sweat rate were calculated from the difference between pre- and post-exercise body weight, corrected for liquid intake. Following caffeine ingestion, when compared with placebo, no significant differences in final rectal temperature or in percent change in plasma volume were found. No significant differences were observed in total water loss (1376 ± 154 vs 1141 ± 158 mL, respectively), sweat rate (12.4 ± 1.1 vs 10.9 ± 0.7 g · m^-2 · min^-1, respectively), rise in rectal temperature (2.1 ± 0.3 vs 1.5 ± 0.4°C, respectively), nor in the calculated rate of heat storage during exercise (134.4 ± 17.7 vs 93.5 ± 22.5 W, respectively). Thus, in spite of the expected rise in oxygen uptake, caffeine ingestion under the conditions of this study does not seem to disturb body fluid balance or affect thermoregulation during exercise performance.