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Caffeine is regarded as a diuretic despite evidence that hydration is not impaired with habitual ingestion. The purpose of this study was to determine whether a caffeinated sports drink impairs fluid delivery and hydration during exercise in warm, humid conditions (28.5 °C, 60% relative humidity). Sixteen cyclists completed 3 trials: placebo (P), carbohydrate-electrolyte (CE), and caffeinated (195 mg/L) sports drink (CAF+CE). Subjects cycled for 120 min at 60–75%VO2max followed by 15 min of maximal-effort cycling. Heart rate and rectal temperature were similar until the final 15 min, when these responses and exercise intensity were higher with CAF+CE than with CE and P. Sweat rate, urine output, plasma-volume losses, serum electrolytes, and blood deuterium-oxide accumulation were not different. Serum osmolality was higher with CAF+CE vs. P but not CE. The authors conclude that CAF+CE appears as rapidly in blood as CE and maintains hydration and sustains cardiovascular and thermoregulatory function as well as CE during exercise in a warm, humid environment.
Millard-Stafford is with the School of Applied Physiology, Georgia Institute of Technology, Atlanta, GA 30332-0356. Cureton and Trilk are with the Dept of Kinesiology, University of Georgia, Athens, GA 30602-6554. Wingo is with the Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, TX 75231. Warren is with the Division of Physical Therapy, Georgia State University, Atlanta, GA 30302-4019. Buyckx is with The Beverage Institute for Health and Wellness, The Coca-Cola Company, Atlanta, GA 30313.