A drinking strategy aiming to replace a given percentage of the sweat losses incurred during exercise should result in reproducible fluid intake volume and, hence, fluid balance from one exercise session to the other performed under similar scenarios. Whether this may also be the case with ad libitum drinking during exercise is unclear. We characterized the repeatability of ad libitum water intake during repeated 1 h exercise sessions and examined its effect over time on fluid balance and selected physiological functions and perceptual sensations. Twelve (3 women) healthy individuals participated in this study. At weekly intervals, they completed four 2 × 30 min walking/jogging exercise bouts (55% V˙O2max, 40 °C, 20-30% relative humidity) interspersed by a 3 min recovery period. During exercise, participants consumed water (20 °C) ad libitum. There were no significant differences among the four exercise sessions for absolute water intake volume (~1000 mL·h-1), percent body mass loss (~0.4%), sweat rate (~1300 mL·h-1) and percent of sweat loss replaced by water intake (~80%). Heart rate, rectal temperature, and perceived thirst and heat stress did not differ significantly between the first and fourth exercise sessions. Perceived exertion was significantly lower during the fourth vs. the first exercise session, but the difference was trivial (<1 arbitrary unit). In conclusion, ad libitum water intake during four successive identical 1 h walking/jogging sessions conducted in the heat will result in similar water intake volumes and perturbations in fluid balance, heart rate, rectal temperature, and perceived thirst, heat stress and exertion.
Keywords: dehydration; drinking pattern; exercise; heat stress; physiological and perceptual functions.