M. Johannsen, J. Cuddy, W. Hailes, M. Schleh, B. Ruby, FACSM

The University of Montana, Missoula, MT

The link between thermoregulation, hydration status, and exercise performance in hot, humid environments is controversial and poorly understood. PURPOSE: The purpose of this study was to evaluate the effects of volume and temperature of ingested fluid on hydration status, thermoregulation and exercise performance. METHODS: Recreationally active males (N=11, 25±1.8 years; VO2max=58.3±1.8 mL/kg/min) completed two 3-hour intermittent exercise trials in the heat (WBGT=35.5 oC with 50% humidity). Participants were given either 1 mL/kg of body weight (BW) of room temperature water (35.5oC; ROOM) or 0.5 mL/kg of an ice slurry mixture (~0oC; COLD) every ten minutes throughout the trial in a randomized cross-over design. Subjects walked on a motorized treadmill at 40% VO2Max for 25-minutes followed by 5-minutes of standing rest. Participants completed a 1-mile time trial as quickly as possible on a non-motorized treadmill (Woodway Curve). After completion of the 1-mile time trial, participants remained seated for the rest of the 1-hour time period. This series of steady state and time trial (TT) segments was repeated three times over each 3-hour trial. Core temperature and heart rate were monitored continuously throughout the 3-hour trials, and used to calculate physiological strain index (PSI). Nude body weight was measured pre and post to calculate sweat rate. RESULTS: There was a difference in BW loss between the trials (2.2±0.7 and 3.0±0.8 kg for the ROOM and COLD, respectively, p<0.05). In contrast, sweat rate was not different between the trials (1.2±0.2 and 1.2±0.2 L/hr, for the ROOM and COLD, respectively). Peak PSI was not different between the trials (9.0±1.3 and 8.8±1.0, for the ROOM and COLD respectively). 1-mile TT performance was not different between trials (ROOM: 9.7±1.3, 10.8±1.4, 12.8±2.4 min for hours 1, 2, and 3, respectively, and COLD: 10.1±1.6, 11.2±2.0, 12.7±2.6 min for hours 1, 2, and 3, respectively) but was impaired over time. CONCLUSION: The necessary volume of fluid ingested may be reduced if the temperature is sufficiently cold enough to aid in the thermoregulatory processes when performing exercise in the heat.

Grant support: United States Air Force FA8650-15-2-6602.

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