Article Title



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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