Effect of Fluid Temperature and Volume on Thermoregulation in the Heat

The link between thermoregulation, hydration status, and exercise performance in hot humid environments is controversial. The purpose of this study was to evaluate the effects of volume and temperature of ingested fluid on hydration status, thermoregulation and exercise performance. Recreationally active males (N=11, 24.7±5.9 years; VO2max=58.2±6.0 mL/kg/min) completed two 3-hour intermittent exercise trials in the heat (31 WBGT=35.5°C with 50% humidity). Participants consumed either 1 mL/kg body weight (BW) of room temperature water (35.5°C; ROOM) or 0.5 mL/kg of an ice slurry mixture (~0°C; COLD) every ten minutes throughout the trials in a randomized crossover design. Subjects walked on a motorized treadmill at 40% VO2max for 25-minutes followed by 5-minutes of standing rest after which a 1.6 km time trial was completed as quickly as possible on a non-motorized treadmill (Woodway Curve). After completion of the time trial, participants remained seated for the rest of the 1-hour time period. This series of steady state (SS) and time trial (TT) segments was repeated three times over each 3hour trial. Core temperature and heart rate were monitored continuously throughout the 3-hour trials and used to calculate physiological strain index (PSI). Nude BW was measured pre and post to calculate sweat loss. Body weight loss was significantly higher for the COLD trial (2.2±0.7 and 3.0±0.8 % for the ROOM and COLD respectively, p<0.05). Total water loss was not different between the trials (1.2±0.2 and 1.2±0.2 L/hr, for the ROOM and COLD, respectively). Time trial performance was not different between treatments (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, 12.8±2.6 min for hours 1, 2, and 3, respectively) but was impaired over time. These data suggest that it is not simply the volume, but the temperature of the ingested fluid that aids in thermoregulation.