Thermal and circulatory responses during exercise: Effects of hypohydration, dehydration, and water intake

This investigation examined the distinct and interactive effects of initial hydration state, exercise-induced dehydration, and water rehydration in a hot environment. On four occasions, 10 men performed a 90-min heat stress test (treadmill walking at 5.6 km/h, 5% grade, 33°C, 56% relative humidity). These heat stress tests differed in pretest hydration [2 euhydrated (EU) and 2 hypohydrated (HY) trials] and water intake during exercise [2 water ad libitum (W) and 2 no water (NW) trials]. HY + NW indicated greater physiological strain than all other trials (P < 0.05- 0.001) in heart rate, plasma osmolality (P(osm)), sweat sensitivity (g/°C · min), and rectal temperature. Unexpectedly, final HY + W and EU + W responses for rectal temperature, heart rate, and P(osm) were similar, despite the initial 3.9 ± 0.2% hypohydration in HY + W. We concluded that differences in pretest P(osm) (295 ± 7 and 287 ± 5 mosmol/kg for HY + W and EU + W, respectively) resulted in greater water consumption (1.65 and 0.31 liter for HY + W and EU + W, respectively), no voluntary dehydration (0.9% body mass increase), and attenuated thermal and circulatory strain during HY + W.