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

Armstrong, Lawrence E., Carl M. Maresh, Catherine V. Gabaree, Jay R. Hoffman, Stavros A. Kavouras, Robert W. Kenefick, John W. Castellani, and Lynn E. Ahlquist. Thermal and circulatory responses during exercise: effects of hypohydration, dehydration, and water intake. J. Appl. Physiol. 82(6): 2028–2035, 1997.—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.