THERMAL AND CIRCULATORY RESPONSES DURING EXERCISE - EFFECTS OF HYPOHYDRATION, DEHYDRATION, AND WATER-INTAKE

This investigation examined the distinct and interactive effects of in itial hydration state, exercise-induced dehydration, and water rehydra tion in a hot environment. On four occasions, 10 men performed a 90-mi n heat stress test (treadmill walking at 5.6 km/h, 5% grade, 33 degree s C, 56% relative-humidity). These heat stress tests differed in prete st hydration [2 euhydrated (EU) and 2 hypohydrated (HY) trials] and wa ter intake during exercise [2 water ad libitum (W) and 2 no water (NW) trials]. HY + NW indicated greater physiological strain than all othe r trials (P<0.05-0.001) in heart rate, plasma osmolality (P-osm), swea t sensitivity (g/degrees C.min), and rectal temperature. Unexpectedly, final HY + Wand 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 KY + W and EU W respectively), no voluntary dehydration (0.9% body mass increase), a nd attenuated thermal and circulatory strain during HY + W.