Effect of hypohydration on core temperature during exercise in temperate and hot environments

The purpose of this study was to compare directly the physiological consequences of 5% hypohydration or euhydration during exercise in both temperate (23 degrees C) and hot (33 degrees C) environments. The subjects were eight male volunteers. Each performed four 1-h exercise bouts at 60% maximum oxygen uptake, one in each of the following conditions: hot-hypohydrated, hot-euhydrated, temperate-hypohydrated, and temperate-euhydrated. Heart rate (HR), rectal temperature (T-re), forearm blood flow, and oxygen uptake were measured after 20, 40, and 60 min exercise. Whole-body sweat rate was also deter mined for each exercise bout. Hypohydration increased T-re significantly (P<0.05) more in the hot environment (0.16 degrees C per 1% decrease in body mass) than in the temperate environment (0.08 degrees C per 1% hypohydration). Furthermore, compared with euhydration, hypohydration decreased forearm blood flow and whole-body sweat rate significantly more during exercise in the hot than in the temperate environment. The reductions in forearm blood flow and whole-body sweat rate appear to have decreased heat loss, thus accounting for the increase in T-re during exercise in the heat while hypohydrated. In conclusion, this study illustrates that the physiological consequences of hypohydration during exercise are exacerbated in the heat.