Reduced sweating response induced by long‐term head‐down bed rest can be prevented by encouraged water intake during head‐down bed rest

Abstract

Many studies on long-term head-down bed rest (HDBR) used as a simulation of microgravity indicate that sweating response is reduced accompanying an elevation of core temperature, suggesting that this alteration of sweating response during HDBR may be caused by dehydration induced. This study was aimed to investigate whether the reduced sweating response is prevented by HDBR in the absence of dehydration. In 12 healthy male volunteers, daily water intake was encouraged to match the urine volume in the previous day during 20 days HDBR. Each subject was immersed his legs in hot water at 42 °C for 45 min in a sitting position before and after the session of HDBR. Core (tympanic) temperature, skin temperature and forearm sweat rates were recorded continuously. Body weight was not significantly changed after HDBR. Higher core and skin temperatures, and greater sweat rate and earlier onset of sweating were observed during the hot water immersion after HDBR. The regression line relating sweat rate to body temperature was not significantly shifted after HDBR, indicating that sweating function was not altered by the HDBR. It was concluded that encouraged water intake (i.e., euhydration) can prevent the reduced sweating response under microgravity condition such as during spaceflight.