Effects of encouraged water drinking on thermoregulatory responses after 20 days of head-down bed rest in humans

Abstract

We tested the hypothesis that encouraged water drinking according to urine output for 20 days could ameliorate impaired thermoregulatory function under microgravity conditions. Twelve healthy men, aged 24 +/- 1.5 years (mean +/- SE), underwent -6 degrees head-down bed rest (HDBR) for 20 days. During bed rest, subjects were encouraged to drink the same amount of water as the 24-h urine output volume of the previous day. A heat exposure test consisting of water immersion up to the knees at 42 degrees C for 45 min after a 10 min rest (baseline) in the sitting position was performed 2 days before the 20-day HDBR (PRE), and 2 days after the 20-day HDBR (POST). Core temperature (tympanic), skin temperature, skin blood flow and sweat rate were recorded continuously. We found that the -6 degrees HDBR did not increase the threshold temperature for onset of sweating under the encouraged water drinking regime. We conclude that encouraged water drinking could prevent impaired thermoregulatory responses after HDBR.

Effects of simulated weightlessness on thermal sweating of human body: An experimental study

“…Another possible reason for the decreased sweat rate lies in the fluid loss induced by real or simulated weightlessness. Sato 14 and Shibasaki 15 hold this point of view. As described by Hall, 41 the sweat of the human body is originally from the interstitial fluid.…”

“…In addition to the above, results of terrestrial studies of weightlessness have also indicated altered sweating. 11–18 During a 13-day, −6° head down bed rest (HDBR) experiment conducted by the National Aeronautics and Space Administration (NASA), the mean core temperature of subjects for the onset of sweating was increased by 0.28°C, 11,12 suggesting an altered sweat response which had further altered thermal regulation in HDBR simulated weightlessness. Another −6° HDBR experiment conducted by NASA 13 found their subjects started sweating at a lower mean skin temperature and that their normal vasodilation response was impaired slightly.…”

“…The changes were ascribed to subjects’ decreased circulatory capability due to the simulated weightlessness, as well as to an alteration in their thermoregulatory system. 13 In addition, studies 14,15 on long-term HDBR simulated weightlessness indicate that the impaired sweating response of subjects and the elevation of core temperature could be ascribed to a fluid loss induced by HDBR. Furthermore, other studies 16–18 claimed that changes in sweating under simulated weightlessness by HDBR were due to changes in vascular functions.…”

Dietary Needs, Approaches and Recommendations to Meet the Demands of Future Manned Space Flights

“…Rapid and dramatic body fluid shifts in space also impair evaporative heat loss. Blunted thirst sensations and lower fluid intakes compound this problem [13,14]. In-flight fluid shifts centralize blood volumes, as a result the body expends more energy to pump blood to the skin surface to enable evaporation.…”

“…Lower fluid intakes evoke dehydration and cause perspiration rates to decline with longer missions, while sweating in space creates a film over the skin that impairs convective heat loss [10,15]. Thus, astronauts often return to Earth from longer flights with 1-5% body and muscle mass losses, hyperthermic and dehydrated [6,10,[14][15][16][17]. Better in-flight nutritional and hydration strategies must be devised to abate these effects.…”