Abstract
Humans’ core body temperature (CBT) is strictly controlled within a narrow range. Various studies dealt with the impact of physical activity, clothing, and environmental factors on CBT regulation under terrestrial conditions. However, the effects of weightlessness on human thermoregulation are not well understood. Specifically, studies, investigating the effects of long-duration spaceflight on CBT at rest and during exercise are clearly lacking. We here show that during exercise CBT rises higher and faster in space than on Earth. Moreover, we observed for the first time a sustained increased astronauts’ CBT also under resting conditions. This increase of about 1 °C developed gradually over 2.5 months and was associated with augmented concentrations of interleukin-1 receptor antagonist, a key anti-inflammatory protein. Since even minor increases in CBT can impair physical and cognitive performance, both findings have a considerable impact on astronauts’ health and well-being during future long-term spaceflights. Moreover, our findings also pinpoint crucial physiological challenges for spacefaring civilizations, and raise questions about the assumption of a thermoregulatory set point in humans, and our evolutionary ability to adapt to climate changes on Earth.
Highlights
Humans’ core body temperature (CBT) is strictly controlled within a narrow range
Recent research suggests that spaceflight induces a pro-inflammatory response, as indicated by increases in interleukin-1 receptor antagonist (IL-1ra)[13], a naturally occurring competitive inhibitor of interleukin 1, which has been shown to play a crucial role in downregulating CBT14
The parameter estimates of the mixed models revealed that CBT was significantly increased throughout the space mission at each time point, reaching a plateau after 75 days of spaceflight, and only returned slowly to baseline during the recovery (Fig. 1a and b)
Summary
Humans’ core body temperature (CBT) is strictly controlled within a narrow range. Various studies dealt with the impact of physical activity, clothing, and environmental factors on CBT regulation under terrestrial conditions. The tight regulation of CBT requires heat transfer mainly via three processes: thermal radiation, convection and evaporation[6,7] While their relative contribution varies by workload, environmental conditions, and hydration[8,9], any inefficiencies of these control mechanisms will result in reduced heat transfer and increase associated the risk of heat stress and hyperthermia. The sensor probe can be applied to the forehead, and has been shown to be highly accurate surrogate compared to nasopharyngeal[20], esophageal[23,24,25,26,27] and artery temperatures[24] Using this technology we tested the hypotheses of impaired thermoregulation during long-term spaceflights by investigating CBT in astronauts at rest and during exercise before, during and after six-months stays on the International Space Station (ISS)
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