SPACE FEVER : CORE BODY TEMPERATURES IN ASTRONAUTS UNDER REST AND EXERCISE ON THE INTERNATIONAL SPACE STATION (ISS)

Introduction In Space, thermoregulation in humans should be altered due to i) changes in convective heat loss, ii) body composition, and iii) a diminished production and/or decreased efficiency of evaporation, especially during physical excercise. We hypothesized that in astronauts for a given absolute level of work, body core temperatures (CBT) should show a faster, higher, and longer lasting increase as compared to a similar exercise protocol on Earth. Material & Methods 11 astronauts (7 male, 4 females) were studied several times before, during, and after a physical exercise protocol on ground and in space on ISS between 2010-2012. Out of these 11 astronauts we received complete pre and post-flight anthropometric and body composition data from 10 astronauts, 6 males and 4 females with an average age of 48.5 ± 3.9 years , height 1.76 m ± 0.07 [m], weight of 77.5 ± 16.2 [kg], body mass index of 24.7 ± 3.4 (kg/[m]2), a lean body mass preflight of 59.5 ± 11.9 [kg], and a fat mass of 18.0 ± 5.3 [kg]. Results It was found that i) CBTs increased much faster in space as compared to baseline values at all time points (p<0.05), ii) during exercise the absolute peakes of CBTs reached were significantly higher than preflight (p<0.05), incidentally above 41.0 °C, and iii) the post- excercise cooling periods were significantly prolonged (p<0.05). However, most unexpected, we observed iv) that the resting CBTs were found to be elevated by 0.5-0.9 °C inflight as compared to preflight (p<0.05), and returned only gradually to control level 30 days after postflight. Discussion & Conclusions We conclude that i) in space already during a short-term physical exercise high CBTs >40.0 °C can be reached in astronauts, ii) post-exercise 'cool-down' periods inflight are prolonged, and iii) most remarkable in space the setpoint of CBT is shifted to a higher level up to 0.9 °C as compared to pre- and post baseline values. The latter shift of core temperature to a higher set- point is presumably induced by immunolological processes and might contribute to our understanding of fore-most isolated observed adaptive changes in humans during long-term space flight such as negative energy balance and peripheral vasoconstriction.