Effect of Exercise on Energy Expenditure and Body Composition in Astronauts Onboard the International Space Station: Considerations for Interplanetary Travel

Objective Body mass (BM) loss and body composition (BC) changes threaten astronauts' health and mission success. However, the energetic contribution of the exercise countermeasure to these changes has never been investigated during long-term missions. We studied energy balance and BC in astronauts during 6-month missions onboard the International Space Station. Methods Before and after at least 3 months in space, BM, BC, total and activity energy expenditure (TEE and AEE) were measured using the doubly labeled water method in 11 astronauts (2011-2017). Physical activity (PA) was assessed by the SensewearPro (R) activity-device. Results Three-month spaceflight decreased BM (- 1.20 kg [SE 0.5]; P = 0.04), mainly due to non-significant fat-free mass loss (FFM; - 0.94 kg [0.59]). The decrease in walking time (- 63.2 min/day [11.5]; P < 0.001) from preflight was compensated by increases in non-ambulatory activities (+ 64.8 min/day [18.8]; P < 0.01). Average TEE was unaffected but a large interindividual variability was noted. Astronauts were stratified into those who maintained (stable_TEE; n = 6) and those who decreased (decreased_TEE; n = 5) TEE and AEE compared to preflight data. Although both groups lost similar BM, FFM was maintained and FM reduced in stable_TEE astronauts, while FFM decreased and FM increased in decreased_TEE astronauts (estimated between-group-difference (EGD) in Delta FFMindex [FFMI] 0.87 kg/m(2), 95% CI + 0.32 to + 1.41; P = 0.01, Delta FMindex [FMI] - 1.09 kg/m(2), 95% CI - 2.06 to - 0.11 kg/m(2); P = 0.03). The stable_TEE group had higher baseline FFMI, and greater baseline and inflight vigorous PA than the decreased_TEE group (P < 0.05 for all). Delta FMI and Delta FFMI were respectively negatively and positively associated with both Delta TEE and Delta AEE. Conclusion Both ground fitness and inflight overall PA are associated with spaceflight-induced TEE and BC changes and thus energy requirements. New instruments are needed to measure real-time individual changes in inflight energy balance components.

Automated summary

This study investigated the energy balance and body composition changes in astronauts during 6-month missions on the International Space Station. The results showed that ground fitness and inflight overall physical activity are associated with spaceflight-induced energy expenditure and body composition changes. New instruments are needed to measure real-time individual changes in inflight energy balance components.