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® 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