Water and sodium balances and their relation to body mass changes in microgravity

Background Since the very beginning of space physiology research, the defic it in body mass that is often observed after landing has always been interp reted as an indication of the absolute fluid loss early during space missio ns. However, in contrast to central hypervolemic conditions on Earth, the a cute shift of blood volume from the legs to the upper part of the body in a stronauts entering mu -gravity (mu -G) has neither stimulated diuresis and natriuresis nor resulted in negative water-and sodium-balances. Design We therefore examined the kinetics of body mass changes in astronaut s (n = 3) during their several weeks aboard the space station MIR. A contin uous diet monitoring was performed during the first mission (EuroMIR94, 30 days). The second mission (MIR97, 19 days) comprised a 15-day metabolic war d period (including predefined constant energy and sodium intake). Water an d sodium balances were calculated and the kinetic of changes in basal conce ntrations of fluid-balance-related hormones during flight were determined. Conclusion The data suggest firstly that loss of body mass during space fli ght is rather a consequence of hypocaloric nutrition. Secondly, mu -G provo kes a sodium retaining hormonal status and may lead to sodium storage witho ut an accompanying fluid retention.