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

Background Since the very beginning of space physiology research, the deficit in body mass that is often observed after landing has always been interpreted as an indication of the absolute fluid loss early during space missions. However, in contrast to central hypervolemic conditions on Earth, the acute shift of blood volume from the legs to the upper part of the body in astronauts 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 astronauts (n = 3) during their several weeks aboard the space station MIR. A continuous diet monitoring was performed during the first mission (EuroMIR94, 30 days). The second mission (MIR97, 19 days) comprised a 15-day metabolic ward period (including predefined constant energy and sodium intake). Water and sodium balances were calculated and the kinetic of changes in basal concentrations of fluid-balance-related hormones during flight were determined. Conclusion The data suggest firstly that loss of body mass during space flight is rather a consequence of hypocaloric nutrition. Secondly, mu -G provokes a sodium retaining hormonal status and may lead to sodium storage without an accompanying fluid retention.