Body mass changes, energy, and protein metabolism in space.

Most astronauts lose body mass during their stay in microgravity. The early hypothesis, which attributed this phenomenon to an increase in diuresis and natriuresis after entering microgravity, is now untenable. Although a fluid shift from the lower to the upper body occurs, it does not lead to a marked fluid loss in the first 2 days of space flight. The continuous day-by-day body mass measurement during the Euromir 94 mission showed that there was a gradual reduction over the entire mission instead of a rapid loss of 2 to 3 kg at the beginning of a mission. The daily energy intake during this mission and the negative energy balances found in the Skylab and LMS-Mission show that lowered body mass is very likely caused by an insufficient energy consumption and its accompanying effects. These include the metabolization of endogenous energy stores, ie, glycogen, protein, and fat. Mobilization of glycogen and protein buffers will also cause the water that is bound to both to be lost. Thus, a gradual decrease in body mass and a concomitant reduction in total body fluid occurs without a significant increase in urine flow or natriuresis. In conclusion, the body mass loss in microgravity is likely a result of undernutrition instead of diuresis and natriuresis caused by the fluid shift.