Abstract
An investigation has been conducted into the changes in body composition which occur during manned spaceflight. The subjects for the investigation were the 9 astronauts taking part in the Skylab programme (1973-4). The principal experimental technique was the preflight and postflight biostereometric analysis of body form, in which total and regional body volumes were measured from a coordinate description of the body, derived from stereoscopic photographs. Data were also obtained from other Skylab experiments, including mineral balance, bioassay of body fluids, bone mineral measurement and inflight body mass determination. After their spaceflights, the subjects were found to have lost water, fat, muscle and bone mineral. An estimated loss of 1.2-1.4 kg of water occured within the first 1-2 days in orbit, in response to a redistribution of body fluid in the absence of gravity. Loss of body fat, usually between one and two kilograms, was observed in at least 7 of the subjects, and was probably caused by an inadequate caloric intake. Most of the astronauts showed a loss of muscle tissue from the legs, usually of less than one kilogram, and apparently due to muscle atrophy, from the relative lack of exercise inflight. A progressive loss of bone mineral was observed, up to 1% of the body total 'being lost in 3 months, although local losses of up to 8% were observed in the os calcis of 3 subjects. The significance of the changes observed is discussed. Except for the loss of bone mineral, all of the changes in body composition appeared to be either self limiting, or preventable by diet or exercise. The loss of bone mineral appears to be continuous, however, and without countermeasures it limits the duration of safe exposure to weightlessness to about 9 months.