Metabolic changes observed in astronauts.

Study of metabolic alterations that occur during space flight can provide insight into mechanisms of physiologic regulation. Results of medical experiments with astronauts reveal rapid loss of volume (2 L) from the legs and a transient early increase in left ventricular volume index. These findings indicate that, during space flight, fluid is redistributed from the legs toward the head. In about 2 days, total body water decreases 2 to 3%. Increased levels of plasma renin activity and antidiuretic hormone while blood sodium and plasma volume are reduced suggest that space flight-associated factors are influencing the regulatory systems. In addition to fluid and electrolyte loss. Skylab astronauts lost an estimated 0.3 kg of protein. Endocrine factors, including increased cortisol and thyroxine and decreased insulin, are favorable for protein catabolism. The body appears to adapt to weightlessness at some physiologic cost. Readaptation to Earth's gravity at landing becomes another physiologic challenge.