SPACEFLIGHT AND PROTEIN-METABOLISM, WITH SPECIAL REFERENCE TO HUMANS

Human space missions have shown that human spaceflight is associated w ith a loss of body protein. Specific changes include a loss of lean bo dy mass, decreased muscle mass in the calves, decreased muscle strengt h, and changes in plasma proteins and amino acids. The major muscle lo ss is believed to be associated with the antigravity (postural) muscle . The most significant loss of protein appears to occur during the fir st month of flight. The etiology is believed to be multifactorial with contributions from disuse atrophy, undernutrition, and a stress type of response. This article reviews the results of American and Russian space missions to investigate this problem in humans, monkeys, and rat s. The relationship of the flight results with ground-based models inc luding bedrest for humans and hindlimb unweighting for rats is also di scussed. The results suggest that humans adapt to spaceflight much bet ter than either monkeys or rats.