Energy requirements of man living in a weightless environment.

The ability to maintain energy balance is a vital factor in maintaining body composition. A negative energy balance requires that body tissue be consumed to sustain biochemical and physiological activity. Such a caloric imbalance coupled with reduced physical activity results in (among other things): a negative balance which can not be reversed by increased protein intake; negative balances for electrolytes; and a suspension of erythrocyte production. Body weight losses were experienced by all astronauts during Gemini and Apollo missions. Data on the magnitude of the changes, together with data on energy consumption, were used to calculate energy imbalances. These data, when compared with results obtained from precise energy balance measurements made on 64 men living in low pressure chambers, show close correlation. When energy requirements are expressed in kilocalories per kilogram of body weight, the difference in energy requirements among the astronauts and chamber subjects was small and not statistically significant. These data indicated that reliable prediction of energy needs for astronauts, during long-term space missions, can be made by studying either the astronauts or healthy subjects in a ground-based environment similar to that of the spacecraft. These data also indicate that changes in body weight and certain other body measurements detected during Gemini and Apollo missions were probably caused, at least in part, by a calorie deficit.