An astronaut will lose lean body mass if energy balance is not maintained. This is especially true if energy expenditure exceeds energy intake. However, astronauts on Skylab lost lean body mass in spite of the apparent adequate intake of energy and protein. Energy requirements could be affected by microgravity and by the limits on physical activity in the close confines of the spacecraft. Before this study, energy requirements during space flight had only been estimated from metabolic balance studies and food intake records. The objectives of this study were to measure total energy expenditure (TEE) during space flight and to compare the results with calculated energy intake.
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Two five-day test sessions (one before and 1 during flight) were completed by each of 13 astronauts on six Shuttle flights lasting eight to 14 days. The astronauts consumed doubly labeled water (labeled with the heavy isotopes 2H and 18O) and periodically provided urine and saliva samples. It was necessary to account for baseline differences in the isotopes of hydrogen and oxygen in the Shuttle's drinking water, which is made by fuel cells from liquid oxygen and hydrogen. Total energy expenditure was determined by indirect calorimetry and was corrected for body weight and fat-free mass. Energy intake was calculated from records of food and fluid intake. One-way repeated-measures ANOVA identified differences in energy intake and expenditure between the preflight and postflight periods. The Student-Newman-Keuls test was used for post-hoc comparisons. Paired t-tests were used to detect any differences between preflight and inflight dietary intake.
Energy expenditures were similar before and during flight. However, energy and fluid intake were lower during flight (energy intake was 23% less; fluid intake was 19% less.) Weight changes ranged from a gain of 1.0 kilogram (kg) to a loss of 3.9 kg, with a mean weight loss of 1.5 kg. In flight energy expenditures were similar to estimates published by the World Health Organization (WHO) of energy requirements for healthy adults. The results indicated that energy expenditure was not changed during space flight and that the WHO estimates of energy requirements were appropriate for estimating the dietary needs of space crews. However, most of the 13 astronauts were in negative energy balance because they had reduced their caloric intake. This is undesirable because the loss of lean body mass reduces work capacity and promotes the loss of electrolytes that are important to muscle function and cardiovascular function. The astronauts also consumed inadequate fluid. The importance of maintaining adequate food and fluid intake should be emphasized in future flights.
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