The objectives of this study were as follows: 1) to measure human energy expenditure (EE) during spaceflight on a shuttle mission by using the doubly labeled water (DLW) method; 2) to determine whether the astronauts were in negative energy balance during spaceflight; 3) to use the comparison of change in body fat as measured by the intake DLW EE, ¹⁸O dilution, and dual energy X-ray absorptiometry (DEXA) to validate the DLW method for spaceflight; and 4) to compare EE during spaceflight against that found with bed rest. Two experiments were conducted: a flight experiment (n = 4) on the 16-day 1996 life and microgravity sciences shuttle mission and a 6° head-down tilt bed rest study with controlled dietary intake (n = 8). The bed rest study was designed to simulate the flight experiment and included exercise. Two EE determinations were done before flight (bed rest), during flight (bed rest), and after flight (recovery). Energy intake and N balance were monitored for the entire period. Results were that body weight, water, fat, and energy balance were unchanged with bed rest. For the flight experiment, decreases in weight (2.6 ± 0.4 kg, P < 0.05) and N retention (2.37 ± 0.45 g N/day, P < 0.05) were found. Dietary intake for the four astronauts was reduced in flight (3,025 ± 180 vs. 1,943 ± 179 kcal/day, P < 0.05). EE in flight was 3,320 ± 155 kcal/day, resulting in a negative energy balance of 1,355 ± 80 kcal/day (15.7 ± 1.0 kcal · kg¹ · day¹, P < 0.05). This corresponded to a loss of 2.1 ± 0.4 kg body fat, which was within experimental error of the fat loss determined by ¹⁸O dilution (1.4 ± 0.5 kg) and DEXA (2.4 ± 0.4 kg). All three methods showed no change in body fat with bed rest. In conclusion, 1) the DLW method for measuring EE during spaceflight is valid, 2) the astronauts were in severe negative energy balance and oxidized body fat, and 3) in-flight energy (E) requirements can be predicted from the equation: E = 1.40 × resting metabolic rate + exercise.

astronauts; nitrogen balance; bed rest

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