Energy expenditure and balance during spaceflight on the space shuttle

The objectives of this study were as follows: 1) to measure human energy ex penditure (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 cha nge in body fat as measured by the intake DLW EE, O-18 dilution, and dual e nergy X-ray absorptiometry (DEXA) to validate the DLW method for spacefligh t; 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 degrees head-d own tilt bed rest study with controlled dietary intake (n = 8). The bed res t study was designed to simulate the flight experiment and included exercis e. Two EE determinations were done before flight (bed rest), during flight (bed rest), and after flight (recovery). Energy intake and N balance were m onitored 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 ba lance of 1,355 +/- 80 kcal/day (-15.7 +/- 1.0 kcal.kg(-1).day(-1), 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 O-18 dilution (-1.4 +/- 0 .5 kg) and DEXA (-2.4 +/- 0.4 kg. All three methods showed no change in bod y fat with bed rest. in conclusion, 1) the DLW method for measuring EE duri ng spaceflight is valid, 2) the astronauts were in severe negative energy b alance and oxidized body fat, and 3) in-flight energy (E) requirements can be predicted from the equation: E = 1.40 x resting metabolic rate + exercis e.