Protein kinetics during and after long-duration spaceflight on MIR

Human spaceflight is associated with a loss of body protein. Bed rest studies suggest that the reduction in the whole body protein synthesis (PS) rate should be ~15%. The objectives of this experiment were to test two hypotheses on astronauts and cosmonauts during long-duration (>3 mo) flights on MIR: that 1) the whole body PS rate will be reduced and 2) dietary intake and the PS rate should be increased postflight because protein accretion is occurring. The ¹⁵N glycine method was used for measuring whole body PS rate before, during, and after long-duration spaceflight on the Russian space station MIR. Dietary intake was measured together with the protein kinetics. Results show that subjects lost weight during flight (4.64 ± i.0 kg, P < 0.05). Energy intake was decreased inflight (2,854 ± 268 vs. 2,145 ± 190 kcal/day, n = 6, P < 0.05), as was the PS rate (226 ± 24 vs. 97 ± 11 g protein/day, n = 6, P < 0.01). The reduction in PS correlated with the reduction in energy intake (r² = 0.86, P < 0.01, n = 6). Postflight energy intake and PS returned to, but were not increased over, the preflight levels. We conclude that the reduction in PS found was greater than predicted from ground-based bed rest experiments because of the shortfall in dietary intake. The expected postflight anabolic state with increases in dietary intake and PS did not occur during the first 2 wk after landing.