Development of insulin resistance by astronauts during spaceflight.

Human spaceflight is associated with the loss of body protein. On Earth, insulin is an important factor in the regulation of muscle protein synthesis and breakdown. The objectives of this study were to determine whether insulin resistance occurs in spaceflight, and if the development of insulin resistance is related to the protein loss. The urinary C-peptide excretion rate was used as a marker for insulin secretion. The experiment was conducted before, during and after the 1991 9.5-d SLS-1 (Columbia) Space Shuttle mission. Dietary intake and urine output were monitored continuously for the four payload crewmembers from 11 d before launch to 7 d after landing for a total of 27 d. Data were obtained on the four payload crewmembers. Results were as follows: 1) the mean inflight C-peptide excretion rates were significantly lower than either the pre- or postflight rates (p < 0.05); and 2) the inflight nitrogen balance decreased as C-peptide excretion increased.