PlanHab: the combined and separate effects of 16 days of bed rest and normobaric hypoxic confinement on circulating lipids and indices of insulin sensitivity in healthy men

Abstract

PlanHab is a planetary habitat simulation study. The atmosphere within future space habitats is anticipated to have reduced Po2, but information is scarce as to how physiological systems may respond to combined exposure to moderate hypoxia and reduced gravity. This study investigated, using a randomized-crossover design, how insulin sensitivity, glucose tolerance, and circulating lipids were affected by 16 days of horizontal bed rest in normobaric normoxia [NBR: FiO2 = 0.209; PiO2 = 133.1 (0.3) mmHg], horizontal bed rest in normobaric hypoxia [HBR: FiO2 = 0.141 (0.004); PiO2 = 90.0 (0.4) mmHg], and confinement in normobaric hypoxia combined with daily moderate intensity exercise (HAMB). A mixed-meal tolerance test, with arterialized-venous blood sampling, was performed in 11 healthy, nonobese men (25-45 yr) before (V1) and on the morning of day 17 of each intervention (V2). Postprandial glucose and c-peptide response were increased at V2 of both bed rest interventions ( P < 0.05 in each case), with c-peptide:insulin ratio higher at V2 in HAMB and HBR, both in the fed and fasted state ( P < 0.005 in each case). Fasting total cholesterol was reduced at V2 in HAMB [−0.47 (0.36) mmol/l; P < 0.005] and HBR [−0.55 (0.41) mmol/l; P < 0.005]. Fasting HDL was lower at V2 in all interventions, with the reduction observed in HBR [−0.30 (0.21) mmol/l] greater than that measured in HAMB [−0.13 (0.14) mmol/l; P < 0.005] and NBR [−0.17 (0.15) mmol/l; P < 0.05]. Hypoxia did not alter the adverse effects of bed rest on insulin sensitivity and glucose tolerance but appeared to increase insulin clearance. The negative effect of bed rest on HDL was compounded in hypoxia, which may have implications for long-term health of those living in future space habitats.