Leptin responses to physical inactivity induced by simulated weightlessness

Physical inactivity induced by head-down bed rest (HDBR) affects body compo sition (BC). Leptin is involved in BC regulation by acting on fuel homeosta sis. We investigated whether leptin and counter-regulatory hormone levels a re affected by a 7-day HDBR. Fasting blood was sampled daily (0700) in male s (n = 8) and on alternating days in females (n = 8) for measurements of le ptin, insulin, norepinephrine (NE), epinephrine (Epi), growth hormone (GH), cortisol, nonesterified fatty acid (NEFA), and glucose. BC was measured by H-2 O-18 dilution. Energy intake (men 10.5 +/- 0.2 MJ/day, women 7.9 +/- 0 .3 MJ/day) and BC were unchanged by HDBR. Increased levels of leptin (men 4 0%, P = 0.003; women 20%, P = 0.050), insulin (men 34%, P = 0.018; women 25 %, P = 0.022), and the insulin-to-glucose ratio (men 30%, P = 0.049; women 25%, P = 0.031) were noted. GH, NE, Epi, and cortisol levels were unaltered . NEFA dropped in both sexes, but glucose decreased only in women. In concl usion, HDBR increased leptin levels independently of stress response, chang es in fat mass, energy intake, or gender. These changes were correlated to the insulin-resistance development in men. Further analyses are required, b ut the results have to be considered for longer HDBR periods with 1) the we ll-described drop in energy intake and 2) the BC changes.