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Fuel Homeostasis during Physical Inactivity Induced by Bed Rest¹

Laboratoire de Physiologie de l’Environnement (UPRES EA 645), Faculté de Médecine Lyon Grange-Blanche (S.B., J.-O.F., C.G.), 69373 Lyon; and Centre de Recherche en Nutrition Humaine de Lyon, Faculté de Médecine Laënnec (S.N., C.P., M.L.), 69372 Lyon, France

Address all correspondence and requests for reprints to: Prof. Claude Gharib, Laboratoire de Physiologie de l’Environnement, Faculté de Médecine Lyon Grange-Blanche, 8 avenue Rockefeller, 69373 Lyon Cedex 08, France. E-mail: gharib{at}cismsun.univ-lyon1.fr

The consequences of physical inactivity on fuel homeostasis were evaluated during 7 days of head-down bed rest (HDBR), a model mimicking weightlessness. Eight men (32.4 ± 1.9 yr; body mass index, 23.9 ± 0.7 kg/m²) and eight women (27.9 ± 0.9 yr; body mass index, 20.9 ± 0.6 kg/m²) underwent an oral glucose tolerance test (OGTT; 1 g/kg) before and after HDBR. The glucose load was labeled with ¹³C and associated with D-[6,6-²H₂]glucose infusion, indirect calorimetry, breath tests, and plasma measurements to determine the glucose turnover and biodisponibility, substrate oxidation, and endocrine responses. Body composition was assessed using H₂¹⁸O dilution. In addition, hormones were measured in daily blood and 24-h urine samples. No change in body composition was noted. Daily fasting insulin increased during HDBR (men, 34%; women, 26%), as did the insulin to glucose ratio (men, 30%; women, 25%). The normetanephrine level dropped (men, 30%; women, 16%), but metanephrine was unchanged. During OGTTs, the insulin response was increased after HDBR (men, 47%; women, 67%), whereas plasma glucose levels were similar. Nonesterified fatty acids and ß-hydroxybutyrate levels were lower. Endogenous glucose production dropped (28%), and exogenous glucose oxidation increased (28%) only in men. Resting energy expenditure was unchanged, but nonproteic respiratory quotient increased (men, 10%; women, 14%). Basal levels of lipid oxidation dropped in both sexes (90%), but those of carbohydrate oxidation increased in men (40%); as did lipogenesis in women (570%). In response to OGTTs, lipid oxidation was 80% reduced in both sexes after HDBR, but carbohydrate oxidation increased (25%) in men. Lipogenesis occurred in men (304%) and women (74%), but the latter had higher absolute levels. Therefore, 7 days of HDBR resulted in 1) reduced sympathetic activity, 2) insulin resistance suggested at the muscle level in men and at both the muscle and liver levels in women, 3) no changes in glucose biodisponibility, suggesting no alterations in the gastrointestinal function, and 4) a shift toward carbohydrate oxidation in men and a net lipogenesis in women. Such results suggest gender differences in response to sedentary life style and warrant further analysis.

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