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Calorie restriction accelerates the catabolism of lean body mass during 2 wk of bed rest^1,2,3

¹ From the Department of Clinical, Technological and Morphological Sciences, Division of Internal Medicine, University of Trieste, Italy (GB, BC, MS, AB, RB, MZ, RA, and GG); the Institute of Cardiology and Sport Medicine, German Sport University, Cologne, Germany (ML); the Department of Sports Medicine and Sports Nutrition, Ruhr-University, Bochum, Germany (PP); and the DLR-Institute of Aerospace Medicine, Cologne, Germany (MH)

Background: Muscle inactivity and low energy intake commonly occur in persons with acute or chronic disease, in astronauts during space flight, and during aging.

Objective: We used a crossover design to investigate the effects of the interactions of inactivity and calorie restriction on whole-body composition and protein kinetic regulation in 9 healthy volunteers.

Design: Lean body mass (LBM) was measured by using dual-energy X-ray absorptionmetry before and at the end of 14-d periods of bed rest (B) and controlled ambulation (A) in patients receiving eucaloric (E) or hypocaloric (H) (80% of total energy expenditure) diets. Whole-body leucine kinetics were determined at the end of the 4 study periods by using a standard stable-isotope technique in the postabsorptive state and during a 3-h infusion of a 0.13 g · kg LBM^–1 · h^–1 amino acid mixture.

Results: In the postabsorptive state, we found a significant (P = 0.04) bed rest x hypocaloric diet interaction for the rate of leucine oxidation, an index of net protein catabolism (A+E: 0.23 ± 0.01; B+E: 25 ± 0.01; A+H: 0.23 ± 0.01; B+H: 0.28 ± 0.01 µmol · min^–1 · kg LBM^–1). Bed rest significantly (P < 0.01) decreased amino acid–mediated stimulation of nonoxidative leucine disappearance, an index of protein synthesis (A+E: 35 ± 2%; B+E: 30 ± 2%; A+H: 41 ± 3%; B+H: 32 ± 2%). B+H decreased LBM by 1.10 ± 0.1 kg, which is significantly (P < 0.01) greater than the decrease seen with A+E, A+H, or B+E.

Conclusion: Calorie restriction enhanced the catabolic response to inactivity by combining greater protein catabolism in the postabsorptive state with an impaired postprandial anabolic utilization of free amino acids.

Key Words: Healthy volunteers • muscle inactivity • protein metabolism • hypocaloric diet • bed rest • leucine kinetics • lean body mass

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