Calorie restriction accelerates the catabolism of lean body mass

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 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 mol min 1 kg LBM ). 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. Am J Clin Nutr 2007;86:366–72.