Short-term bed rest impairs amino acid-induced protein anabolism in humans.

Diminished muscular activity is associated with alterations of protein metabolism. The aim of this study was to evaluate the effect of short-term muscle inactivity on regulation of whole-body protein deposition during amino acid infusion to simulate an experimental postprandial state. We studied nine healthy young volunteers at the end of 14 day periods of strict bed rest and of controlled ambulation using a cross-over design. Subjects received a weight-maintaining diet containing 1 g protein kg(-1) day(-1). l[1-(13)C]leucine was used as a marker of whole-body protein kinetics in the postabsorptive state and during a 3 h infusion of an amino acid mixture (0.13 g amino acid (kg lean body mass)(-1) h(-1)). In the postabsorptive state, bed rest decreased (P < 0.05) the rate of leucine disposal (R(d)) to protein synthesis and tended to decrease leucine rate of appearance (R(a)) from proteolysis, whereas the rate of leucine oxidation did not change significantly. Amino acid infusion increased leucine R(d) to protein synthesis and oxidation and decreased leucine R(a) from proteolysis in both the bed rest and ambulatory conditions. Changes from basal in leucine R(d) to protein synthesis were lower (P < 0.05) during bed rest than those in the ambulatory period, whereas changes in leucine R(a) from proteolysis and oxidation were not significantly different. During amino acid infusion, net leucine deposition into body protein was 8 +/- 3% lower during bed rest than during the ambulatory phase. In conclusion, short-term bed rest leads to reduced stimulation of whole-body protein synthesis by amino acid administration. Results of this study were, in part, presented at the meeting, Experimental Biology, 2004, Washington DC

Calorie restriction accelerates the catabolism of lean body mass during 2 wk of bed rest.

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) (approximately 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 x kg LBM(-1) x 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 micromol x min(-1) x 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