Leucine partially protects muscle mass and function during bed rest in middle-aged adults

Background: Physical inactivity triggers a rapid loss of muscle mass and function in older adults. Middle-aged adults show few phenotypic signs of aging yet may be more susceptible to inactivity than younger adults. Objective: The aim was to determine whether leucine, a stimulator of translation initiation and skeletal muscle protein synthesis (MPS), can protect skeletal muscle health during bed rest. Design: We used a randomized, double-blind, placebo-controlled trial to assess changes in skeletal MPS, cellular signaling, body composition, and skeletal muscle function in middle-aged adults (n = 19; age +/- SEM: 52 +/- 1 y) in response to leucine supplementation (LEU group: 0.06 g . kg(-1) . meal(-1)) or an alanine control (CON group) during 14 d of bed rest. Results: Bed rest decreased postabsorptive MPS by 30% +/- 9% (CON group) and by 10% +/- 10% (LEU group) (main effect for time, P < 0.05), but no differences between groups with respect to pre-post changes (group x time interactions) were detected for MPS or cell signaling. Leucine protected knee extensor peak torque (CON compared with LEU group: -15% +/- 2% and -7% +/- 3%; group x time interaction, P < 0.05) and endurance (CON compared with LEU: -14% +/- 3% and -2% +/- 4%; group x time interaction, P < 0.05), prevented an increase in body fat percentage (group x time interaction, P < 0.05), and reduced whole-body lean mass loss after 7 d (CON compared with LEU: -1.5 +/- 0.3 and -0.8 +/- 0.3 kg; group x time interaction, P < 0.05) but not 14 d (CON compared with LEU: -1.5 +/- 0.3 and -1.0 +/- 0.3 kg) of bed rest. Leucine also maintained muscle quality (peak torque/kg leg lean mass) after 14 d of bed-rest inactivity (CON compared with LEU: -9% +/- 2% and +1% +/- 3%; group x time interaction, P < 0.05). Conclusions: Bed rest has a profoundly negative effect on muscle metabolism, mass, and function in middle-aged adults. Leucine supplementation may partially protect muscle health during relatively brief periods of physical inactivity. This trial was registered at clinicaltrials.gov as NCT00968344.