Older adults are at increased risk of being bedridden and experiencing negative health outcomes including the loss of muscle tissue and functional capacity. We hypothesized that supplementing daily meals with a small quantity (3-4 g/meal) of leucine would partially preserve lean leg mass and function of older adults during bed rest. During a 7-day bed rest protocol, followed by 5 days of inpatient rehabilitation, healthy older men and women (67.8 ± 1.1 yr, 14 men; 6 women) were randomized to receive isoenergetic meals supplemented with leucine (LEU, 0.06 g/kg/meal; n = 10) or an alanine control (CON, 0.06 g/kg/meal; n = 10). Outcomes were assessed at baseline, following bed rest, and after rehabilitation. Body composition was measured by dual-energy X-ray absorptiometry. Functional capacity was assessed by knee extensor isokinetic and isometric dynamometry, peak aerobic capacity, and the short physical performance battery. Muscle fiber type, cross-sectional area, signaling protein expression levels, and single fiber characteristics were determined from biopsies of the vastus lateralis. Leucine supplementation reduced the loss of leg lean mass during bed rest (LEU vs. CON: -423 vs. -1035 ± 143 g; P = 0.008) but had limited impact on strength or endurance-based functional outcomes. Similarly, leucine had no effect on markers of anabolic signaling and protein degradation during bed rest or rehabilitation. In conclusion, providing older adults with supplemental leucine has minimal impact on total energy or protein consumption and has the potential to partially counter some, but not all, of the negative effects of inactivity on muscle health.NEW & NOTEWORTHY Skeletal muscle morphology and function in older adults was significantly compromised by 7 days of disuse. Leucine supplementation partially countered the loss of lean leg mass but did not preserve muscle function or positively impact changes at the muscle fiber level associated with bed rest or rehabilitation. Of note, our data support a relationship between myonuclear content and adaptations to muscle atrophy at the whole limb and single fiber level.
Keywords: aging; bed rest; dietary supplementation; nutrition.