BACKGROUND: Essential amino acids (EAA) and aerobic exercise (AE) acutely and independently stimulate skeletal muscle protein anabolism in older adults. OBJECTIVE: In this Phase 1, double-blind, placebo-controlled, randomized clinical trial, we determined if chronic EAA supplementation, AE training, or a combination of the two interventions could improve muscle mass and function by stimulating muscle protein synthesis. METHODS: We phone-screened 971, enrolled 109, and randomized 50 independent, low-active, nonfrail, and nondiabetic older adults (age 72 +/- 1 years). We used a 2x2 factorial design. The interventions were: daily nutritional supplementation (15 g EAA or placebo) and physical activity (supervised AE training 3 days/week or monitored habitual activity) for 24 weeks. Muscle strength, physical function, body composition, and muscle protein synthesis were measured before and after the 24-week intervention. RESULTS: Forty-five subjects completed the 24-week intervention. VO2peak and walking speed increased (p < 0.05) in both AE groups, irrespective of supplementation type, but muscle strength increased only in the EAA+AE group (p < 0.05). EAA supplementation acutely increased (p < 0.05) muscle protein synthesis from basal both before and after the intervention, with a larger increase in the EAA+AE group after the intervention. Total and regional lean body mass did not change significantly with any intervention. CONCLUSIONS: In nonfrail, independent, healthy older adults AE training increased walking speed and aerobic fitness, and, when combined with EAA supplementation, it also increased muscle strength and EAA-stimulated muscle protein synthesis. These increases occurred without improvements in muscle mass.
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