BACKGROUND: The progressive loss of skeletal muscle mass with aging is attributed to a disruption in the regulation of skeletal muscle protein turnover. OBJECTIVE: We investigated the effects on whole-body protein balance and mixed-muscle protein synthesis rates of the ingestion of carbohydrate with or without protein and free leucine after simulated activities of daily living. DESIGN: Eight elderly (75 +/- 1 y) and 8 young (20 +/- 1 y) lean men were randomly assigned to 2 crossover experiments in which they consumed either carbohydrate (CHO) or carbohydrate plus protein and free leucine (CHO+Pro+Leu) after performing 30 min of standardized activities of daily living. Primed, continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover and the protein fractional synthetic rate in the vastus lateralis muscle over a 6-h period. RESULTS: Whole-body phenylalanine and tyrosine flux were significantly higher in the young than in the elderly men (P < 0.01). Protein balance was negative in the CHO experiment but positive in the CHO+Pro+Leu experiment in both groups. Mixed-muscle protein synthesis rates were significantly greater in the CHO+Pro+Leu than in the CHO experiment in both the young (0.082 +/- 0.005%/h and 0.060 +/- 0.005%/h, respectively; P < 0.01) and the elderly (0.072 +/- 0.006%/h and 0.043 +/- 0.003%/h, respectively; P < 0.01) subjects, with no significant differences between groups. CONCLUSIONS: Co-ingestion of protein and leucine with carbohydrate after activities of daily living improves whole-body protein balance, and the increase in muscle protein synthesis rates is not significantly different between lean young and elderly men.