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Co-ingestion of protein and leucine stimulates muscle protein synthesis rates to the same extent in young and elderly lean men^1,2,3

¹ From the Departments of Human Biology (RK, RJFM, APG, and LJCvL) and Movement Sciences (LV and LJCvL), Nutrition and Toxicology Research Institute Maastricht (NUTRIM), Maastricht University, Maastricht, Netherlands; Numico Research BV, Wageningen, Netherlands (MG); the Department of Internal Medicine, Academic Hospital Maastricht, Maastricht, Netherlands (EP); and the School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom (AJMW).

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.

Key Words: Protein metabolism • sarcopenia • muscle • aging

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