Co-ingestion of Protein and Leucine Stimulates Muscle Protein Synthesis in Young and Elderly Men

Abstract

Aging is associated with a progressive loss of skeletal muscle mass, which is attributed to a disruption in the regulation of skeletal muscle protein turnover, which results in a chronic imbalance between muscle protein synthesis and breakdown rate. Knowledge of differences in responses of young and elderly subjects under normal daily living conditions are of crucial importance to our understanding of the etiology of sarcopenia and are currently lacking in the literature. PURPOSE: We investigated the response to the ingestion of carbohydrate with or without protein and free leucine following simulated activities of daily living (ADL) on whole-body protein balance and mixed muscle protein synthesis rates. METHODS: Eight elderly (75±1 y) and eight young (20±1 y) male subjects were randomly assigned to 2 trials in which they consumed either carbohydrate (CHO) or carbohydrate, protein and free leucine (CHO+PRO+leu) after performing 30 min of standardized ADL activities. 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 as well as fractional protein synthetic rate (FSR) in the vastus lateralis muscle over a 6 h period. RESULTS: Plasma insulin responses were significantly higher in the CHO+PRO+leu trial than in the CHO trial in both the young and elderly subjects (p<0.05). Whole-body phenylalanine and tyrosine fluxes were significantly higher in the young versus the elderly (p<0.01). Protein balance was negative in the CHO trial, but positive in the CHO+PRO+leu trial in both groups. In the CHO trial, muscle FSR was significantly lower in the elderly compared with the young subjects (p<0.05). Mixed muscle protein synthesis rates were significantly greater in the CHO+PRO+leu compared with the CHO trial in both the young (0.082±0.005%.h−1 vs. 0.060±0.005%h−1, respectively; p<0.01), and elderly subjects (0.072±0.006%h−1 vs. 0.043±0.003 %h−1 respectively; p<0.01), with no differences between groups. CONCLUSIONS: Co-ingestion of protein hydrolysate and leucine with carbohydrate following ADL activities improves whole-body protein balance and increases muscle protein synthesis rates to the same extent in young and elderly men.