Age Specific Changes in Protein Synthesis and Plasma Amino Acid Profiles Following Intact Protein Ingestion

Abstract

Muscle protein synthesis is acutely stimulated by an increase in precursor availability. While young and elderly respond similarly to ingestion of a supplement containing essential amino acids, there are limited data quantifying the respective changes in plasma amino acid concentrations and protein synthesis following ingestion of a normal dietary protein source. PURPOSE: To compare changes in plasma amino acid concentrations and mixed muscle fractional synthesis rate (FSR) following ingestion of a 4 oz serving of lean beef in healthy young and elderly volunteers. METHODS: Peripheral venous blood samples and vastus lateralis muscle biopsy samples were obtained during a primed, constant infusion of L-ring- 13C6 phenylalanine phenylalanine. Plasma amino acid concentrations were measured using High Performance Liquid Chromatography (HPLC; Waters 2695 Separation Module, 2475 Multi wavelength Detector). Mixed muscle fractional synthetic rate (FSR) was calculated during the post-absorptive period and for 5 h following ingestion of a 4 oz serving of lean beef (90% lean; 30% fat) in young (n=10; 173±4 cm; 88.4±4.6yrs; 28.5±2.8 % body fat; 60.9±3.5 % lean muscle mass) and elderly (n=10; 169±3 cm; 69.9±3.8 yrs; 27.3±1.8 % body fat; 48.4±3.3 % lean muscle mass) volunteers. RESULTS: Beef ingestion increased mixed muscle FSR values in both groups by 27.9±7.5% (Young) and 29.1±9.3% (Elderly) respectively. Plasma amino acid concentrations (essential, non-essential and total) peaked approximately 100 minutes following nutrient ingestion in both young and elderly. The magnitude of the increase in plasma amino acid concentration was significantly greater in elderly individuals, reflecting a smaller plasma volume and lower lean muscle mass. CONCLUSIONS: Despite differences in precursor availability, a standard 4 oz serving of beef stimulates muscle protein synthesis in both young and elderly individuals. Further investigations are needed to examine the efficiency of muscle protein anabolism following ingestion of different amounts of dietary protein in young and elderly individuals. This project is funded (DPJ) by beef and veal producers and importers through their $1-per-head check off and was produced for the Cattlemen's Beef Board and State beef councils by the National Cattlemen's Beef Association. Support also provided by M01 RR 00073 from the National Center for Research Resources, NIH, USPHS.