Ingestion of sufficient dietary protein is a fundamental prerequisite for muscle protein synthesis and maintenance of muscle mass and function. Elderly people are often at increased risk for protein-energy malnutrition, sarcopenia, and a diminished quality of life. This study sought to compare changes in muscle protein synthesis and anabolic efficiency in response to a single moderate serving (113 g; 220 kcal; 30 g protein) or large serving (340 g; 660 kcal; 90 g protein) of 90% lean beef. Venous blood and vastus lateralis muscle biopsy samples were obtained during a primed, constant infusion (0.08 μmol/kg/min) of L-[ring-13C6] phenylalanine in healthy young (n=17; 34±3 years) and elderly (n=17; 68±2 years) individuals. Mixed muscle fractional synthesis rate was calculated during a 3-hour postabsorptive period and for 5 hours after meal ingestion. Data were analyzed using a two-way repeated measures analysis of variance with Tukey's pairwise comparisons. A 113-g serving of lean beef increased muscle protein synthesis by approximately 50% in both young and older volunteers. Despite a threefold increase in protein and energy content, there was no further increase in protein synthesis after ingestion of 340 g lean beef in either age group. Ingestion of more than 30 g protein in a single meal does not further enhance the stimulation of muscle protein synthesis in young and elderly.
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T. B. Symons is an assistant professor, Graduate Center for Gerontology, University of Kentucky, Lexington; at the time of the study, he was a postdoctoral fellow, Division of Rehabilitation Sciences, The University of Texas Medical Branch, Galveston.
M. Sheffield-Moore is an associate professor, Department of Internal Medicine, The University of Texas Medical Branch, Galveston.
D. Paddon-Jones is an associate professor, Departments of Physical Therapy and Internal Medicine, The University of Texas Medical Branch, Galveston.
R. R. Wolfe is a professor, University of Arkansas for Medical Sciences, Little Rock; at the time of the study he was a professor, Department of Surgery, The University of Texas Medical Branch, Galveston.
Accepted: January 30, 2009
© 2009 American Dietetic Association. Published by Elsevier Inc. All rights reserved.