B.B. Rasmussen*,***, R.R. Wolfe*, E. Volpi**,****

*Departments of Surgery and **Internal Medicine, University of Texas Medical Branch, Galveston, TX 77550; ***Department of Kinesiology, University of Southern California, Los Angeles, CA 90089; ****Department of Medicine, Division of Endocrinology and Diabetes, University of Southern California, Los Angeles, CA 90033, USA. Correspondence: Blake B. Rasmussen, Ph.D., University of Southern California, Department of Kinesiology, University Park Campus, 3560 Watt Way, PED 107, Los Angeles, CA 90089-0652.
Phone: (213) 821-2846. Fax: (213) 740-7909. Email: blakeras@usc.edu

BACKGROUND: Muscle protein synthesis is stimulated in the elderly when amino acid availability is increased. OBJECTIVE: To determine which mode of delivery of amino acids (intravenous vs. oral ingestion) is more effective in stimulating the rate of muscle protein synthesis in elderly subjects. DESIGN: Fourteen elderly subjects were assigned to one of two groups. Following insertion of femoral arterial and venous catheters, subjects were infused with a primed, continuous infusion of L-[ring-2H5] phenylalanine. Blood samples and muscle biopsies were obtained to measure muscle protein fractional synthesis rate (FSR) with the precursor-product model, phenylalanine kinetics across the leg with the three-pool model, and whole body phenylalanine kinetics. Protein metabolism parameters were measured in the basal period, and during the administration of oral amino acids (n=8) or a similar amount of intravenous amino acids (n=6). RESULTS: Enteral and parenteral amino acid administration increased amino acid arterial concentrations and delivery to the leg to a similar extent in both groups. Muscle protein synthesis as measured by both FSR, and the three-pool model, increased during amino acid administration (P < 0.05 vs. basal) in both groups with no differences between groups. Whole body proteolysis did not change with the oral amino acids whereas it increased slightly during parenteral amino acid administration. CONCLUSIONS: Increased amino acid availability stimulates the rate of muscle protein synthesis independent of the route of administration (enteral vs. parenteral).

Keywords: Amino acids, aging, muscle protein synthesis, stable isotopes.