Oral and intravenously administered amino acids produce similar effects on muscle protein synthesis in the elderly

Abstract

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).

Figure 1

Percent change from basal of phenylalanine arterial concentration and delivery to the leg. Open bars, oral ingestion of amino acids (OAA); closed bars, intravenous infusion of amino acids (IV).

Figure 2

Muscle protein fractional synthetic rate (FSR) as measured by the precursor-product method. Open bars, oral ingestion of amino acids; closed bars, intravenous infusion of amino acids.