Amino acids stimulate muscle protein anabolism in the elderly regardless of route of administration

It is unclear if exogenous amino acids can stimulate muscle protein anabolism in the elderly despite the significant reduction in muscle mass and function associated with the aging process. We measured phenylalanine net balance across the leg in two groups of healthy elderly subjects during the basal post-absorptive state and the intra-venous (IVAA, n=6, age 71±2 yrs, mean±SE) or oral (OAA, n=5, age 68±1 yrs) administration of an amino acid mixture (3 mg-kg-min"1, phenylalanine 0.8 nmol-kg~'-min"1). In addition, phenylalanine rate of disappearance in the leg was measured in both groups by the means of i.v. [ring-2H5]phenylalanme infusion and phenylalanine splanchnic extraction was measured in the OAA group by adding [ring-13C6]phenylalanine to the oral mixture. Amino acid administration resulted in a significant (p<0.02) improvement of leg net balance from negative to positive in both groups (basal: IVAA -81±24, OAA -32±11; amino acid administration: IVAA 94±45, OAA 15±3, nmol/min-100 ml of leg) with no differences between the two groups. Phenylalanine rate of disappearance significantly increased as well (p<0.02). Phenylalanine extraction by the splanchnic tissue in the OAA group was 46±3 %. Our results indicate that exogenous amino acids stimulate muscle protein anabolism in healthy elderly subjects. Furthermore, since the same amount of amino acids induces a similar muscle response regardless of the i.v. or oral administration route and despite a -50 % splanchnic extraction during the oral intake, it is possible to hypothesize that the splanchnic tissue plays a role in amino acid disposal during i v. amino acid intake as well.