Oral amino acids stimulate muscle protein anabolism in the elderly despitehigher first-pass splanchnic extraction

Muscle protein synthesis and breakdown and amino acid transport; were measu red in 7 healthy young (30 +/- 2 yr) and 8 healthy elderly (71 +/- 2 yr) vo lunteers in the postabsorptive state and during the oral administration of an amino acid mixture with L-[ring-H-2(5)]phenylalanine infusion, femoral a rtery and vein catheterization, and muscle biopsies. Phenylalanine first-pa ss splanchnic extraction was measured by adding L-[ring-C-13(6)] phenylalan ine to the mixture. In the postabsorptive state, no differences in muscle a mino acid kinetics were detected between young and elderly volunteers. Phen ylalanine first-pass splanchnic extraction was significantly higher in the elderly (P < 0.003) during ingestion of amino acids, but the delivery to th e leg increased to the same extent in both groups. Phenylalanine transport into the muscle, muscle protein synthesis, and net balance increased signif icantly (P < 0.01) and similarly in both the young and the elderly. We conc lude that, despite an increased splanchnic first-pass extraction, muscle pr otein anabolism can be stimulated by oral amino acids in the elderly as wel l as in the young.