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Aging is associated with diminished accretion of muscle proteins after the ingestion of a small bolus of essential amino acids^1,2,3

¹ From the Departments of Surgery (CSK and RRW), Internal Medicine (MS-M), and Anesthesiology (AA), the University of Texas Medical Branch, Galveston, TX, and the AminoScience Laboratories, Ajinomoto Co Inc, Kawasaki, Japan (HK)

Background: Previous evidence suggests that aging in healthy persons does not result in decreased incorporation of muscle proteins after a bolus ingestion of 15 g essential amino acids (EAAs).

Objective: We sought to examine whether ingestion of a smaller bolus of EAAs is associated with diminished accretion of muscle proteins in the elderly when compared with the young.

Design: Eleven elderly subjects ( ± SEM: 68 ± 2 y) and 8 young control subjects ( ± SEM: 31 ± 2 y) were studied in the postabsorptive state and for 3.5 h after a bolus ingestion of 7 g EAAs. Muscle protein accretion and synthesis were measured with the femoral arteriovenous phenylalanine net balance technique during a constant infusion of L-[ring-²H₅]phenylalanine.

Results: Similar to previous observations, no significant differences in the postabsorptive phenylalanine net balance were observed between the groups. However, the mean (±SEM) net phenylalanine uptake after EAA ingestion was significantly less in the elderly (9.9 ± 3.7 mg/leg) than in the young (25.1 ± 3.7 mg/leg; P < 0.05). The mean (±SEM) rate of disappearance of phenylalanine during the same period significantly increased above basal rates in the young (36 ± 3 compared with 30 ± 3 nmol · min^–1 · 100 mL leg volume^–1; P < 0.05) but not in the elderly (30 ± 3 compared with 28 ± 5 nmol · min^–1 · 100 mL leg volume^–1; P > 0.05).

Conclusions: These data indicate that aging results in a diminished accretion of muscle proteins after ingestion of a small dose of EAAs. These findings may have practical implications with respect to the amount of protein contained in supplements given to the elderly for enhancing the stimulation of muscle protein synthesis.

Key Words: Sarcopenia • elderly • amino acids • nutrition • metabolism • stable isotope

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