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ORIGINAL RESEARCH COMMUNICATION |
1 From the Division of Rehabilitation Sciences (TBS) and the Departments of Surgery (SES, TLC, DLC, and RRW) and Physical Therapy and Internal Medicine (DPJ), The University of Texas Medical Branch, Galveston, TX
Background: Sarcopenia is a debilitating condition afflicting the elderly that may be facilitated by insufficient or ineffectual intake of dietary protein. We previously showed that free-form essential amino acids acutely stimulate muscle protein synthesis in both the young and the elderly. However, the ability of an actual protein-rich food to stimulate anabolism in the young and the elderly has not been explored.
Objective: We aimed to characterize changes in plasma amino acid concentrations and to quantify muscle protein synthesis in healthy young (41 ± 8 y old; n = 10) and elderly (70 ± 5 y old; n = 10) persons after ingestion of a 113-g (4-oz) serving of lean beef.
Design: Venous blood samples and vastus lateralis muscle biopsy samples were obtained during a primed (2.0 µmol/kg) constant infusion (0.08 µmol·kg–1·min–1) of L-[ring-13C6] phenylalanine. Plasma amino acid concentrations were measured and a mixed-muscle fractional synthesis rate (FSR) was calculated during the premeal period and for 5 h after beef ingestion.
Results: Mixed-muscle FSR increased by 
51% in both the elderly (mean ± SE measurements: 0.072 ± 0.004%/h and 0.108 ± 0.006%/h before and after the meal, respectively) and the young (0.074 ± 0.005%/h and 0.113 ± 0.005%/h before and after the meal, respectively) after beef ingestion (P < 0.001). Plasma amino acid concentrations peaked at 100 min after beef ingestion in both age groups but were substantially higher in the elderly (2185 ± 134 nmol/mL compared with 1403 ± 96 nmol/mL; P < 0.001).
Conclusion: Despite differences in the concentration of amino acids in the plasma precursor pool, aging does not impair the ability to acutely synthesize muscle protein after ingestion of a common protein-rich food.
Key Words: Nutrition • stable isotopes • sarcopenia • diet • beef • amino acids
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