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 Ȁ 8 y old; n ҃ 10) and elderly (70 Ȁ 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 mol/kg) constant infusion (0.08 mol⅐kg Ҁ1 ⅐min Ҁ1 ) of L-[ring-13 C 6 ] phenylalanine. Plasma amino acid concentrations were measured and a mixedmuscle fractional synthesis rate (FSR) was calculated during the premeal period and for 5 h after beef ingestion. Results: Mixed-muscle FSR increased by Ȃ51% in both the elderly (mean Ȁ SE measurements: 0.072 Ȁ 0.004%/h and 0.108 Ȁ 0.006%/h before and after the meal, respectively) and the young (0.074 Ȁ 0.005%/h and 0.113 Ȁ 0.005%/h before and after the meal, respectively) after beef ingestion (P 0.001). Plasma amino acid concentrations peaked at Ȃ100 min after beef ingestion in both age groups but were substantially higher in the elderly (2185 Ȁ 134 nmol/mL compared with 1403 Ȁ 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.Am J Clin Nutr 2007;86:451-6.

Relative to essential amino acids (EAAs), carbohydrate (CHO) ingestion stimulates a delayed response of net muscle protein balance (NBAL). We investigated if staggered ingestion of CHO and EAA would superimpose the response of NBAL following resistance exercise, thus resulting in maximal anabolic stimulation. Eight recreationally trained subjects completed 2 trials: combined (COMB — drink 1, CHO+EAA; drink 2, placebo) and separated (SEP — drink 1, CHO; drink 2, EAA) post-exercise ingestion of CHO and EAA. Drink 1 was administered 1 h following an acute exercise bout and was followed 1 h later by drink 2. A primed, continuous infusion of l-[ring-13C6]-phenylalanine was combined with femoral arteriovenous sampling and muscle biopsies for the determination of muscle protein kinetics. Arterial amino acid concentrations increased following ingestion of EAA in both conditions. No difference between conditions was observed for phenylalanine delivery to the leg (COMB: 167 ± 23 μmol·min−1·(100 mL leg vol)−1 × 6 h; SEP: 167 ± 21 μmol·min−1·(100 mL leg vol)−1 × 6 h, P > 0.05). In the first hour following ingestion of the drink containing EAA, phenylalanine uptake was 50% greater for the SEP trial than the COMB trial. However, phenylalanine uptake was similar for COMB (110 ± 19 mg) and SEP (117 ± 24 mg) over the 6 h period. These data suggest that whereas separation of CHO and EAA ingestion following exercise may have a transient physiological impact on NBAL, this response is not reflected over a longer period. Thus, separation of CHO and EAA ingestion is unnecessary to optimize post-exercise muscle protein metabolism.

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