Dose-Dependent Increases in Whole-Body Net Protein Balance and Dietary Protein-Derived Amino Acid Incorporation into Myofibrillar Protein During Recovery from Resistance Exercise in Older Men

Background: Age-related decline in skeletal muscle mass is at least partly attributed to anabolic resistance to food intake. Resistance exercise sensitizes skeletal muscle tissue to the anabolic properties of amino acids.Objective: The present study assessed protein digestion and amino acid absorption kinetics, whole-body protein balance, and the myofibrillar protein synthetic response to ingestion of different amounts of protein during recovery from resistance exercise in older men.Methods: Forty-eight healthy older men [mean +/- SEM age: 66 +/- 1 y; body mass index (kg/m(2)): 25.4 +/- 0.3] were randomly assigned to ingest 0, 15, 30, or 45 g milk protein concentrate after a single bout of resistance exercise consisting of 4 sets of 10 repetitions of leg press and leg extension and 2 sets of 10 repetitions of lateral pulldown and chest press performed at 75-80% 1-repetition maximum. Postprandial protein digestion and amino acid absorption kinetics, whole-body protein metabolism, and myofibrillar protein synthesis rates were assessed using primed, continuous infusions of L-[ring-H-2(5)]-phenylalanine, L-[ring-H-2(2)]-tyrosine, and L-[1-C-13]-leucine combined with ingestion of intrinsically L-[1-C-13]phenylalanine and L-[1-C-13]-leucine labeled protein.Results: Whole-body net protein balance showed a dose-dependent increase after ingestion of 0, 15, 30, or 45 g of protein (0.015 +/- 0.002, 0.108 +/- 0.004, 0.162 +/- 0.008, and 0.215 +/- 0.009 mu mol Phe . kg(-1) . min(-1), respectively; P <0.001). Myofibrillar protein synthesis rates were higher after ingesting 30 (0.0951% +/- 0.0062%/h, P = 0.07) or 45 g of protein (0.0970% +/- 0.0062%/h, P <0.05) than after 0 g (0.0746% +/- 0.0051%/h). Incorporation of dietary protein-derived amino acids (L-[1-13C]-phenylalanine) into de novo myofibrillar protein showed a dose-dependent increase after ingestion of 15, 30, or 45 g protein (0.0171 +/- 0.0017, 0.0296 +/- 0.0030, and 0.0397 +/- 0.0026 mole percentage excess, respectively; P <0.05).Conclusions: Dietary protein ingested during recovery from resistance exercise is rapidly digested and absorbed. Whole-body net protein balance and dietary protein-derived amino acid incorporation into myofibrillar protein show dosedependent increases. Ingestion of >= 30 g protein increases postexercise myofibrillar protein synthesis rates in older men.