Dose-response effects of dietary protein on muscle protein synthesis during recovery from endurance exercise in young men: a double-blind randomized trial

Background: Protein ingestion increases skeletal muscle protein synthesis rates during recovery from endurance exercise. Objectives: We aimed to determine the effect of graded doses of dietary protein co-ingested with carbohydrate on whole-body protein metabolism, and skeletal muscle myofibrillar (MyoPS) and mitochondrial (MitoPS) protein synthesis rates during recovery from endurance exercise. Methods: In a randomized, double-blind, parallel-group design, 48 healthy, young, endurance-trained men (mean +/- SEM age: 27 +/- 1 y) received a primed continuous infusion of L-[ring-H-2(5)]phenylalanine, L-[ring-3,5-H-2(2)]-tyrosine, and L-[1-C-13]-leucine and ingested 45 g carbohydrate with either 0 (0 g PRO), 15 (15 g PRO), 30 (30 g PRO), or 45 (45 g PRO) g intrinsically L-[1-C-13]-phenylalanine and L-[1-C-13]-leucine labeled milk protein after endurance exercise. Blood and muscle biopsy samples were collected over 360 min of postexercise recovery to assess whole-body protein metabolism and both MyoPS and MitoPS rates. Results: Protein intake resulted in similar to 70%-74% of the ingested protein-derived phenylalanine appearing in the circulation. Whole-body net protein balance increased dose-dependently after ingestion of 0, 15, 30, or 45 g protein (mean +/- SEM: -0.31 +/- 0.16, 5.08 +/- 0.21, 10.04 +/- 0.30, and 13.49 +/- 0.55 mu mol phenylalanine . kg(-1) . h(-1), respectively; P < 0.001). 30 g PRO stimulated a similar to 46% increase in MyoPS rates (%/h) compared with 0 g PRO and was sufficient to maximize MyoPS rates after endurance exercise. MitoPS rates were not increased after protein ingestion; however, incorporation of dietary protein-derived L-[1-C-13]-phenylalanine into de novo mitochondrial protein increased dose-dependently after ingestion of 15, 30, and 45 g protein at 360 min postexercise (0.018 +/- 0.002, 0.034 +/- 0.002, and 0.046 +/- 0.003 mole percentage excess, respectively; P < 0.001). Conclusions: Protein ingested after endurance exercise is efficiently digested and absorbed into the circulation. Whole-body net protein balance and dietary protein-derived amino acid incorporation into mitochondrial protein respond to increasing protein intake in a dose-dependent manner. Ingestion of 30 g protein is sufficient to maximize MyoPS rates during recovery from a single bout of endurance exercise.