['It has been speculated that the amount of leucine in a meal largely determines the post-prandial muscle protein synthetic response to food intake. The present study investigates the impact of leucine co-ingestion on subsequent post-prandial muscle protein accretion following the ingestion of a single bolus of dietary protein in elderly males.', 'Twenty-four elderly men (74.3±1.0 y) were randomly assigned to ingest 20 g intrinsically L-[1-(13)C]phenylalanine-labeled casein protein with (PRO+LEU) or without (PRO) 2.5 g crystalline leucine. Ingestion of specifically produced intrinsically labeled protein allowed us to create a plasma phenylalanine enrichment pattern similar to the absorption pattern of phenylalanine from the ingested protein and assess the subsequent post-prandial incorporation of L-[1-(13)C] phenylalanine into muscle protein.', 'Plasma amino acid concentrations increased rapidly following protein ingestion in both groups, with higher leucine concentrations observed in the PRO+LEU compared with the PRO group (P<0.01). Plasma L-[1-(13)C]phenylalanine enrichments increased rapidly and to a similar extent in both groups following protein ingestion. Muscle protein-bound L-[1-(13)C]phenylalanine enrichments were significantly greater after PRO+LEU when compared with PRO at 2 h (72%; 0.0078±0.0010 vs. 0.0046±0.00100 MPE, respectively; P<0.05) and 6 h (25%; 0.0232±0.0015 vs. 0.0185±0.0010 MPE, respectively; P<0.05) following protein ingestion. The latter translated into a greater muscle protein synthetic rate following PRO+LEU compared with PRO over the entire 6 h post-prandial period (22%; 0.049±0.003 vs. 0.040±0.003% h(-1), respectively; P<0.05).'] Does leucine co-ingestion improve post-prandial muscle protein accretion in elderly men?