Leucine co-ingestion improves post-prandial muscle protein accretion in elderly men

Background & aims: 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. Methods: Twenty-four elderly men (74.3 ± 1.0 y) were randomly assigned to ingest 20 g intrinsically L-[1-13C]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-[1-13C] phenylalanine into muscle protein. Results: 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-[1-13C]phenylalanine enrichments increased rapidly and to a similar extent in both groups following protein ingestion. Muscle protein-bound L-[1-13C]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). Conclusion: Leucine co-ingestion with a bolus of pure dietary protein further stimulates post-prandial muscle protein synthesis rates in elderly men.