Carbohydrate co-ingestion with protein does not further augment post-prandial muscle protein accretion in older men

Background: A blunted muscle protein synthetic response to protein ingestion may contribute to the age related loss of muscle tissue. We hypothesized that the greater endogenous insulin release following co-ingestion of carbohydrate facilitates post-prandial muscle protein accretion after ingesting a meal-like bolus of protein in older males. Methods: Twenty-four healthy older men (75+/-1 y) were randomly assigned to ingest 20 g intrinsically L-[1-C-13] phenylalanine-labeled casein protein with (PRO-CHO) or without (PRO) 40 g carbohydrate. Ingestion of specifically produced intrinsically L-[1-C-13] phenylalanine labeled protein allowed us to assess post-prandial incorporation of dietary protein derived amino acids into muscle protein. Blood samples were collected at regular intervals, with muscle biopsies being obtained prior to and 2 and 6 h after protein ingestion. Results: Plasma glucose and insulin concentrations showed a greater increase in PRO-CHO compared with PRO (P<0.001). Muscle protein-bound L-[1-C-13] phenylalanine enrichments tended to increase to a greater extent in PRO-CHO compared with PRO during the first 2 h after protein ingestion (0.0072+/-0.0013 vs 0.0046+/-0.010 MPE, respectively; P=0.13). However, 6 h after protein ingestion, differences in muscle protein-bound L-[1-C-13] phenylalanine enrichments were no longer observed between experiments (0.0213+/-0.0024 vs 0.0185+/-0.0010 MPE, respectively; P=0.30). Conclusions: This study shows that carbohydrate ingestion may accelerate, but does not further augment postprandial incorporation of dietary protein derived amino acids into muscle protein in healthy elderly men.