Carbohydrate Coingestion Delays Dietary Protein Digestion and Absorption but Does Not Modulate Postprandial Muscle Protein Accretion

Background: Dietary protein digestion and absorption is an important factor modulating muscle protein accretion. However, there are few data available on the effects of co-ingesting other macronutrients with protein on digestion and absorption kinetics and the subsequent muscle protein synthetic response. Objective: To determine the impact of carbohydrate co-ingestion with protein on dietary protein digestion and absorption and muscle protein accretion in healthy young and older men. Design: Twenty-four healthy young (age: 21+/-1 y, BMI: 21.8+/-0.5 kg.m-2) and 25 older (age: 75+/-1 y, BMI: 25.4+/-0.6 kg.m-2) men received a primed continuous L-[ring-2H5]-phenylalanine and L-[ring-3,5-2H2]-tyrosine infusion and ingested 20 g intrinsically L-[1-13C]-phenylalanine-labeled protein with (Pro+CHO) or without (Pro) 60 g carbohydrate. Plasma samples and muscle biopsies were collected in a postabsorptive and postprandial state. Results: Carbohydrate co-ingestion delayed the appearance of exogenous phenylalanine in the circulation (P=0.001). Dietary protein-derived phenylalanine availability over the 5 h postprandial period was lower in the older (62+/-2%) when compared with the young subjects (74+/-2%; P=0.007), with no differences between conditions (P=0.20). Carbohydrate co-ingestion did not modulate postprandial muscle protein synthesis rates (0.035+/-0.003 vs 0.043+/-0.004 and 0.033+/-0.002 vs 0.035+/-0.003 %.h-1 following Pro vs Pro+CHO in the young and older group, respectively). In accordance, no differences in muscle protein-bound L-[1-13C]-phenylalanine enrichments were observed between conditions (0.020+/-0.002 vs 0.020+/-0.002 and 0.019+/-0.003 vs 0.022+/-0.004 mole percent excess following Pro vs Pro+CHO in the young and older subjects, respectively). Conclusion: Carbohydrate co-ingestion with protein delays dietary protein digestion and absorption, but does not modulate postprandial muscle protein accretion in healthy young or older men.