Casein Ingestion Does Not Increase Muscle Connective Tissue Protein Synthesis Rates

Abstract

Purpose This study aimed to assess the effect of dietary protein ingestion on intramuscular connective tissue protein synthesis rates during overnight recovery from a single bout of resistance exercise. Methods Thirty-six healthy, young males were randomly assigned to one of three treatments. One group ingested 30 g intrinsically L-[1-13C]-phenylalanine-labeled casein protein before sleep (PRO, n = 12). The other two groups performed a bout of resistance exercise in the evening and ingested either placebo (EX, n = 12) or 30 g intrinsically L-[1-13C]-phenylalanine-labeled casein protein before sleep (EX + PRO, n = 12). Continuous intravenous infusions of L-[ring-2H5]-phenylalanine and L-[1-13C]-leucine were applied, and blood and muscle tissue samples were collected to assess connective tissue protein synthesis rates and dietary protein-derived amino acid incorporation in the connective tissue protein fraction. Results Resistance exercise resulted in higher connective tissue protein synthesis rates when compared with rest (0.086 ± 0.017%·h-1 [EX] and 0.080 ± 0.019%·h-1 [EX + PRO] vs 0.059 ± 0.016%·h-1 [PRO]; P < 0.05). Postexercise casein protein ingestion did not result in higher connective tissue protein synthesis rates when compared with postexercise placebo ingestion (P = 1.00). Dietary protein-derived amino acids were incorporated into the connective tissue protein fraction at rest, and to a greater extent during recovery from exercise (P = 0.002). Conclusion Resistance exercise increases intramuscular connective tissue protein synthesis rates during overnight sleep, with no further effect of postexercise protein ingestion. However, dietary protein-derived amino acids are being used as precursors to support de novo connective tissue protein synthesis.