Casein Ingestion Does Not Increase Muscle Connective Tissue Protein Synthesis Rates.

PURPOSE: To assess the impact 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-C]-phenylalanine-labeled casein protein prior to sleep (PRO, n=12). The other two groups performed a bout of resistance exercise in the evening and ingested either a placebo (EX, n=12) or 30 g intrinsically L-[1-C]-phenylalanine-labeled casein protein prior to sleep (EX+PRO, n=12). Continuous intravenous infusions of L-[ring-H5]-phenylalanine and L-[1-C]-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 to rest (0.086+/-0.017 (EX) and 0.080+/-0.019 (EX+PRO) vs 0.059+/-0.016 (PRO) %.h; P<0.05). Post-exercise casein protein ingestion did not result in higher connective tissue protein synthesis rates when compared to post-exercise 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 impact of post-exercise protein ingestion. However, dietary protein-derived amino acids are being utilized as precursors to support de novo connective tissue protein synthesis.