Protein–leucine ingestion after strenuous endurance exercise accentuates muscle protein synthesis and improves recovery of muscle performance.
Purpose: The objective of this study is to determine whether a low-dose protein–leucine blend ingested after endurance exercise enhances skeletal muscle myofibrillar protein fractional synthetic rate (FSR).
Method: In a crossover design, 12 trained men completed 100 min of high-intensity cycling, then ingested either 70/15/180/30 g of protein/leucine/carbohydrate/fat (15LEU), 23/5/180/30 g of 5LEU, or 0/0/274/30 g of CON beverages in randomized order in four servings during the first 90 min of a 240-min recovery period. Muscle biopsies were collected at 30 and 240 min into recovery with FSR determined by L-[ring-13C6]phenylalanine incorporation and mTORC1 pathway phosphorylation by Western blot.
Results: The 33% (90% CL, ±12%) increase in FSR with 5LEU (mean, SD: 0.080, 0.014%·h−1) versus CON (0.060, 0.012%·h−1) represented near-maximal FSR stimulation. Tripling protein–leucine dose (15LEU: 0.090, 0.11%·h−1) negligibly increased FSR (13%, ±12% vs 5LEU). Despite similar FSR, mTORC1Ser2448 phosphorylation only increased with 15LEU at 30 min, whereas p70S6KThr389, rpS6Ser240/244, and 4E-BP1γSer112 phosphorylation increased with protein–leucine quantity at one or both time points. Plasma leucine and essential amino acid concentrations decreased during recovery in CON but increased with protein–leucine dose. Serum insulin was increased in 15LEU versus CON (60%, ±20%) but was unaffected relative to 5LEU. Regression analysis revealed p70S6K–rpS6 phosphorylation moderately predicted FSR, but the associations with plasma leucine and essential amino acids were small.
Conclusions: Ingesting 23 g of protein with 5 g of added leucine achieved near-maximal FSR after endurance exercise, an effect unlikely attributable to mTORC1–S6K–rpS6 signaling, insulin, or amino acids. Translating the effects of protein–leucine quantity on protein synthesis to optimizing adaptation and performance requires further research.