Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men

Key points Essential amino acids (EAAs) stimulate increased rates of myofibrillar protein synthesis (MPS). Leucine is a key regulator of MPS in rodents; however, its importance relative to the other EAAs is not clear. About 20 g of protein maximally stimulates MPS after resistance exercise in young men, but we do not know if smaller doses can be made better by adding certain amino acids. We report that a suboptimal dose of whey protein (6.25 g) supplemented with either leucine or a mixture of EAAs without leucine stimulates MPS similar to 25 g of whey protein under resting conditions; however, only 25 g of whey sustains exercise-induced rates of MPS. Adding leucine or a mixture of EAAs without leucine to a suboptimal dose of whey is as effective as 25 g whey at stimulating fed rates of MPS; however, 25 g of whey is better suited to increase resistance exercise-induced muscle anabolism. Abstract Leucine is a nutrient regulator of muscle protein synthesis by activating mTOR and possibly other proteins in this pathway. The purpose of this study was to examine the role of leucine in the regulation of human myofibrillar protein synthesis (MPS). Twenty-four males completed an acute bout of unilateral resistance exercise prior to consuming either: a dose (25 g) of whey protein (WHEY); 6.25 g whey protein with total leucine equivalent to WHEY (LEU); or 6.25 g whey protein with total essential amino acids (EAAs) equivalent to WHEY for all EAAs except leucine (EAA-LEU). Measures of MPS, signalling through mTOR, and amino acid transporter (AAT) mRNA abundance were made while fasted (FAST), and following feeding under rested (FED) and post-exercise (EX-FED) conditions. Leucinaemia was equivalent between WHEY and LEU and elevated compared to EAA-LEU (P =0.001). MPS was increased above FAST at 13 h post-exercise in both FED (P < 0.001) and EX-FED (P < 0.001) conditions with no treatment effect. At 35 h, only WHEY remained significantly elevated above FAST in EX-FED (WHEY 184%vs. LEU 55% and EAA-LEU 35%; P= 0.036). AAT mRNA abundance was increased above FAST after feeding and exercise with no effect of leucinaemia. In summary, a low dose of whey protein supplemented with leucine or all other essential amino acids was as effective as a complete protein (WHEY) in stimulating postprandial MPS; however only WHEY was able to sustain increased rates of MPS post-exercise and may therefore be most suited to increase exercise-induced muscle protein accretion.