Leucine is regarded as an anabolic trigger for the mTORC1 pathway and the stimulation muscle protein synthesis rates. More recently, there has been an interest in underpinning the relevance of branched-chain amino acid (BCAA)-containing dipeptides and their intact absorption into circulation to regulate muscle anabolic responses. We investigated the effects of dileucine and leucine ingestion on postprandial muscle protein turnover. Ten healthy young men (age: 23 ± 3 yr) consumed either 2 g of leucine (LEU) or 2 g of dileucine (DILEU) in a randomized crossover design. The participants underwent repeated blood and muscle biopsy sampling during primed continuous infusions of l-[ring-13C6]phenylalanine and l-[15N]phenylalanine to determine myofibrillar protein synthesis (MPS) and mixed muscle protein breakdown rates (MPB), respectively. LEU and DILEU similarly increased plasma leucine net area under the curve (AUC; P = 0.396). DILEU increased plasma dileucine AUC to a greater extent than LEU (P = 0.013). Phosphorylation of Akt (P = 0.002), rpS6 (P < 0.001), and p70S6K (P < 0.001) increased over time under both LEU and DILEU conditions. Phosphorylation of 4E-BP1 (P = 0.229) and eEF2 (P = 0.999) did not change over time irrespective of condition. Cumulative (0-180 min) MPS increased in DILEU (0.075 ± 0.032%·h-1), but not in LEU (0.047 ± 0.029%·h-1; P = 0.023). MPB did not differ between LEU (0.043 ± 0.030%·h-1) and DILEU conditions (0.051 ± 0.027%·h-1; P = 0.659). Our results showed that dileucine ingestion elevated plasma dileucine concentrations and muscle protein turnover by stimulating MPS in young men.NEW & NOTEWORTHY The role of dipeptides as anabolic agents remains unresolved in humans. We show that the ingestion of 2 g dileucine increased plasma dileucine concentrations and resulted in an enhancement of muscle protein turnover by stimulating an increase in muscle protein synthesis rates in healthy young males. The ingestion of 2 g leucine, however, did not stimulate an increase in muscle protein turnover. Our work provides the first insights into the effects of dipeptides on human protein metabolism.
Keywords: BCAA; anabolism; dipeptides; muscle protein breakdown; muscle protein synthesis.