Does Leucine Supplementation Contribute to Muscle Growth in Exercising, Healthy Adults?

Protein intake and resistance exercise are the cornerstone to muscle growth. Following protein ingestion, increased blood and/or intramuscular leucine concentration is considered the main nutrient-derived driver of muscle protein synthesis, leading to a speculation that leucine supplementation could improve muscle mass in exercising individuals. In this study, investigators tested whether supplemental dietary leucine could be beneficial to muscle and strength gains in exercising adults. 

They found that 10 g/day (2x5 g/day) of supplemental leucine did not further enhance the effects of a 12-week supervised resistance training program in young, healthy adults consuming adequate amounts of protein (>1.6 g/kg/day), as similar gains in thigh muscle area and lower-limb maximal strength were observed between leucine- and placebo-supplemented groups. The authors contend that the already sufficient protein provision may have saturated the muscle protein synthetic response, an effect known as muscle full, mitigating any possible additional effects of leucine supplementation.

Although the results cannot be extrapolated to conditions of suboptimal protein intake, the authors speculate that in a theoretical scenario where the supplemental leucine is not paralleled by adequate dietary protein intake, no additive gains in muscle hypertrophy and strength would also be expected.