Isolated Leucine and Branched-Chain Amino Acid Supplementation for Enhancing Muscular Strength and Hypertrophy: A Narrative Review

Abstract

Branched-chain amino acids (BCAA) are one of the most popular sports supplements, marketed under the premise that they enhance muscular adaptations. Despite their prevalent consumption among athletes and the general public, the efficacy of BCAA has been an ongoing source of controversy in the sports nutrition field. Early support for BCAA supplementation was derived from extrapolation of mechanistic data on their role in muscle protein metabolism. Of the three BCAA, leucine has received the most attention because of its ability to stimulate the initial acute anabolic response. However, a substantial body of both acute and longitudinal research has now accumulated on the topic, affording the ability to scrutinize the effects of BCAA and leucine from a practical standpoint. This article aims to critically review the current literature and draw evidence-based conclusions about the putative benefits of BCAA or leucine supplementation on muscle strength and hypertrophy as well as illuminate gaps in the literature that warrant future study.

L-Carnitine Combined with Leucine Supplementation Does Not Improve the Effectiveness of Progressive Resistance Training in Healthy Aged Women

“…Therefore, LC supplementation does not appear to affect fasting serum IGF-1 level in people aged <80 years (46,47) but may be effective in centenarians. Importantly, in longitudinal studies, older people may not ingest sufficient protein (55) or energy (56). Although energy intake and dietary composition did not differ between groups in our study, a protein intake ~1.0g/kg/ day may limit the extent of adaptation to RET (57).…”

The Effect of Leucine Supplementation on Sarcopenia-Related Measures in Older Adults: A Systematic Review and Meta-Analysis of 17 Randomized Controlled Trials

“…This may mean that follow-up studies can set multiple leucine supplementation doses to explore the association between leucine supplementation doses and the risk of sarcopenia. In addition, if the population's total protein intake is already enough, leucine supplementation may not provide additional benefits (52).…”

Oral Branched-Chain Amino Acids Supplementation in Athletes: A Systematic Review

“…More recently, contemporary research suggests that 1.6 g.kg −1 .day −1 of protein intake is sufficient to optimize fat-free mass [ 47 ]. If an adequate intake of protein is ingested, the effects of BCAA on strength and hypertrophy are questioned [ 6 ]. The participants in the previous study [ 46 ], likely to meet the individual recommendations of protein intake and, consequently, the BCAA supplementation had a significant effect on cortisol, testosterone and creatine kinase.…”

“…Notably, BCAAs bypass metabolism in the liver and are oxidized in skeletal muscle [ 3 , 4 ]. Leucine, in particular, activates the mammalian target of rapamciyn-1 (mTOR), an anabolic signal that mediates muscle protein synthesis [ 2 , 5 ], which in turn is related to adaptations in strength and hypertrophy [ 6 ]. To this end, it is hypothesized that BCAAs are beneficial for performance, recovery and body composition [ 7 ].…”

“…A recent statement claims that acceptable levels of BCAA intake should be prioritized with exercise participation [ 8 ]. However, oral ingestion of BCAAs is highly questionable with reference to optimizing performance and protein synthesis [ 6 , 9 , 10 ]. Recent systematic reviews show that BCAA supplementation tended to attenuate muscle soreness and is an indicator of muscle damage [ 10 , 11 ].…”