Phys Sportsmed. 2026 Feb 8. doi: 10.1080/00913847.2026.2627863. Online ahead of print.
ABSTRACT
BACKGROUND: Branched-chain amino acids (BCAAs) – leucine, isoleucine, and valine – are widely used in sports nutrition, yet their effects on endurance performance remain uncertain. Most studies emphasize biochemical markers without linking them to functional outcomes. This is the first systematic review to evaluate whether biochemical alterations induced by BCAA or leucine supplementation are associated with actual performance or recovery benefits in endurance athletes.
METHODS: A systematic review was conducted in accordance with PRISMA 2020, the Cochrane Handbook for Systematic Reviews of Interventions, and the GRADE approach. Searches were performed in PubMed, Embase, and Web of Science up to 11 July 2024. Eligible studies included endurance runners or athletes, used BCAA or leucine supplementation, and reported outcomes related to performance, recovery, or adverse effects. Risk of bias was assessed using the ROBINS-I tool.
RESULTS: From 152 records, 15 studies met inclusion criteria. No consistent improvement was observed in performance, fatigue, or recovery. Only two studies reported statistically significant differences. One trial found a 42% reduction in muscle soreness (p < 0.05), though with inadequate control for protein intake and confounders. Biochemical changes included: increase 140% valine (p < 0.01), low plasma glucose (p < 0.01), increase free fatty acids (p < 0.001), and raise 25% protein synthesis post-exercise (95% CI: 20-30%, p = 0.01). Mental performance improved after 12 km and 30 km runs (p < 0.05), but no functional performance gains were consistently observed.
CONCLUSION: BCAA and leucine supplementation do not result in meaningful improvements in endurance performance or muscle recovery. Despite biochemical alterations, current evidence – limited by low methodological quality, surrogate outcomes, and risk of bias – does not support the use of BCAA as an effective strategy for endurance athletes.
PMID:41655197 | DOI:10.1080/00913847.2026.2627863
