Branched-chain amino acid supplementation and endurance performance: reporting guidelines and systematic review of biochemical vs clinical evidence

Background Branched-chain amino acids (BCAAs) are widely used in endurance sports, yet their impact on performance remains unclear. This review evaluated whether biochemical changes from BCAA or leucine supplementation translate into functional benefits.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.Results From 152 records, 15 studies met inclusion criteria. No consistent improvement was observed in performance, fatigue, or recovery. Only two studies reported 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.