Synopsis

Source of validity: Scientific
Rating (out of 5): 2

Branched-Chain Amino Acids (BCAAs)—leucine, isoleucine, and valine—are popular supplements among endurance athletes, with theoretical benefits including reduced exercise-induced muscle breakdown and delayed fatigue. The scientific rationale stems from the idea that BCAAs may help prevent central fatigue by reducing brain uptake of tryptophan and subsequent serotonin production, which is linked to perceived exertion during prolonged exercise.

However, the clinical evidence is mixed and generally not robust. Some small studies have reported minor improvements in endurance performance or reductions in perceived exertion when supplementing with BCAAs, but many well-controlled trials have failed to show significant performance benefits in endurance sports such as running, cycling, or triathlon. Reviews and meta-analyses (e.g., Jackman et al., 2010; Gleeson, 2005) generally conclude that while BCAAs may reduce muscle soreness and markers of muscle damage, their effect on actual endurance performance is minimal or absent in most cases.

In summary, BCAAs are used for endurance support based on plausible mechanisms and some limited scientific data, but the overall strength of evidence is low. While not solely a product of tradition, the scientific justification for BCAA supplementation to meaningfully enhance endurance is weak, leading to a rating of 2 out of 5.

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