The evidence for BCAA/leucine supplementation supporting endurance performance is equivocal.⁴ A recent study⁵ suggests that 20 g of BCAA ingested 1hr prior to a ramp test on a treadmill can delay fatigue. However, like much of  the BCAA literature around endurance performance/fatigue there are significant flaws in the research. In the case of ⁵ the placebo is not matched for calories. A thorough analysis of the most recent research on BCAA and endurance performance is needed to determine their efficacy for promoting performance. There is a great deal of heterogeneity in the literature around supplementation protocols and dosing strategies and so firm recommendations on this are difficult. There is an argument however, that because BCAAs become significant substrates during prolonged exercise that supplementing may prevent muscle damage/breakdown, but there is little evidence to support this notion. For instance, 20 g of BCAA supplemented before and during a 100 km race (3 g 1hr before followed by 17 g throughout a 100 km race) did not affect performance or markers of muscle damage.⁶

There is building evidence for BCAA supplementation to augment the response to damaging exercise. A meta-analysis of the literature from 2007-2013 suggests that BCAA supplementation may significantly reduce the severity of delayed onset muscle soreness following damaging exercise when compared to placebo treatments.⁷ Additionally, a systematic literature review carried out on research published up to August 2017 suggests that there may be a modest benefit of BCAA supplementation for markers of muscle damage.⁸ In an analysis of the dosing strategies in this systematic review⁸ it is suggested that a daily intake greater than 200 mg*kg*day-1 (~16 g) for at least 7 days prior to the damaging exercise may alleviate some of the impacts of muscle damage on muscle performance (force decrement, plasma CK). However, it should be noted that the placebo in all the included trials is devoid of any protein. So, it remains to be seen if BCAA supplementation would be better/worse than intact protein for this outcome measure. Furthermore, the systematic review suggests efficacy of BCAA supplementation only if the damage is low-moderate.

Because BCAA/leucine are critical for signalling to increase muscle protein synthesis³, it has long been thought that BCAA/leucine supplementation may enhance muscle protein synthesis and therefore growth in response to nutrition/exercise. However, the case for supplementing BCAA/leucine in isolation seems to be weak at best. For instance, when 5.6 g (equivalent content in 20 g of Whey) of BCAA were supplemented following a session of resistance exercise the resulting increase in muscle protein synthesis was only 22%.⁸ With intact protein we would expect this stimulatory response on muscle protein synthesis to be at least double that. So, whilst BCAAs, when taken in isolation following resistance exercise, can stimulate muscle protein synthesis they probably should not be recommended over whole foods containing sufficient high-quality protein. However, there may be a case for utilising BCAA/leucine to “top up” the anabolic potential of sub-optimal meals. The leucine content of a meal seems to be the key driver of the anabolic response (muscle protein synthesis) to that meal. Approximately 2.5 g of leucine per meal (equivalent to ~20 g of whey protein) seems to be sufficient to maximise muscle protein synthesis.³ Furthermore, when a suboptimal dose of whey protein (6.25 g whey, 0.75 g of leucine) in a mixed macronutrient beverage is “topped up” with leucine to contain 3 g of leucine, it produces a similar muscle protein synthesis response in the recovery from resistance exercise  as 25 g of whey protein (3g of leucine).⁹ These data suggest that leucine could be used to enhance the anabolic potential of certain meals that may not, on their own, maximise muscle protein synthesis. This could take the form of supplementing sub-optimal meals (plant-based meals, meals with less than 20-30 g of high-quality protein) with up to 3 g of additional leucine. However, we do not know if this strategy would support muscle growth in the long term.