Leucine plays an important role in muscle protein synthesis, while isoleucine induces glucose uptake into cells. Further research is needed to determine valine’s role in a BCAA supplement.
A BCAA supplement is commonly taken to promote muscle growth, but studies have shown that supplementation with BCAAs alone results in a submaximal muscle protein synthesis (MPS) response. This is because all of the essential amino acids must be available in sufficient amounts for a maximal MPS response to occur.
Supplementation with BCAA can help to increase the MPS response if it's taken alongside a meal that contains an inadequate amount of essential amino acids, but the increase in MPS is still less than that of an adequate amount of protein from a high-quality source (e.g., whey protein).
BCAAs are important to ingest on a daily basis, but many protein sources, such as meat and eggs, already provide sufficient BCAAs. Consequently, supplementation does not enhance gains in muscle strength or hypertrophy when adequate protein is consumed.
Supplementation with BCAAs shows some promise for delaying mental and physical fatigue during prolonged exercise. Supplementation has also been found to reduce markers of exercise-induced muscle damage and muscle soreness 24 – 48 hours after exercise, but it may not speed up the recovery of muscle performance.