BCAA (branched-chain amino acids)
Three essential amino acids (leucine, isoleucine, valine) called "branched-chain" because their chemical structure includes a side chain, and making up 35% of the amino acids in muscle tissue.
BCAAs play a major role in muscle anabolism (muscle building). Leucine is particularly important because it activates mTOR, a key signalling pathway for protein synthesis. Isoleucine and valine also support blood sugar stability and prevent muscle catabolism (breakdown).
A fresh angle (sport and ageing): during intense exercise or calorie restriction (dieting), the body draws on muscle for energy. BCAAs, taken BEFORE or DURING training, block this catabolism and support recovery.
But beware the myth of isolated BCAA supplements: a complete whey protein provides the BCAAs plus all the other EAAs. Eating eggs, chicken or protein-rich lentils is enough unless you are a very high-level athlete. The supplement industry has over-invented the need for BCAAs.
With ageing, BCAAs become important because muscle anabolic sensitivity declines (a phenomenon called anabolic resistance). Keeping BCAA intake high through food (or supplements if practical) helps prevent sarcopenia.