BCAAs

The three branched-chain amino acids, leucine, isoleucine, and valine, are so named because each has a side chain that branches off the main carbon backbone. ^[3] They are three of the nine essential amino acids, meaning the human body cannot synthesize them and must obtain them from food; they account for a large fraction of the amino acids found in muscle protein. ^[2] Dietary sources include meat, dairy, eggs, and other protein-rich foods.

Of the three, leucine has drawn the most attention because it can directly stimulate muscle protein synthesis. ^[2] Leucine activates a cellular signaling hub called mTOR, specifically the complex mTORC1, which switches on the machinery that builds new muscle protein. ^[2] This mechanism is the scientific basis behind marketing BCAAs, and especially leucine, as muscle-building supplements.

Whether isolated BCAA supplements actually build muscle in people is contested. A prominent review argued that, because constructing new muscle protein requires all of the essential amino acids, supplying only three of them cannot maximally stimulate synthesis; taking BCAAs alone provides a signal to build but not all of the building blocks. ^[1] The practical conclusion of much of the research is that when total protein intake is already adequate, adding isolated BCAAs offers little further benefit, since dietary protein already contains them.

BCAAs have also been studied for reducing fatigue during endurance exercise. According to the central fatigue theory, BCAAs compete with the amino acid tryptophan for transport into the brain, which can lower the brain's production of serotonin and modestly delay the sense of tiredness. ^[3] The overall effect on performance, however, appears small.

Blood levels of BCAAs are of interest in metabolism research, since elevated levels have been associated with obesity and insulin resistance, and a rare inherited disorder called maple syrup urine disease results from an inability to break them down. As supplements, BCAAs are sold as powders and capsules and are generally regarded as safe for healthy people at normal intakes.

BCAAs are very safe for healthy people, with GI discomfort being about the worst common complaint. Very high chronic intakes have been theoretically linked to altered insulin signaling, but this isn't a practical concern at normal doses. They're not appropriate as a protein substitute since they lack the other essential aminos.