The three branched-chain amino acids (BCAAs), leucine, isoleucine, and valine, are not synthesized in the body, and therefore are classified as essential amino acids that must be supplied in the diet.1 They are required for protein synthesis and neurotransmitter synthesis. The basic structure of all amino acids consists of an amine (-NH2) group and a carboxyl (-COOH) group attached to a single carbon atom: also present is an organic side chain, and it is the structures of these side chains that give the different amino acids their characteristic structures. The structures of the BCAAs are shown in Figure 13.1, and they are so called because they each possess a short, branched hydrocarbon chain as the side group attached to the alpha carbon of the amino acid molecule. They are the only essential amino acids
organic side group
General structure of an amino acid
FIGURE 13.1 The general structure of amino acids and the structures of the branched-chain amino acids: leucine, isoleucine, and valine.
that are oxidized to a significant extent during exercise, and they must therefore be replenished by the diet. In the late 1970s, BCAAs were suggested to be the third fuel for skeletal muscle after carbohydrate and fat, and BCAAs are sometimes supplied to athletes in energy drinks to provide extra fuel. Claims have also been made that BCAA supplementation can reduce net protein breakdown in muscle during exercise, reduce fatigue, enhance performance via effects on the brain, and speed up the repair of muscle following exercise-induced muscle damage.
However, the majority of studies, using various exercise and treatment designs and several forms of administration of BCAAs (infusion, oral, and with and without carbohydrates), have failed to find a performance-enhancing effect. Leucine does appear to provide a stimulatory signal for muscle protein synthesis and thus has an anticatabolic effect during and after exercise in humans, but there is only very limited scientific evidence to support the claim that BCAA supplements may accelerate the repair of muscle damage or reduce muscle soreness after exercise. Acute intakes of BCAA supplements of up to about 30 g/day seem to be well tolerated and without ill effect, though the suggested reasons for taking such supplements have not received much support from well-controlled scientific studies.
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