Leucine, isoleucine and valine are unique amongst the essential amino acids because they possess a branched variable group. Known as the branched chain amino acids (BCAAs) they account for roughly 15 % of the total amino acids in skeletal muscle protein. Humans possess a pool of free branched chain amino acids in plasma, but this is very small compared the large amount contained in skeletal muscle. The BCAAs (particularly leucine) have been researched in relation to their ability to stimulate protein synthesis. Research has shown that valine is able to stimulate protein synthesis because it can stimulate the insulin signalling pathways inside cells via activation of the mTOR kinsase (here). Skeletal muscle BCAAs undergo extensive catabolism during exercise which suggest that exercise increase the requirement of dietary BCAAs
BCAA catabolism occurs in the mitochondria (figure 1), where the BCAA undergo transamination followed by oxidative decarboxylation to produce succinyl CoA, succinyl CoA and acetyl CoA, and acetyl CoA and acetoacetate from valine, isoleucine and leucine respectively. Therefore, only leucine is completely oxidised in skeletal muscle for energy. During times of fasting, or during exercise, levels of leucine in the blood and muscle rise and the muscle increases its catabolism of lecucine to provide a greater percentage of its energy needs. At the same time as leucine oxidation occurs, pyruvate oxidation is inhibited and pyruvate is converted to lactate and exported from the muscle to the liver, where it can be synthesised into glucose in order to maintain blood sugar levels. Leucine catabolism thus spares important gluconeogenic precursors in time of exercise or fasting.
Figure 1. The catabolism of the branched chain amino acids. BCKAD requires thiamin as TPP, niacin as NADH, Mg2+ and CoA from pantothenate.
Research published in the Journal of Nutrition in 20061, fed 16 female and 14 male non-exercising subjects a solution containing 5g of BCAA mixture or a placebo containing 5g of dextrin cornstarch. The subjects then performed a squat exercise protocol designed to induce delayed onset muscle soreness (DOMS) in the leg muscles. Peak muscle soreness for the female subjects was lower when consuming the BCAA supplements compared to the placebo. There was also a trend for reduced soreness in the male subjects, but this did not reach statistical significance, possibly because of the lower ingested dose of BCAA based on the larger size of the male subjects. This research, and others suggest that supplementation with high concentrations of BCAA before intense extercise may decrease recovery time and prevent the onset of delayed onset muscle soreness.
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