In 1981 Buse reported that in rats the BCAAs may be rate limiting for muscle protein synthesis. Data supporting this hypothesis have been obtained from studies of the responses of rats. The concept that the BCAAs may have a unique capacity to stimulate muscle protein synthesis has been put forward for more than 35 years. Leucine is not only a precursor for muscle protein synthesis, but also may play a role as a regulator of intracellular signaling pathways that are involved in the process of protein synthesis (e.g., ). The branched-chain amino acids leucine, isoleucine and valine are three of the nine EAAs. For synthesis of new muscle protein, all the EAAs, along with the eleven non-essential amino acids (NEAAs) that can be produced in the body, must be present in adequate amounts. Muscle protein is in a constant state of turnover, meaning that protein synthesis is occurring continuously to replace protein lost as a consequence of protein breakdown. Nine of the twenty are considered essential amino acids (EAAs), meaning they cannot be produced by the body in physiologically significant amounts, and therefore are crucial components of a balanced diet. There are a total of twenty amino acids that comprise muscle protein. We conclude that the claim that consumption of dietary BCAAs stimulates muscle protein synthesis or produces an anabolic response in human subjects is unwarranted. The catabolic state in which the rate of muscle protein breakdown exceeded the rate of muscle protein synthesis persisted during BCAA infusion. Both of these intravenous infusion studies found that BCAAs decreased muscle protein synthesis as well as protein breakdown, meaning a decrease in muscle protein turnover. An extensive search of the literature has revealed no studies in human subjects in which the response of muscle protein synthesis to orally-ingested BCAAs alone was quantified, and only two studies in which the effect of intravenously infused BCAAs alone was assessed. Realistically, a maximal increase in muscle protein synthesis of 30% is an over-estimate because the obligatory oxidation of EAAs can never be completely suppressed. Theoretically, the maximal stimulation of muscle protein synthesis in the post-absorptive state in response to BCAAs alone is the difference between muscle protein breakdown and muscle protein synthesis (about 30% greater than synthesis), because the other EAAs required for synthesis of new protein can only be derived from muscle protein breakdown. In this brief review the theoretical and empirical bases for that claim are discussed. A multi-million dollar industry of nutritional supplements has grown around the concept that dietary supplements of BCAAs alone produce an anabolic response in humans driven by a stimulation of muscle protein synthesis.
The branched chain amino acids (BCAAs) are leucine, valine and isoleucine.
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