Does consumption of dietary BCAAs stimulates muscle protein synthesis or produces an anabolic response in humans?

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The branched chain amino acids (BCAAs) are leucine, valine and isoleucine.

There are a total of twenty amino acids that comprise muscle protein. 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.

The BCAA –  leucine, isoleucine and valine are three of the nine EAA. 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, ( the process whereby biological cells generate new proteins ).

The concept that the BCAA may have a unique capacity to stimulate muscle protein synthesis has been put forward for more than 35 years.

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The sale of BCAA as nutritional supplements has become a multi-million dollar business. At the center of the marketing for these products is the widely-believed claim that consumption of BCAA stimulates muscle protein synthesis, and as a result improves an anabolic response. The selling argument beeing used over and over again is that the metabolic goal of consuming BCAA supplements is to maximize the anabolic state. It is widely asserted that BCAAs induce an anabolic state by stimulating muscle protein synthesis.

When all evidence and theory is considered together, it is reasonable to conclude that there is no credible evidence that ingestion of a dietary supplement of BCAA alone results in a physiologically-significant stimulation of muscle protein. In fact, available evidence indicates that BCAA actually decrease muscle protein synthesis. All EAA must be available in abundance for increased anabolic signaling to translate to accelerated muscle protein synthesis.

As in the case of consumption of BCAA alone, there are limited studies of the co–ingestion of BCAAs with other nutrients. When BCAA or an isonitrogenous mixture of threonine, methionine and histidine were administered to human subjects along with carbohydrate, the rate of muscle protein synthesis decreased equally in both groups, indicating no unique role of the BCAA. Similarly, consumption of a mixture of BCAAs to carbohydrate after resistance exercise did not increase the anabolic signaling factors to any greater extent than carbohydrate alone. Thus, available evidence does not support the notion of a special anabolic effect of the BCAAs when given with carbohydrate.

BCAA may enhance the anabolic effect of a protein meal. The addition of 5 g of BCAAs to a beverage containing 6.25 g whey protein increased muscle protein synthesis to a level comparable to that induced by 25 g of whey protein. This result suggests that one or more of the BCAA might be rate limiting for the stimulation of muscle protein synthesis by whey protein.

A physiologically-significant increase in the rate of muscle protein synthesis requires adequate availability of all amino acid precursors. The source of EAAs for muscle protein synthesis is the free intracellular pool. Intracellular free EAAs that are available for incorporation into protein are derived from muscle protein breakdown. Under normal conditions about 70% of EAA released by muscle protein breakdown are reincorporated into muscle protein. The efficiency of reincorporation of EAA from protein breakdown back into muscle protein can only be increased to a limited extent. For this fundamental reason, a dietary supplement of BCAA alone cannot support an increased rate of muscle protein synthesis. The availability of the other EAA will rapidly become rate limiting for accelerated protein synthesis. The few studies in human subjects have reported decreases, rather than increases, in muscle protein synthesis after intake of BCAA.

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