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The BCAA breakdown

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Aaron Deere is a sports nutritionist, functional medicine consultant and advanced personal trainer. He is based in London.

Leucine, isoleucine and valine are three essential amino acids collectively known as branched-chain amino acids or BCAAs.

BCAAs cannot be synthesised by your body, so they must therefore be consumed from the diet.
It is estimated that the three branched chain amino acids make up approximately 33% of all the amino acids in the body. A great proportion of these three amino acids found in skeletal muscle, where they act as both structural elements and play regulatory roles in muscle protein synthesis, particularly during exercise[1].

Adequate intake of BCAAs is therefore required for normal growth and development, and to maintain body protein at steady state, with an intake of BCAAs below the requirements limiting growth and development. It is no surprise then that BCAAs are also highly prominent in dietary protein, with the three branched chain aminos accounting for approximately 20% to 25% of most dietary proteins[2].

Metabolic actions
One of the primary ways BCAA metabolism differs from that of the other amino acids is that the first two steps in the oxidation of each of these three amino acids are catalysed by two common enzymes as a group and not as individual amino acids.

One of the key distinguishing features of this BCAA catabolism is that a relatively small fraction of the capacity occurs in the liver with more than half the capacity occurs in the skeletal muscle, further highlighting the role and importance of the BCAAs in muscle protein synthesis and energy production. Skeletal muscle is a major site for the catabolism of the BCAA. This represents an important difference between these and the other dietary amino acids which are primarily disposed of in the liver and intestine.

For BCAAs to be completely catabolised it requires a number of enzymatic steps, with most of these reactions occurring in the mitochondria of muscle cells. As with most amino acids, the final catabolic step results in the production of acetyl-CoA, propionyl-CoA and succinyl-CoA, resulting in the BCAAs being considered as glucogenic – meaning that they can be converted into glucose – with energy being made available via the citric acid cycle[3].

This is why BCAAs are referred to as quite energy-rich. Their oxidation produces considerable quantities of ATP, although these still remain minor fuels compared with carbohydrate or fat.

Supplement considerations
BCAA supplementation around exercise is popular with many people involved in strength and conditioning programs. While supplementing with BCAAs is largely regarded as safe, it has been purported that high dose branched-chain amino acid supplement protocols can lead to fatigue and loss of coordination. Many of the initial steps of BCAA catabolism involve B-vitamins, especially B6.

When this is combined with the fact that the end fate of BCAA catabolism is the conversion to acetyl-CoA and entry into the citric acid cycle, another process highly dependent on B-vitamins, there is the potential for B-vitamin depletion with high-dose BCAA protocols.

The mechanism behind cognitive dysfunction relates to the fact that a dysregulation of the BCAA catabolic pathways could mimic the neurological symptoms of hereditary diseases of BCAA metabolism, which is largely neural dysfunctions revolving around brain neurotransmitter synthesis.

1. R. Rajendram et al. (eds.), Branched Chain Amino Acids in Clinical Nutrition: Volume 1, Nutrition and Health, DOI 10.1007/978-1-4939-1923-9_2
 2. Hutson, S. M., Sweatt, A. J., & LaNoue, K. F. (2005). Branched-chain amino acid metabolism: implications for establishing safe intakes. The Journal of nutrition, 135(6), 1557S-1564S.
3. Brosnan, J. T., & Brosnan, M. E. (2006). Branched-chain amino acids: enzyme and substrate regulation. The Journal of nutrition, 136(1), 207S-211S.


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