Isoleucine

Branched Chain Amino Acids (Isoleucine, Leucine, Valine) are the three branched chain amino acids (BCAAs). Branched chain amino acids (BCAA) are essential amino acids and must be obtained from the diet (mainly meat, grains, and dairy).

Not only do the BCAAs account for almost 50% of muscle protein, but they have many metabolic functions.

BCAAs act as substrates for protein synthesis, energy production, neurotransmitter production, glucose metabolism, and the immune response. They are also involved in stimulation of albumin and glycogen synthesis, improvement of insulin resistance, inhibition of free radical production, and hepatocyte apoptosis with liver regeneration.

Unlike most amino acids, the initial step of BCAA metabolism does not take place in the liver. After dietary intake, BCAAs remain in circulation and are taken up by skeletal muscle, the heart, kidney, diaphragm, and adipose tissue for immediate metabolism. BCAAs are transaminated into α-keto acids and used within the tissues or released into circulation. The liver and other organs can then further catabolize these α-keto acids. The complete oxidation of valine yields succinyl CoA, and leucine and isoleucine produce acetyl CoA for use in the citric acid cycle. Isoleucine also produces propionyl CoA and succinyl CoA.

Skeletal muscle is a major site of BCAA utilization. During exercise, catabolism of the BCAAs is elevated; β-aminoisobutyric acid (β-AIB) is a metabolite of valine released during exercise. There is much published literature on the use of BCAAs for muscle protein synthesis, however it’s been shown that BCAA supplementation alone does not enhance muscle protein synthesis better than the consumption of a complete, high quality food protein containing the full spectrum of essential amino acids. 

Of the three BCAA, leucine may have the most immediate impact. Leucine is one of the few amino acids that is completely oxidized in the muscle for energy, generating more ATP molecules than glucose. Additionally, leucine can be used to synthesize fatty acids in adipose tissue, and generates HMG CoA, an intermediate in the synthesis of cholesterol.

Leucine also stimulates insulin secretion and promotes protein synthesis in the liver, muscle, and skin. BCAAs, grouped in patterns and as single biomarkers, have been studied as predictors of obesity, insulin resistance, type 2 diabetes, metabolic syndrome, and cardiovascular disease outcomes.

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