Branched Chain Amino Acids (Isoleucine, Leucine, Valine) Isoleucine, leucine and 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 a-keto acids and used within the tissues or released into circulation.
The liver and other organs can then further catabolize these a-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 which is evaluated in the B-Vitamin Marker section below.
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.
Low levels of essential BCAA may indicate a poor-quality diet, or maldigestion due to deficient digestive peptidase activity or pancreatic dysfunction.
Low levels of leucine can be seen after significant aerobic exercise or strength training.
Supplementation with zinc, vitamin B3, and vitamin B6, has improved outcomes in various conditions where low levels of BCAA’s have been associated.
High protein intake may elevate BCAAs. In the catabolism of BCAAs, branched chain aminotransferase and branched chain alpha ketoacid dehydrogenase complex (BCKDC) require several cofactors such as vitamin B6, vitamin B1, and lipoic acid. Therefore, functional need for these cofactors may contribute to high levels of BCAAs.
Lastly, BCAAs can be elevated due to a rare inborn error of metabolism. Maple Syrup Urine Disease is an inherited disorder of branched chain amino acid metabolism due to deficiency of the BCKDC complex.
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