isoleucine

Isoleucine is one of the 20 standard amino acids used by cells to synthesize proteins. It is an essential amino acid in humans, meaning it cannot be synthesized at adequate levels and must be obtained from the diet. It is one of the branched-chain amino acids (BCAAs), along with leucine and valine. Isoleucine exists in two enantiomeric forms, but the L-isomer is specifically incorporated into proteins, whereas D-enantiomers are rare in nature. The side chain is a branched, aliphatic sec-butyl group, giving it hydrophobic character.

In metabolism, isoleucine is transaminated by branched-chain amino acid aminotransferase to the corresponding keto acid and further degraded primarily in skeletal muscle and other tissues. Its breakdown products feed into the citric acid cycle. Isoleucine contributes to protein synthesis, tissue repair, and energy production during fasting or exercise. It may influence glucose uptake and insulin signaling, though its most notable metabolic link is in concert with other BCAAs and the mTOR pathway, especially leucine.

Dietary sources include meat, fish, eggs, dairy products, soy, legumes, nuts, seeds, and whole grains. Typical dietary intake across populations provides sufficient isoleucine for most individuals; those with increased protein requirements or endurance training may seek higher intakes under professional guidance. In clinical and nutritional settings, isoleucine is included in medical foods and parenteral nutrition formulas as part of complete amino acid mixtures.