leucine

Leucine is one of the nine essential amino acids for humans. It is a branched-chain amino acid (BCAA) along with isoleucine and valine, and it must be obtained from the diet because the body cannot synthesize it. Leucine has a hydrophobic, aliphatic side chain and a chemical formula of C6H13NO2. It plays a key role in protein synthesis and tissue maintenance, and it contributes to growth and repair as part of dietary protein.

In metabolism, leucine is primarily catabolized in muscle. It participates in the initial transamination by branched-chain amino acid transferase and then enters the branched-chain α-ketoacid dehydrogenase complex for further breakdown. Leucine can also act as a signaling molecule that stimulates the mTORC1 pathway, promoting translation and muscle protein synthesis, particularly after feeding or resistance exercise. Its ketoacid derivative is alpha-ketoisocaproate (KIC), which can be further metabolized for energy.

Dietary sources include meat, fish, dairy products, eggs, soy, and other legumes and grains. Typical Western diets supply leucine in amounts that support normal protein synthesis, and many athletes use leucine- or BCAA-containing supplements to support training adaptation, though effects vary.

Clinical and nutritional considerations: deficiencies are rare in healthy individuals but can occur in severe malnutrition. A metabolic disorder, maple syrup urine disease, results from deficient activity of the BCKD complex and leads to elevated levels of leucine and other BCAAs, with potential neurologic consequences if untreated. High leucine intake from supplements should be approached with caution in people with kidney disease or certain metabolic disorders, and medical advice is recommended.