Leucine is an α-amino acid used in the biosynthesis of proteins. Leucine is an essential hydrophobic amino acid. It is used in the Leucine may be used some people as a supplement to build muscle. Leucine is also found in fish, meat, and poultry. In the pharmaceutical industry, L-leucine is used for parenteral and enteral nutrition and feeding, and is also used as a flavoring product and tablet lubricant in manufacturing. Leucine is an mTOR activator. It is a dietary amino acid with the capacity to directly stimulate muscle protein synthesis. As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats. Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men. Leucine potently activates the mammalian target of rapamycin kinase that regulates cell growth. Infusion of leucine into the rat brain has been shown to decrease food intake and body weight via activation of the mTOR pathway.

Diiodotyrosine (DIT) is a regulatory ligand for thyroid peroxidase, enzyme, which is involved in the production of thyroxine (T4) and triiodothyronine (T3), the thyroid hormones. Diiodotyrosine was proposed to be a new marker of leukocyte phagocytic activity in sepsis and severe infections.