Leucine, also L-Leucine, is an essential proteinogenic amino acid. It acts as a building block for proteins; the body is unable to produce it autonomously. This is why it is necessary to take sufficient quantities of the amino acid with daily nutrition.
Together with Isoleucine and Valine, Leucine represents the so-called BCAA (branched chain amino acids). Most of these branched chain amino acids are found in the muscle tissue.