Leucine is utilized in the liver, adipose tissue, and muscle tissue. In adipose and muscle tissue, leucine is used in the formation of sterols, and the combined usage of leucine in these two tissues is seven times greater than its use in the liver.^[4]

Leucine is the only dietary amino acid that has the capacity to stimulate muscle protein synthesis.^[5] 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.^[6] While once seen as an important part of the three branch chained amino acids in sports supplements, leucine has since earned more attention on its own as a catalyst for muscle growth and muscular insurance. Supplement companies once marketed the "ideal" 2:1:1 ratio of leucine, iso-leucine and valine; but with furthered evidence that leucine is the most important amino acid for muscle building, it has become much more popular as the primary ingredient in dietary supplements.^[7]

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.^[8]

Leucine toxicity, as seen in decompensated Maple Syrup Urine Disease (MSUD), causes delirium and neurologic compromise, and can be life-threatening.

In yeast genetics, mutants with a defective gene for leucine synthesis (leu2) are transformed with a plasmid that contains a working leucine synthesis gene (LEU2) and grown on minimal media. Leucine synthesis then becomes a useful selectable marker.