Leucine

An essential amino acid — one of the building blocks of protein, which the body cannot make and must obtain from food. In cosmetics it is used as a conditioning agent for skin and hair. It appears in formulas as part of an amino acid blend far more often than on its own, and its role is supportive rather than active.

Amino acids are part of natural moisturising factor, the mixture of water-binding compounds the skin makes as its cells mature. Applying them topically adds to that pool at the surface, which helps hold water in the outer layer. That is the honest extent of it. Leucine's role in muscle protein synthesis, which comes up often in nutrition, has no bearing on what a topical application does. On hair it reduces static by helping the surface hold a little moisture.