Amino acids

Amino acids are small molecules that link together to form peptides and, in longer chains, proteins — keratin in the outer skin and hair, collagen and elastin deeper down. Around twenty of them are used by the body to build those proteins, and the same names turn up on cosmetic ingredient lists.

They matter for the skin surface in a second way. As cells move up through the stratum corneum, a protein called filaggrin is broken down into free amino acids and related molecules, and this mixture — the natural moisturising factor — is part of how the outer layer holds on to water. Skin low in it tends to feel tight and rough.

Topically, amino acids act mostly as humectants that draw and hold water at the surface, as conditioning agents in haircare, and as the starting point for mild amino-acid-derived cleansers. What they do not do is rebuild collagen in the deeper skin the way marketing sometimes implies: the visible benefit is hydration, softness and appearance, supported by the lipids and occlusives around them.