Threonine (2-amino-3-hydroxybutanoic acid) is a small, polar α-amino acid used in cosmetics for skin-identical hydration and conditioning. Free amino acids are major constituents of the skin’s Natural Moisturizing Factor (NMF), supporting corneocyte water binding, enzymatic desquamation, and supple feel. In leave-on systems (toners, gel serums, emulsions, sheet masks), threonine contributes lightweight humectancy without occlusion and pairs well with osmolytes (glycerin, betaine), NMF components (PCA, urea), and barrier lipids. Its zwitterionic nature across common skin pH ranges enables straightforward water-phase incorporation and compatibility with acid-leaning formulations where pH stability matters.

In hair/scalp care, threonine is categorized for hair conditioning and antistatic action, helping reduce combing forces and flyaways while improving tactile softness—especially on chemically stressed fibers. In rinse-off or leave-in formats, threonine complements cationic polymers and hydrolyzed proteins: the amino acid provides moisture attraction at the cuticle surface, while cationic/peptide partners enhance deposition and film formation for better alignment and control. These interactions are valuable in color-care masks, repair conditioners, and smoothing shampoos intended to refine fiber feel without heavy build-up.