Threonine
Also known as Threonine, L-Threonine, 2-Amino-3-hydroxybutanoic acid
“CIR Expert Panel says: safe as used in cosmetics.”
Threonine (CAS 72-19-5; C4H9NO3) is a proteinogenic α-amino acid bearing a hydroxyl group on its side chain (β-carbon), making it a polar, water-binding humectant used in cosmetics as a skin- and hair-conditioning agent. The CIR Expert Panel assessed all 21 α-amino acids as a group (Burnett et al., IJT 32(6 Suppl):41S-64S, 2013; PMID 24335967), naming Threonine explicitly in the study scope and concluding these amino acids are safe as cosmetic ingredients in the present practices of use and concentration; the QRT records Threonine with Finding=S (Safe), no qualifying conditions, citation IJT 32(S4):41-64, 2013. Table 4 of the same assessment lists Threonine at 170 reported cosmetic uses at concentrations up to 0.05%. A 2025 randomized controlled trial (Matsumoto et al., PMID 40419835) established an oral NOAEL of 12 g/day for L-threonine in healthy adults, corroborating its broad biological tolerability; the cosmetic topical exposures at ≤0.05% are orders of magnitude below any systemic concern threshold.
Proteinogenic α-amino acid with a hydroxyl group on the β-carbon side chain (–CH(OH)–CH3); the hydroxyl group enables strong hydrogen bonding with water molecules, contributing to humectant and skin-conditioning activity
Endogenous natural moisturizing factor (NMF) component: free amino acids including threonine are constituents of the stratum corneum that maintain skin hydration and plasticity; topical application replenishes NMF
Hydroxyl side chain is biochemically significant: threonine is one of only three standard amino acids with a hydroxyl-bearing side chain (alongside serine and tyrosine), which also makes it a site for O-linked glycosylation in proteins
CIR Expert Panel concluded threonine is safe as used in cosmetics in the present practices of use and concentration:41-64, 2013; QRT Finding=S); reported at 170 cosmetic uses up to 0.05% concentration (Burnett et al., 2013 Table 4)
Broad tolerability confirmed by randomized controlled trial: oral NOAEL of 12 g/day in healthy adults (Matsumoto et al., 2025)
- · No known sensitization or irritation at cosmetic use concentrations — the CIR panel found no adverse dermal or ocular signals for α-amino acids as a class
- · Oral NOAEL is 12 g/day (Matsumoto et al., 2025); topical cosmetic exposures at ≤0.05% are not a systemic concern
CIR Quick Reference Table (12/2017, revised 07/2018) — Threonine row: Finding=S, Citation=IJT 32(S4):41-64, 2013
“Threonine | S | | IJT 32(S4):41-64, 2013”— QRT-122017revised072018.pdf, p. 133