Isoleucine
Also known as Isoleucine, L-Isoleucine, 2-Amino-3-methylpentanoic acid
“CIR Expert Panel says: safe as used in cosmetics.”
Isoleucine (CAS 73-32-5; L-Isoleucine, a branched-chain essential proteinogenic α-amino acid) is used in cosmetics as a skin- and hair-conditioning humectant. The CIR Expert Panel assessed all 21 α-amino acids and their simple salts in cosmetics (Burnett et al., IJT 32(6 Suppl):41S-64S, 2013; PMID 24335967) and concluded they are safe in the present practices of use and concentration; Isoleucine is explicitly named in the assessment. The QRT records Isoleucine with Finding=S (Safe), no qualifying conditions, citing IJT 32(Suppl. 4):41-64, 2013. As one of the three branched-chain amino acids (BCAAs, alongside Leucine and Valine), Isoleucine carries a sec-butyl side chain and is an essential amino acid not synthesized by the human body.
Branched-chain essential α-amino acid: one of three BCAAs (with Leucine and Valine), bearing a sec-butyl side chain; not synthesized by the human body and obtained from diet
Natural moisturizing factor (NMF) component: free amino acids including Isoleucine are endogenous constituents of the stratum corneum that contribute to skin hydration and barrier function
Skin- and hair-conditioning humectant: attracts and binds water; used as a low-molecular-weight humectant in cosmetic formulations
CIR Expert Panel concluded Isoleucine is safe as used in cosmetics in the present practices of use and concentration of the 2013 safety assessment:41-64, 2013; QRT Finding=S)
- · No sensitization or irritation concerns identified in the CIR group assessment at reported cosmetic use concentrations
- · No known adverse effects at cosmetic use concentrations in the 2013 CIR group assessment
CIR Quick Reference Table (October 2024) — Isoleucine row: Finding=S, Citation=IJT 32(Suppl. 4):41-64, 2013
“Isoleucine | S | | IJT 32(Suppl. 4):41-64, 2013”— QRT-Update-100824_0.pdf, p. 141