Serine
Also known as Serine, L-Serine, 2-Amino-3-hydroxypropanoic acid
“CIR Expert Panel says: safe as used in cosmetics.”
Serine (CAS 56-45-1) is a proteinogenic α-amino acid bearing a hydroxyl group on its side chain (C3H7NO3), which distinguishes it from glycine and alanine and underlies its water-binding capacity. 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), concluding they are safe as cosmetic ingredients in the practices of use and concentration of the assessment; the QRT records Serine with Finding=S (Safe), no qualifying conditions, citation IJT 32(S4):41-64, 2013. Serine is an endogenous component of the stratum corneum's natural moisturizing factor (NMF); repetitive frictional trauma was shown to deplete serine from the superficial stratum corneum, elevating transepidermal water loss (Wong et al., PMID 29243811), confirming its role in maintaining skin hydration and barrier function. With 334 reported cosmetic uses at up to 2% concentration (Table 4, Burnett 2013), Serine is the most heavily used α-amino acid in this sub-batch.
Proteinogenic α-amino acid with a hydroxyl side chain (–CH2OH); the hydroxyl group enables strong hydrogen bonding with water molecules, contributing to humectant activity
Endogenous NMF component: free serine is an abundant constituent of the stratum corneum that maintains skin hydration and plasticity; topical application replenishes depleted NMF
Phospholipid precursor: serine is a biosynthetic precursor to phosphatidylserine, a key phospholipid in cell membranes, supporting skin cell membrane integrity
Highest cosmetic usage among the α-amino acids in this sub-batch: 334 total reported uses at concentrations up to 2% (Burnett et al., 2013 Table 4)
Skin-conditioning and humectant agent: attracts and retains moisture in formulations; used in leave-on and rinse-off products
CIR Expert Panel concluded Serine is safe as used in cosmetics in the present practices of use and concentration:41-64, 2013; QRT Finding=S)
- · No known sensitization or irritation at cosmetic use concentrations
NMF depletion studies show that external removal of serine from the stratum corneum (e.g., via repetitive friction) can transiently compromise barrier function, but this is a physical stripping effect, not ingredient toxicity
CIR Quick Reference Table (12/2017, revised 07/2018) — Serine row: Finding=S, Citation=IJT 32(S4):41-64, 2013
“Serine | S | | IJT 32(S4):41-64, 2013”— QRT-122017revised072018.pdf, p. 116