Encyclopedia · protein or peptide
Amino acids
Free amino acids in conditioners are small enough to enter hair, but there is little evidence that they strengthen it.
What it is
The building blocks of protein, added to conditioners singly (arginine, serine, glycine) or as blends. Oral amino-acid supplements for hair are covered elsewhere.
How it works
Small polar molecules can diffuse into the fibre and hydrogen-bond to keratin. In a Unilever-co-authored UV study, hydrolysed keratin broke down into peptides and free amino acids, which coincided with deeper penetration and preserved tensile strength. Fan 2025
What the studies show
- UV breakdown of keratin hydrolysate to free amino acids and peptides coincided with better penetration and preserved tensile strength, in a laboratory study. Fan 2025
How it's used in studies
We found no free-amino-acid hair regimen in the literature.
Side effects and safety
Low risk when applied to hair.
Bottom line
A mild supportive ingredient where marketing outpaces data. Which amino acids matter, and at what level, is unstudied. We are very unsure about any benefit.
Studies 1
Fan J et al. Performance and mechanism of hydrolyzed keratin for hair photoaging prevention. Molecules. 2025
Tested: hydrolysed keratin treatment before UV
Result: Hydrolysed keratin deposited on the cuticle and partly penetrated; treated hair kept its tensile strength after UV while untreated hair lost 14.32%.
Read the study ↗Related
- Hydrolyzed keratinKeratin fragments, usually from wool or feathers, that coat damaged hair and in lab tests partly entered it and raised wet strength; the evidence is small and industry-linked.
- Sodium PCAA salt of an amino-acid derivative found in skin's natural moisturising factor, used as a humectant; no hair study was found.
- PanthenolA small vitamin B5 alcohol that enters the hair cortex and raised break stress in a lab study; the only humectant here with real hair-fibre data.