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Proteins & peptides

Keratin

Also listed as hydrolyzed keratin, keratin protein, keratin amino acids

Protein fragments (usually from wool or feathers) that coat and fill damaged hair so it feels smoother and breaks less. A cosmetic effect that washes out.

Limited evidenceTypical strength: Usually under 1-2% in shampoos, conditioners and treatments

What it is

Hair is itself made of keratin. Products add keratin broken into smaller pieces ('hydrolysed'), which stick to the hair surface and, if small enough, slip a little way into damaged cuticle. Hair feels smoother, looks shinier and may break less while the coating lasts.

This is different from salon 'keratin smoothing' or 'Brazilian' treatments, whose straightening effect usually comes from formaldehyde-releasing chemicals heated into the hair, not from keratin.

How strong is the evidence?

Limited evidence

Laboratory tests, mostly by ingredient makers, show hydrolysed keratin binds to hair and improves strength, gloss and combing. There are no independent trials on long-term hair health, and nothing shows it rebuilds hair structure or affects growth.

What it's used for, checked

Smoother, shinier, less frizzy hair

Holds up

A real but temporary conditioning effect.

Repairs damaged hair

Mixed

It fills and coats damaged areas for a while; hair cannot actually repair itself.

Makes hair grow

False

It acts on the hair shaft, not the follicle.

Works well with

  • silk-protein
  • quinoa-protein

Who should be careful

  • Keratin in shampoo and conditioner is very low risk.
  • Salon keratin smoothing treatments are a different product: the FDA has warned that many release formaldehyde when heated, causing eye, nose and throat irritation. Ask what is in them, and consider avoiding them during pregnancy.
  • Heavy protein products can make fine hair feel stiff; alternate with a moisturising conditioner.