Hair & scalp· in 5 products

Keratin & proteins

Also listed as Hydrolyzed keratin, Keratin amino acids, Hydrolyzed wheat protein and Hydrolyzed silk

Moderate evidence

Keratin is the main structural protein of hair; hair products use hydrolyzed keratin and other proteins broken into small peptides and amino acids. They strengthen and smooth the feel of damaged hair temporarily.

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What it does

The short version.

Small hydrolyzed proteins adsorb to the hair surface and, to a degree, penetrate damaged areas of the cuticle and cortex. That can increase the hair's resistance to breakage, reduce porosity and frizz, and improve shine, especially on bleached or heat-damaged hair. Effects wash out over time and need repeat use. 'Keratin treatments' in salons are different: they are semi-permanent smoothing services that may use formaldehyde-releasing chemicals.

Moderate evidence

Lab studies on hair fibers show improved strength and surface properties; consumer-level clinical data are limited and effects are cosmetic and temporary.

In our directory: It’s a key ingredient in 5 products we review, most often treatments and styling. Those products are mostly built for shine, frizz control and oil control. In formulas, it most often shares the label with argan oil, panthenol and biotin.

Reach for it for

Damage repair Split ends Frizz control Shine Heat protection

Usually a good fit for

Damaged hair Color-treated hair Fine hair Curly hair Wavy hair

How to use it

Getting it right.

Use protein masks or leave-ins on damp hair weekly or as directed. Alternate with moisturizing products if hair begins to feel stiff.

Layering

Friends & frenemies.

Pairs well with

Bond builders Argan oil Panthenol Shea butter

Before you start

Overuse can leave some hair, especially low-porosity or coarse hair, feeling stiff or brittle ('protein overload'). Salon keratin smoothing treatments are a different product and some release formaldehyde, which the FDA has warned can cause eye, nose and breathing irritation.

References

Educational, not medical advice. Ingredient guides summarise published research and expert guidance; formulas and concentrations differ between products.