Keratin: The Protein Your Hair Is Made Of

The word keratin can describe the structural proteins already in hair, fragments of protein added to a formula, or the marketing name of a salon service. Those are not interchangeable things. None of them, on its own, tells you where a product goes or what it changes.

The material is organized at several scales

In the cortex, type I and type II keratin chains assemble into coiled-coil structures and then into intermediate filaments. Those filaments sit within a matrix of keratin-associated proteins. The resulting fiber-and-matrix composite, not a solid rod of one ingredient, carries much of the strand’s tensile load.

The cuticle contains keratin too, but with a different organization and extensively cross-linked outer layers. Around and between these protein structures are lipids, including those in the cell membrane complex, as well as water and pigment. Hair is protein-rich without being a uniform block of protein.

That composition gives damage more than one address. Oxidation can alter disulfide bonds and other amino-acid residues. Weathering and grooming can remove or disrupt surface lipids and cuticle cells. Water changes the mechanical contribution of the matrix. None of these events is usefully summarized as a general shortage of keratin.

Damage is not a single missing ingredient

Consider three observations that are often gathered under “protein damage.”

Roughness and snagging may reflect surface erosion, changed friction, deposits, or a combination of them. They do not tell you how much protein remains in the cortex.

Excessive wet extension concerns the response of the internal composite to water and force. It cannot be translated into a protein percentage, and conditioning feel does not settle the question.

Breakage can begin at a local flaw as well as within chemically altered material. It is an outcome that calls for attention to location and history, not an automatic instruction to add a protein product.

This distinction is not an argument against protein-derived ingredients. It is an argument for judging them by a defined effect rather than by the idea that hair must always need more of what it already contains.

What hydrolyzed keratin actually changes

Hydrolysis breaks protein into smaller peptides. An ingredient list that says “hydrolyzed keratin” does not disclose one universal molecular size, charge, distribution of fragments, concentration, or finished formula. All of those can influence deposition and penetration.

Malinauskyte and colleagues compared several molecular-weight classes of wool-derived hydrolyzed keratin on freshly relaxed, highly textured hair. Their low- and mid-weight materials reached deeper regions under the study conditions, while the high-weight peptide was mainly associated with the surface and outer layers. The mid- and high-weight treatments also changed some mechanical measurements; the lowest-weight material did not produce the same stabilizing pattern.

Two lessons are more useful than a simplistic “small is better” rule. First, penetration and performance are separate measurements. Second, a surface-associated protein film can still have a useful mechanical or conditioning effect. The experiment involved a particular hair substrate, ingredient set, concentrations, and exposure times, not a universal cutoff for every keratin product.

“Keratin treatment” is not a mechanism

A service name may describe the intended cosmetic result more clearly than the chemistry used to achieve it. Likewise, finding keratin on an ingredient list does not establish that the product substitutes for lost structural material. Ask whether the claim concerns surface feel, combing force, appearance, water response, or a measured mechanical property.

A stiff or coated feel after using a product is also not, by itself, proof of the popular diagnosis “protein overload.” Formula buildup, insufficient distribution, other ingredients, and the condition of the hair can produce an unwanted result. Simplify the routine and assess the response before adding another corrective treatment.

Choose by evidence, not by the word keratin

1. Name the object of the claim. Is the discussion about the proteins naturally organized inside hair, a hydrolyzed ingredient, or a service with a keratin label?
2. Name the intended outcome. Less snagging and easier styling are surface-relevant outcomes. A change in tensile behavior is a different claim and needs a suitable test.
3. Look for the test conditions. Which hair was used, what damage preceded treatment, what served as the control, and what was actually measured? Ingredient presence cannot answer those questions.
4. Keep only what improves the routine. More protein-derived products are not automatically better, and removing a useful conditioner on the basis of an unverified overload theory is not automatically corrective.

If chemical damage and changing hair behavior are part of the decision, the free consultation takes about a minute. It asks a few questions about color, heat, washing, and how your hair behaves, then asks for your email before showing a Damage Profile with a suggested order of care.

References

• Harland DP, Popescu C, Richena M, et al. The susceptibility of disulfide bonds to modification in keratin fibers undergoing tensile stress. Biophys J. 2022;121(11):2168-2179. Full text.
• Malinauskyte E, Shrestha R, Cornwell PA, Gourion-Arsiquaud S, Hindley M. Penetration of different molecular weight hydrolysed keratins into hair fibres and their effects on the physical properties of textured hair. Int J Cosmet Sci. 2021;43(1):26-37. Full text.
• Rogers GE. Known and Unknown Features of Hair Cuticle Structure. Cosmetics. 2019;6(2):32. Full text.
• Grosjacques C, Babiel S, Hodes J, Bobier A, Cavagnino A, Baraibar M. Carbonylation of hair proteins: A robust biomarker of molecular and structural oxidative damage in hair fibres. Int J Cosmet Sci. 2025;47(4):604-625. Full text.