Hair is composed of over 95% keratin, a structural protein that gives it strength, suppleness, and shine. Understanding keratin's role in the hair's life cycle and knowing how to preserve it allows you to choose your hair care products more effectively and avoid mistakes that weaken the hair fiber. Here's everything you need to know.
What is keratin?
Keratin is a natural insoluble protein composed of amino acids, notably cysteine (a sulfur-containing amino acid that forms disulfide bonds responsible for hair strength). It is synthesized by specialized cells called keratinocytes, located in the hair bulb and supplied by the blood capillaries of the hair papilla.
It forms the basic material not only of hair, but also of nails and the superficial layer of skin. In hair, it provides shine, suppleness, and resistance to tension and external aggressions.
What is the role of keratin in the hair cycle?
The hair life cycle comprises three distinct phases, and keratin is only synthesized during the first one.
During the anagen phase (2 to 7 years depending on genetics), the keratinocytes in the hair bulb actively produce keratin. This is the period of hair growth, during which the hair fiber lengthens by approximately 1 cm per month.
The catagen phase (2 to 6 weeks) marks the cessation of keratin production. The bulb gradually detaches from the papilla and rises towards the surface of the scalp.
The telogen phase (2 to 6 months) is the resting phase: the hair remains in place until a new hair grows out and naturally pushes it out. It is during this phase that natural shedding occurs (50 to 100 hairs per day is normal).
Keratin, once synthesized, is a dead substance and does not regenerate on its own. It gradually wears down throughout the hair growth cycle due to mechanical, thermal, and chemical damage. Therefore, it is essential to protect and nourish it.
What is the structure of hair?
The hair shaft (the visible part of the hair) consists of three concentric layers, all composed largely of keratin.
The cuticle is the outermost layer. It's made up of overlapping scales, like roof tiles, which protect the inner layers. When the cuticle is intact and tightly closed, the hair is smooth, shiny, and strong. When it's damaged—by heat, coloring, aggressive brushing, or UV rays—the scales lift, and the hair becomes dull, porous, and prone to split ends and breakage. To understand how split ends form and how to get rid of them, our article "How to Get Rid of Split Ends" provides practical solutions.
The cortex is the thickest, middle layer. It is composed of keratinized cells and melanin (the pigment that gives hair its color). It is what gives hair its elasticity, tensile strength, and fundamental structure. Permanent hair dyes and chemical perms affect the cortex by oxidizing the pigments, which permanently weakens this layer.
The medulla is the core of the hair. It is present discontinuously, particularly in thick hair, and does not have a significant impact on the mechanical properties of the fiber.
How to preserve and supplement keratin?
Keratin does not regenerate in the already formed parts of the hair. However, it is possible to protect and strengthen it through two complementary approaches: external cosmetic treatments and internal nutrition.
Keratin-based cosmetic treatments
Cosmetic products enriched with hydrolyzed keratin (small keratin fragments capable of penetrating the hair fiber) help fill in the "holes" in the cuticle and smooth damaged scales. Keratin shampoos, masks, and leave-in treatments are effective options for porous or damaged hair. "Brazilian straightening" is a professional keratin application technique that temporarily seals the cuticle, but it uses very high temperatures, and some formulas may contain formaldehyde—best avoided. For gentler alternatives to repair damaged hair, our article on how to repair damaged hair offers natural solutions.
Dietary supplementation
Water-soluble keratin, available as dietary supplements (capsules, gummies), works from within on both layers of the hair—the cortex and the cuticle—unlike cosmetic treatments that only act on the cuticle. Studies have shown that keratin supplementation improves hair tensile strength, reduces porosity, and decreases hair loss during the telogen phase.
Keratin synthesis also depends on key nutrients: zinc and vitamin B6 are essential for transporting keratin to the hair root. Foods rich in zinc (seafood, wheat germ, nuts, meats) and vitamin B6 (bananas, poultry, chickpeas) should be favored. Biotin (vitamin B8) also plays a central role in keratin production. To learn more about this nutrient, our article on the benefits of biotin for hair details its action and dietary sources.
Frequently asked questions about keratin and hair
Is the keratin in cosmetic products the same as that in hair?
No. The natural keratin in hair is an insoluble protein with a very complex helical structure. The keratin used in cosmetics is called "hydrolyzed": it has been broken down into small peptides to penetrate the hair fiber. It temporarily repairs the cuticle but does not replenish the natural keratin in the cortex. This is why keratin treatments need to be repeated regularly.
Can keratin production be increased naturally?
Yes, to some extent. A diet rich in protein (sulfur-containing amino acids), zinc, biotin, B vitamins, and sulfur supports keratin synthesis during the anagen phase. Dietary supplements containing water-soluble keratin, biotin, and zinc can complement this approach, particularly in cases of identified deficiency. However, no treatment can "create" keratin in the already formed, dead part of the hair.
Does keratin straightening damage hair?
Professional keratin straightening treatments use very high temperatures (200-230°C) which can weaken the hair fiber despite the apparent protection of keratin. Some formulas containing formaldehyde (or its derivatives) pose health risks. If you wish to straighten your hair, opt for heatless methods or natural smoothing treatments based on plant oils and silk proteins.