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Understanding Hair Structure: Why Keratin Is Essential to Hair Health
To take good care of your hair, you must first understand what it is made of. Its strength, shine, and flexibility are not the result of chance: everything is based on a complex and unique structure, dominated by an essential protein… keratin .
The three layers of the hair fiber
A hair is not a simple fiber. It is composed of three distinct layers:
- The cuticle : the outermost layer, formed of scales that fit together like tiles. It protects the hair fiber and gives hair its shine and softness to the touch.
- The cortex : This is the main part of the hair, responsible for its strength and color (pigments). It also gives the hair its mechanical strength.
- The medulla : the innermost layer, present especially in thick hair.
Keratin, the heart of hair strength
Hair strength is mainly based on its chemical composition. Hair primarily contains:
- keratin proteins,
- lipids,
- and water.
Keratin is key: it is a unique protein, rich in cysteine, a sulfur-containing amino acid. This richness in cysteine distinguishes hair keratin from skin keratin (7.6% versus 2.9%).
Why is this important? Because cysteine forms disulfide bonds, true "bridges" that connect the keratin chains. These bridges give hair its strength, resilience, and durability.
An architecture reinforced by multiple links
Hair fiber is a true biological feat:
- Covalent bonds and disulfide bridges provide strength.
- Ionic and hydrogen bonds provide flexibility and elasticity.
- Hydrophobic forces contribute to cohesion.
It is thanks to this complex network that hair can be both strong and flexible. Its behavior varies depending on humidity: it can lengthen, swell, or soften.
But be careful: when the disulfide bridges are destroyed (for example during bleaching or chemical straightening), the fiber loses its strength and becomes brittle.
The role of lipids and water
Besides keratin, hair contains:
1 to 9% lipids, which provide shine, suppleness and a “silky” effect. They also act as a natural lubricant.
Up to 32% water, which contributes to flexibility and allows the formation of hydrogen bonds with proteins.
Why repairing your hair's keratin is essential
Daily aggressions (sun, pollution, UV rays, heat from appliances, frequent washing, chemical products such as coloring or straightening) damage the hair structure.
Result: dry, dull, brittle hair that is difficult to style.
The solution? Provide the hair fiber with bioavailable sulfur amino acids , capable of restoring disulfide bonds and regenerating damaged keratin.
This is precisely what we do by combining organic wool keratin and snail slime in our Mademoiselle Agathe hair care: two natural ingredients rich in active peptides, capable of penetrating deep down to restore hair's original strength and beauty.
Hair health depends on a subtle balance between keratin, lipids, and water. Keratin, thanks to its richness in cysteine and disulfide bonds, is the central element of hair strength and shine.
By choosing treatments formulated with real animal keratin , compatible with human hair fiber, you offer your hair authentic and lasting repair — well beyond the surface effects of plant phytokeratin.
Malinauskyte E, Shrestha R, Cornwell PA, Gourion-Arsiquaud S, Hindley M. Penetration of different molecular weight hydrolyzed keratins into hair fibers and their effects on the physical properties of textured hair. Int J Cosmet Sci. 2021 Feb;43(1):26-37. doi:10.1111/ics.12663. Epub 2020 Oct 15. PMID: 32946595; PMCID: PMC7820954.
Fernandes et al., 2012 – Int. J. Cosmetic Science – Keratin-based peptide strengthens relaxed hair (mechanical & thermal recovery) link
Barba et al., 2010 – Fibers and Polymers – Wool keratin proteins restore major properties of pretreated hair.
Basit et al., 2018 – Biotechnology Reports – Recombinant keratin K31 improves smoothness, diameter and resistance of damaged hair.
Daniels et al., 2016 – IFSCC Magazine – Comparative study: hydrolyzed keratin vs wheat vs collagen. Keratin = best for wet tensile strength.
Cruz et al., 2017 – Int. J. Biol. Macromol. – Small peptides interact via hydrophobic bonds and disulfide bridges with human keratins.
Essendoubi et al., 2019 – Int. J. Cosmetic Science – Confocal Raman: monitoring α-helix/β-sheet conformations of hair keratin with active ingredients.
Fan et al., 2025 – Molecules – Hydrolyzed keratin prevents UV-induced hair photoaging.
