Identified in the 19th century, keratin is a natural insoluble protein produced by specialised cells called keratinocytes. Rich in sulfur, it is composed of 18 amino acids, predominated by cysteine (1).
Keratin exists in two distinct forms within the animal kingdom: beta-keratin (primarily in reptiles and birds) and alpha-keratin (in mammals, including humans) (2). The vegetable alternative, phytokeratin is obtained by hydrolysing plant proteins, mainly those from wheat and almonds.
In the human body, keratin is concentrated in the integumentary system, which includes the hair, skin and nails, where it plays a protective and structural role (3). Long used in the cosmetics and beauty industry, it is a common ingredient in hair care products: strengthening shampoos, masks, serums, and Brazilian smoothing treatments (for eliminating frizz) (4-5)!
A key component of hair, keratin actually accounts for almost 95% of the composition of hair fibres, where it is present in different layers of the hair shaft (except the central medulla, which serves no particular function).
In the cuticle, the hair’s outermost layer, keratin binds the scales together, keeping them smooth and flat. In so doing, it makes this protective sheath more impervious to external assaults and at the same time, boosts its natural shine. A lack of keratin usually results in dull-looking hair, split ends or porous, brittle hair (6-7).
It is also present in pigmented form in the cortex, the hair shaft’s middle, and thickest, layer. This consists of a network of keratin filaments which follows the line of the capillary shaft, maintaining the hair’s elasticity (8).
Important: synthesis of keratin by the hair bulb is not a renewing process. It takes place solely during the anagen phase of the hair cycle, which corresponds to the period of hair growth. If hair is mistreated (using a too-hot hairdryer, repeated blow-drying, wearing elastic bands or hair slides ...), it cannot self-repair (9).
Produced in the matrix unguis (the base of the nail hidden under the skin), keratin gives nails their shine, strength and smooth appearance (10).
Weak, brittle, split or ridged nails can be a sign of a lack of keratin, though there are many other potential contributory factors to nail fragility: exposure to corrosive chemicals, trauma, fungal infections, poor diet (11)…
The skin’s first line of defense, the epidermis is composed primarily of keratinocytes. These migrate from the epidermis’ deepest basal layer to its outermost one, the stratum corneum, becoming increasingly keratinised as they do so (12).
At the end of their journey, they differentiate into corneocytes, hard, flattened and keratin-filled dead cells, which fit together to form a cornified envelope. Smooth and strong, this has remarkable biomechanical properties (which ensure the skin’s resistance to friction), keeps the skin hydrated and prevents invasion by pathogens. Its exceptional cohesion is made possible by specific adhesive junctions (called ‘tight junctions’ and ‘desmosomes’) that keratin works to strengthen (13).
At the end of the cycle, corneocytes also undergo a process of desquamation, which is essential for the elimination of dead cells and skin renewal (14).
We can therefore see how the skin’s appearance is intrinsically linked to the condition of the stratum corneum and even more so, its keratinisation. More specifically, an insufficiently dense cornified envelope increases the risk of irritation and over-sensitive skin (15).
If you’re keen to have radiant hair, shiny nails, and a glowing complexion, then alongside a varied, balanced diet, keratin supplements can provide a significant beauty boost:
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