Why do some hairs withstand chemical and thermal stress while others break at the slightest brushing? Why does loss of shine always precede breakage? The answer lies not in care routines, but in biology. Hairswiss returns to the fundamentals: understanding hair structure at the molecular level is the prerequisite for choosing the right actives and the right protocols.
The Hair Fiber: A Three-Layer Architecture
Hair is a dead, keratinized structure — it does not repair itself from within like skin. Its health therefore depends entirely on the condition of its three constituent layers, each with a specific function and vulnerability.
The Cuticle: First Line of Defense
The cuticle is formed by 6 to 10 overlapping layers of flattened dead cells, composed mainly of hard keratin with a high cysteine content. These scales overlap like tiles, with the tip pointing toward the hair end. Their integrity directly determines shine (a closed cuticle reflects light uniformly), friction resistance, and the fiber’s capacity to retain water.
The cuticle is the first target of chemical aggression. Bleaching, for example, imposes an alkaline pH (9 to 11) that forces the scales open to allow hydrogen peroxide to reach the melanin in the cortex. With each procedure, a fraction of cuticle cells is irreversibly destroyed.
The Cortex: The Structural Mass of the Hair
The cortex accounts for 80 to 90% of the fiber volume. It is composed of keratin macrofibrils — bundles of helical proteins (α-helices) linked together by disulfide bonds (S-S) between cysteine residues. It is precisely these disulfide bonds that give hair its mechanical strength and elasticity: a healthy hair can stretch 20 to 30% before breaking.
Permanent chemical treatments (bleaching, perming, Brazilian straightening) work by reducing and then re-oxidizing these disulfide bonds. Each cycle irreversibly weakens the protein matrix of the cortex. The measure of this degradation is expressed through hair porosity: the more exposed the cortex, the faster the fiber absorbs water but loses it just as quickly — a sign of a compromised internal architecture.
The Medulla: The Central Core
Present only in thick hair, the medulla is a central cellular channel whose function remains partially unclear. It plays no decisive role in mechanical strength, but may contribute to thermal regulation of the fiber.
The Hair Follicle: The Production Factory
The quality of the hair produced depends directly on the state of the hair follicle, a complex epidermal structure that passes through three cyclical phases:
- Anagen phase (active growth): lasts 2 to 7 years depending on genetics and general health. The follicle actively synthesizes keratin from sulfur-containing amino acids (cysteine, methionine) derived from diet. This is where nutrition plays a direct and measurable role.
- Catagen phase (involution): 2 to 3 weeks. The follicle retracts and ceases production.
- Telogen phase (rest): 3 to 4 months, then the hair falls out and the cycle begins again. On average, 50 to 100 hairs fall naturally each day — a number well beyond this range may signal a nutritional deficiency, oxidative stress, or hormonal imbalance.
The Role of Nutrients: Biochemistry, Not Slogans
- Biotin (vitamin B7): essential coenzyme in the synthesis of fatty acids and keratin. Proven deficiencies are rare in well-nourished adults. Supplements have a demonstrated effect only in cases of real deficit.
- Iron (ferrous, Fe²⁺): indispensable for oxygen transport to the follicle via hemoglobin. Serum ferritin below 30 ng/mL correlates with telogen effluvium (diffuse hair loss). It is one of the most frequent deficiencies in women of childbearing age.
- Zinc: cofactor of enzymes involved in protein synthesis and cell division. The hair follicle is one of the fastest-renewing tissues in the body — it is therefore particularly sensitive to zinc deficiency.
- Sulfur-containing amino acids (cysteine, methionine): direct constituents of keratin. Cysteine alone accounts for approximately 14% of the amino acids in hair keratin. An insufficient protein intake is directly reflected in the strength of the hair produced.
Heat, UV and Mechanical Stress: Degradation at the Molecular Level
- Heat (>150°C): beyond this threshold, keratin α-helices denature irreversibly — they lose their three-dimensional configuration and with it their elastic resistance. At 230°C (a typical professional straightener temperature), degradation is significant from the very first use. Effective heat protectants form an occlusive film that distributes heat and slows conduction toward the cortex.
- UV radiation: UVB and UVA photons cause photolysis of disulfide bonds and oxidation of methionine residues into sulfoxides. Result: loss of mechanical strength, yellowing of blonde hair (oxidation of pheomelanin), and progressive drying of the cuticle.
- Mechanical friction: aggressive brushing on wet hair is particularly destructive — the water-saturated fiber sees its stretch resistance reduced by 30%. Cuticle scales lifted by friction are the first to detach, triggering irreversible surface degradation.
What This Means for Choosing Professional Treatments
Understanding hair biology allows you to read formulations differently. An effective treatment must not simply “nourish” the fiber in a vague sense — it must target a precise structural level: hydrolyzed proteins for the cortex, cationic polymers (such as Regenine) for the cuticle, humectants (hyaluronic acid, panthenol) for intrafibrillary hydration, reconstructive lipids (18-MEA, penetrating oils) for the cuticle’s intercellular cement.
Hairswiss regularly publishes detailed analyses of professional hair actives — their chemistry, their mechanism of action, and their clinical relevance. Because informed choices begin with a solid understanding.
Professional Product Grounded in Hair Biology
The Reconstructive Shampoo 250 by Edelstein, available on cliCHair.ch, translates hair biology principles into concrete formulation: hydrolyzed keratin (peptides 300–5,000 Da) to fill protein gaps in the cortex, mild surfactants that respect the physiological pH of the cuticle (4.5–5.5), and humectants that preserve intrafibrillary bound water. A shampoo designed according to the chemical logic of hair structure — not marketing logic.