Keratin: Composition and Role in Hair Restoration

Keratin is a family of structural proteins that primarily make up hair, nails, and the stratum corneum. Unlike many other proteins in the body, keratin forms a strong framework that is virtually non-renewable and resistant to mechanical, chemical, and thermal stress. This is why the condition of keratin directly affects the appearance of hair and nails. [1]

Hair and nails are structures made of so-called "hard" keratin, which contains a very high concentration of the amino acid cysteine. Cysteine forms disulfide bonds, making keratin fibers particularly rigid and resilient. "Soft" keratins are present in the epidermis of the skin and provide elasticity and barrier properties to the stratum corneum. [2]

In hair, keratin is organized into a complex system of filaments, which are collected in the cortex and surrounded by the cuticle. It is this fibrous structure that determines the hair's strength, elasticity, and ability to withstand tension and thermal stress. The better the keratin architecture is preserved, the less hair breaks and splits. [3]

Keratin is one of the most stable proteins in the body: it is virtually uninvolved in rapid metabolic processes and remains in hair and nails for months and years. Therefore, any damage to keratin (bleaching, frequent thermal straightening, aggressive curling) accumulates and cannot be completely reversed by any cosmetic procedure. [4]

Keratin is attractive to the cosmetics industry for two reasons. First, it is a naturally occurring protein in humans, and keratin-based products are marketed as "natural." Second, its chemical structure allows for the creation of derivatives that can partially strengthen and smooth the surface of hair and nails, improving appearance without surgical intervention. [5]

Table 1. Where in the body is keratin used and what does it do?

Structure	Keratin type	Main function
Hair	Hard keratin	Strength, tensile and heat resistance
Nails	Hard keratin	Stiffness and protection of fingertips
The stratum corneum of the skin	Soft keratin	Barrier, protection against moisture loss and irritants
Eyelashes and eyebrows	Hard keratin	Eye protection, hairline shaping
Epithelial cells	Different keratins	Maintaining cell shape and strength

Chemical composition and structure of keratin

Chemically, keratin is an intermediate filament protein. It is composed of amino acid chains that form helical regions and more rigid domains. Keratin is distinguished by its very high content of the sulfur-containing amino acid cysteine, which forms strong disulfide bridges between the chains and makes the structure nearly indestructible under normal conditions. [6]

There are many genetically different keratins. They are conventionally divided into type I (more acidic) and type II (more neutral or basic) keratins. In hair, these proteins assemble into heterodimers, then into larger filaments, which form the basis of the hair cortex. Errors in the synthesis of individual keratins can lead to hereditary hair and skin diseases. [7]

Hard keratin in hair and nails differs from soft skin keratin in its cysteine content and the density of disulfide bonds. The more such bridges, the more rigid and chemically stable the structure becomes. This is good for strength, but makes keratin nearly insoluble and difficult to use directly in cosmetics without prior processing. [8]

Heat and harsh chemical treatments break down some of the disulfide bonds, and the keratin structure shifts from its predominant alpha-helical form to more chaotic and less ordered configurations. Research using infrared and spectroscopic hair imaging has shown that intense heat and chemical treatments alter the ratio of helical and beta structures, which correlates with a loss of strength and increased frizz. [9]

Modern cosmetic developments are increasingly attempting to specifically interact with the keratin structure. Peptides and small molecules capable of strengthening or partially restoring specific areas of keratin, preserving the alpha helix and reducing heat damage, are being studied. This approach remains at the scientific and early clinical level, but is already being actively discussed in the professional literature. [10]

Table 2. Key features of keratin as a protein

Parameter	Significance for hair and nail properties
High cysteine content	Disulfide bond formation and rigidity
Structure of intermediate filaments	Resistance to mechanical stress
Hard vs. soft keratin	Differences in strength and solubility
Low solubility	The need for chemical modification in cosmetics
Sensitivity to high heat and chemicals	Changes in secondary structure and hair fragility

Cosmetic forms of keratin: hydrolyzed, oxidized, soluble

Native keratin from hair, wool, or feathers is insoluble in water and most mild solvents, so modified forms are used in cosmetics. The most common is hydrolyzed keratin—a product of the controlled breakdown of the original protein into shorter chains and peptides. These fragments dissolve better, can partially penetrate the surface layers of the hair, and form a film on the cuticle. [11]

In addition to hydrolyzed keratin, oxidized, sulfonated, and soluble keratin are also used. These forms are produced through chemical reactions that partially break disulfide bonds or add new functional groups that increase solubility and alter the charge of the molecule. This allows for more precise adjustment of the keratin's interaction with hair, skin, and other formula ingredients. [12]

The safety of such ingredients is regularly assessed by the independent expert group Cosmetic Ingredient Review, which analyzes toxicological data and real-world use in cosmetics. In a recent review, this group examined eight keratin derivatives and concluded that they are safe at current uses and concentrations, primarily as conditioning agents for skin and hair. [13]

From a practical standpoint, what matters most to consumers is not the advertised name, but the specific form on the ingredient list. Hydrolyzed keratin and its analogs are most often added to shampoos, conditioners, masks, and leave-in sprays, where they act as film-forming and strengthening components. Oxidized and sulfonated forms are often used in more specialized formulas for hair restoration and coloring. [14]

The raw materials for these ingredients are typically sourced from animal byproducts: wool, feathers, and hooves. From a safety standpoint, this doesn't reduce, and sometimes even improves, the process's controllability: the manufacturer receives standard raw materials, undergoes purification and testing, and the final formula contains only carefully characterized protein fragments. Vegan alternatives rely on plant-based peptides that mimic the properties of keratin. [15]

Table 3. The main forms of keratin in cosmetics and their features

Ingredient	How to get	Main function in cosmetics
Hydrolyzed Keratin	Hydrolysis of hard keratin	Film formation, reduction of fragility
Hydrolyzed Hair Keratin	Hydrolysis of keratin from hair	Improved compatibility with human hair
Oxidized Keratin	Partial oxidation with bond cleavage	Increased solubility, charge modification
Sulfonated Keratin	Introduction of sulfonic groups	Improving water solubility
Soluble Keratin	Various modifications for complete solubility	Skin and hair conditioning

The Role of Keratin in Hair Products

The primary role of keratin in hair products is conditioning and protection. Hydrolyzed keratin and related peptides are able to adsorb onto the hair surface, especially in damaged areas where the cuticle is raised and the structure is more porous. This allows hair to retain moisture better, become less brittle, and feel smoother. [16]

According to industry reviews, hydrolyzed keratin is found in hundreds of shampoo, mask, and conditioner formulas, primarily rinse-off hair products. Typical concentrations are in the low percentage range, sufficient to form a thin film and interact with the cuticle without causing excessive protein stripping when used with reasonable frequency. [17]

The effectiveness of such ingredients is confirmed in laboratory models and ex vivo experiments. Research shows that treating hair with products containing keratin peptides can partially preserve the alpha-helical structure of keratin during thermal damage, reduce protein loss during washing, and reduce the degree of damage after coloring and heat treatment. This is not a "miracle repair," but a statistically significant improvement in the mechanical properties of hair. [18]

An important detail: keratin in shampoos or masks primarily works on the surface of existing hair, not in the hair follicles. It doesn't stimulate new hair growth or treat alopecia, but rather improves the appearance and behavior of existing hair. If hair loss is significant or changes in quality occur, it's important to begin with an examination and treatment, and consider keratin products as a supplement to your basic care. [19]

It's especially important to distinguish between regular keratin-based hair care products and so-called "keratin straightening," where the effect is achieved not so much by the keratin itself as by aggressive chemicals and high heat. While these treatments may contain keratin, the primary ingredients are formaldehyde or glyoxylic acid, which alter the hair structure rather than simply condition the cuticle. [20]

Table 4. How keratin works in different types of hair products

Product type	The role of keratin
Shampoo	Light conditioning, reducing dryness
Conditioner and balm	Smoothing the cuticle, making combing easier
Mask	More pronounced film formation and protection
Leave-in spray	Antifreeze effect, friction protection
Professional care without straightening	Strengthening the surface, reducing brittleness

Keratin in skin and nail products

In skin products, hydrolyzed keratin and related fragments are used as emollients and film-forming ingredients. They help form a thin protective layer that reduces transepidermal water loss and makes skin feel smoother. Unlike hair, the emphasis here shifts from strength to barrier function and comfort. [21]

Some skin and nail formulas use combinations of keratin with other proteins and moisturizing ingredients. In this case, the keratin fragments enhance the feeling of density and smoothness, while the other ingredients provide hydration and a lipid barrier component. Systematic reviews of such products show that keratin most often acts as a supporting conditioning agent rather than the primary active ingredient. [22]

Keratin fragments are logical to use in nail products because the nail plate itself is composed of hard keratin. However, in real-life conditions, large molecules have limited penetration into the nail, so the primary effect of such products is on the surface: reduced fragility due to film formation, reduced roughness, and improved adhesion of decorative coatings. [23]

The safety assessment of keratin and its derivatives in skin and nail products is conducted using the same principles as for hair. The Cosmetic Ingredient Review expert panel analyzed data on systemic toxicity, irritation, sensitization, and phototoxicity and concluded that keratin ingredients are safe for current use and concentrations. The main restrictions relate only to compliance with general cosmetic safety standards. [24]

Online databases of consumer organizations that monitor potential ingredient risks also rate hydrolyzed keratin as a low-risk ingredient for carcinogenicity, reprotoxicity, and immunotoxicity. However, there is always a risk of individual allergic reaction, especially in people with severe atopy or sensitive skin, so patch testing before active use remains a sensible precaution. [25]

Table 5. Use of keratin in skin and nail products

Product category	Why is keratin added?
Hand and body creams	Film formation, smooth feeling
Skin serums	Additional air conditioning
Nail strengthening products	Reducing brittleness, leveling the surface
Moisturizing foot products	Softening of rough skin, protection
Multifunctional tools	Marketing focus on "protein care"

Keratin and Keratin Treatment Safety: Where the Real Danger Lies

It's important to clearly distinguish keratin as a cosmetic ingredient from "keratin" hair straightening treatments. The former refers to hydrolyzed or modified keratin in shampoos, masks, or creams, which act primarily on the surface of the hair or skin and have a proven safety profile. The latter are complex compounds that, when heated, release toxic substances and can affect the entire body. [26]

Traditional keratin straightening treatments often contained formaldehyde or its precursors, which release formaldehyde into the air when heated by a straightening iron. Research has shown that formaldehyde concentrations in salons during such treatments can exceed acceptable levels, leading to eye and respiratory irritation, headaches, and a potential increased risk of cancer with chronic exposure. [27]

In the wake of formaldehyde bans, many manufacturers have switched to formulas containing glyoxylic acid, marketed as "safe and formaldehyde-free." However, a series of clinical cases in recent years have shown that such products can cause acute kidney injury with the deposition of calcium oxalate crystals, sometimes in young and previously healthy individuals. This has already led to discussions about the need for stricter regulation of such products. [28]

For stylists and clients, the main recommendation from regulators and professional associations is simple: don't rely solely on marketing claims like "formaldehyde-free" and "organic straightening," but rather investigate the actual ingredients, the presence of glyoxylic acid, and ventilation conditions. Many official documents explicitly state that even mislabeled products can release formaldehyde when heated and require strict respiratory protection standards. [29]

Against this backdrop, hair care products containing hydrolyzed keratin in standard concentrations appear significantly safer. Expert reviews have not identified any serious systemic risks associated with their use, and the main potential problems are limited to skin irritation or individual allergies. The key message for consumers is simple: the danger stems not from the word "keratin" itself on the packaging, but from the combination of keratin with aggressive reagents and high temperatures during straightening procedures. [30]

Table 6. Keratin ingredients and keratin treatments: fundamental differences

Object	What is this?	Main risks
Hydrolyzed keratin in shampoo	Conditioning ingredient	Local irritation or allergy
Keratin in a mask or serum	Film-forming strengthening component	Potential protein overload with abuse
Keratin straightening with formaldehyde	Chemical straightening with heat	Toxic fumes, irritation, possible carcinogenic risk
Straightening with glyoxylic acid	Alternative straightening with acid	Potential kidney damage, systemic toxicity
Home "pseudo-salon" kits	Professional compositions without control	Unpredictable exposure and burns

Prospects for using keratin and typical myths

Modern research is actively exploring the subtle interactions of keratin fibers with peptides, polymers, and other active ingredients. It has already been shown that certain peptides can stabilize the secondary structure of keratin and reduce heat-induced hair damage. Based on these findings, so-called "bond repair agents" are being developed that work not only through keratin but also through specific linking molecules. [31]

At the same time, the field of keratin biomaterials is developing: films, sponges, and fibers are being created from recycled keratin for medical applications, tissue engineering, and, potentially, even more complex cosmetic applications. While these are primarily laboratory and experimental solutions, they demonstrate how multifunctional this protein can be beyond the traditional realm of hair care. [32]

At the consumer level, it's important to debunk several persistent myths. The first is that the keratin in shampoo "fills" the hair from the inside and completely restores its structure. In reality, the effect is limited to the surface layers and a partial reduction in breakage, while the damaged deeper sections of the hair shaft remain damaged until new hair grows. [33]

The second myth is that any "keratin" treatment is automatically beneficial because keratin is "natural." As data on formaldehyde and glyoxylate straightening shows, it is the combination of keratin with harsh chemicals and high temperatures that poses the greatest health risks. Therefore, the sensible approach is to use keratin in gentle treatment formulas and be extremely cautious with any harsh treatments, even if they are advertised as organic. [34]

The third important point is the role of nutrition and overall health. Even the most advanced keratin treatments cannot compensate for protein and key nutrient deficiencies. When protein is deficient, the body prioritizes vital organs, and hair and nails are the last to suffer: they become brittle, thin, and grow less well. In such cases, it's best to first adjust your diet and, if necessary, consult a doctor, and only then consider keratin treatments as a supplement. [35]

Table 7. Keratin in reality: what it can and cannot do

Statement	The real state of affairs
"Keratin completely restores hair"	Partially improves the surface and reduces brittleness
"Keratin treatments are always safe."	Risks depend on the reagents and heating
"Without keratin, hair cannot be healthy."	General nutrition and gentle care are more important.
"Formaldehyde has been replaced, everything is now safe."	Glyoxylic acid is also associated with serious risks.
"Keratin treats alopecia"	Works on the hair shaft, not the follicle