Keratin

Keratin: A Comprehensive Reference

1. Identity, Nomenclature, and Natural Sources

Chemical and Biological Names

The word keratin is derived from the Greek word kera, meaning "horn," and historically the term stood for all of the proteins extracted from skin modifications such as horns, claws, and hooves. With deeper scientific understanding, keratin is now redefined as all intermediate filament (IF)-forming proteins with specific physicochemical properties produced in any vertebrate epithelium. In humans, keratins are encoded by 54 genes.

Keratins and keratinous materials are subdivided into two different classes of secondary protein structure: alpha (α)-keratin and beta (β)-keratin. The α-helical conformation of keratin is the principal constituent of wool and other mammalian features such as hair, quills, fingernails, horns, hooves, and the epidermal layer of skin. Other keratin configurations have also been reported, including the β- and γ-keratins occurring in different animal species. β-Keratin represents an extended keratin configuration, a structural protein mainly occurring in the epidermis of reptiles and feathers.

Natural Sources

Keratins, as a group of insoluble and filament-forming proteins, mainly exist in certain epithelial cells of vertebrates. Keratinous materials are made up of cells filled with keratins, and they are among the toughest biological materials — found in human hair, wool, and horns of mammals, and feathers, claws, and beaks of birds and reptiles, where they function in protection, defense, hunting, and as armor.

Skin and its appendages, such as feathers, claws, horns, hooves, and wool, are rich in keratin protein (~80% protein). This protein is exclusively produced by animals and is the most abundant biomaterial after collagen. Commercially, the predominant sources used in dietary supplements are poultry feathers and sheep wool, with some preparations derived from human hair.

Common Forms and Preparations

Keratin is naturally water-insoluble and indigestible, necessitating harsh extraction techniques that reduce palatability, digestibility, bioavailability, and alter nutritive composition, making the native protein unsuitable for human consumption. Dietary keratins are best extracted from wool by microwaving or from feathers by acid/alkaline hydrolysis.

Soluble keratins can be extracted from hair fibers by oxidation or reduction. If an oxidative treatment is employed, the resulting keratins are referred to as "keratoses"; if a reductive treatment is used, the resulting keratins are referred to as "kerateines." This terminology also refers to the extracted keratin derivatives produced by oxidative and/or reductive treatment, including keratose, alpha-keratose, gamma-keratose, kerateine, alpha-kerateine, or gamma-kerateine.

Acid hydrolysis is the most efficient method for breaking down keratin into free amino acids of L-form and small peptides (below 800 Da). Enzymatic hydrolysis of wool keratin results in low molecular weight peptides (10–44 kDa) that are highly soluble in water. Microbial/enzyme-based hydrolysates provide better alternatives due to improved digestibility and nutrient bioavailability.

In the dietary supplement market, keratin is available as:

• Oral capsules and tablets containing hydrolyzed keratin (keratein or keratose fractions), standardized by proprietary processes
• Powders intended to be dissolved in water or mixed with beverages
• Liquid sachets — aqueous solutions containing keratin (e.g., 5% keratin) provided as concentrated liquid sachets for addition to food, beverage, or water
• Soft gelatin capsules — soft gelatin capsules containing keratin have been developed and patented as ingestible supplements.

Notable proprietary ingredients include Cynatine® HNS (a solubilized keratin derived from wool), Kera-Diet® (a natural keratin hydrolysate from poultry feathers), and KeraGEN-IV® (a functionalised keratin from New Zealand wool). The feather keratin hydrolysate (FKH) used in some preparations results in a characteristic mix of free L-amino acids (≥ 83.5%).

2. Traditional and Historical Use

Human civilizations have a long history of exploitation of keratin-rich tissues for the fabrication of daily life tools and ornaments, such as the use of horn sheaths as drinking vessels, mammalian and reptile skin as leather covers and clothing, and feathers as bedding materials and clothing, among many others.

From as early as the 16th century, there are reports of the use of pyrolyzed hair's ash for blood clotting and wound healing in the Chinese Compendium of Materia Medica, and since the 19th century there has been knowledge that diverse dermic structures such as hairs, feathers, and hooves consist of similar substances referred to as "horn" or "keratin." The preparation documented in the Compendium of Materia Medica (Bencao Gangmu, compiled by Li Shizhen in the late 16th century) was known as Xue Yu Tan (pyrolyzed human hair ash). The stated indications for Xue Yu Tan were accelerated wound healing and blood clotting.

Around 1849, the word "keratin" appears in the scientific literature to describe the material that made up hard tissues such as animal horns and hooves — the term deriving from the Greek kera, meaning horn. This new protein intrigued scientists because it did not behave like other proteins; for example, the normal methods used for dissolving proteins were ineffective with keratin.

During the years from 1905 to 1935, many methods were developed to extract keratins using oxidative and reductive chemistries. By the late 1920s many techniques had been developed for breaking down the structures of hair, horns, and hooves; scientists concluded that many different forms of keratin were present in these extracts, and that the hair fiber must be a complex structure, not simply a strand of protein. In 1934, a key research paper described different types of keratins, distinguished primarily by having different molecular weights.

The use of keratin-rich animal by-products as nutritional supplements for livestock has a much longer history. Keratin extracted from animal by-products such as feathers, horns, hooves, and hair is frequently added into animal feed as an inexpensive source of dietary protein. Human nutraceutical applications are substantially more recent, emerging primarily in the 21st century as extraction and hydrolysis technologies improved.

3. Key Constituents and Active Compounds

Structural Composition

In comparison with other fibrous proteins, keratin is distinguished by its high content of cysteine residues, which form disulfide bonds, supplying better mechanical, thermal, and chemical properties and water stability. Keratin is a natural protein with a high content of cysteine residues (7–13%) and is widely found in hair, wool, horns, hooves, and nails. The great stability of keratin results from the numerous disulfide bonds of cystine. The amino acid composition of keratin differs from that of collagen; cystine may account for up to 24 percent of the total amino acids.

α-keratin is a polypeptide chain, typically high in alanine, leucine, arginine, and cysteine, that forms a right-handed α-helix. Two of these polypeptide chains twist together to form a left-handed helical structure known as a coiled coil. These coiled-coil dimers, approximately 45 nm long, are bonded together with disulfide bonds, utilizing the many cysteine amino acids found in α-keratins. The dimers then align, their termini bonding with the termini of other dimers, and two of these new chains bond length-wise through disulfide bonds to form a protofilament. Two protofilaments aggregate to form a protofibril, and four protofibrils polymerize to form the intermediate filament.

β-keratins consist of β-strands that can be laterally packed in parallel or antiparallel sense, linked by hydrogen bonding, forming four β-strands that distort and create the β-sheet. Two β-sheets then assemble to form the filaments of β-keratin. This conformation gives a higher stiffness than α-keratins.

Hard alpha-keratins, such as those found in nails, have a higher cysteine content in their primary structure. This causes an increase in disulfide bonds that stabilize the keratin structure, allowing it to resist a higher level of force before fracture.

Amino Acid Profile

Keratins are structural, thiol-rich proteins, which comprise 90% of total poultry feather weight. The protein contains elevated phenylalanine, leucine, lysine, tyrosine, valine, proline, isoleucine, and aspartic/glutamic acid.

Keratin can be considered a natural source of cysteine, which is present as the dimer cystine, including linked S–S bonds that readily hydrolyze to liberate the monomer cysteine. Cysteine is one of the four sulfur-containing amino acids, the others being methionine, homocysteine, and taurine; however, only cysteine and methionine are incorporated into proteins. Unlike methionine, cysteine residues on the backbone of keratin polypeptides have a highly chemically reactive thiol (–SH) functional group at physiological pH, forming disulfide bridges in their oxidized form.

The amino acid profile of keratin isolates indicates suitability for bioactive peptide production, with a high content of hydrophobic amino acids and cysteine for high antioxidant potential, and a high proline content enhancing peptide bioavailability. However, supplementation with histidine, lysine, methionine, and tryptophan is recommended for a better-balanced profile of essential amino acids. Most keratins contain only low concentrations of several essential amino acids (histidine, lysine, and tryptophan), which suggests the overall quality of keratin protein is lower than other protein sources.

Cell-Binding Motifs

Keratin possesses abundant cell-binding motifs such as leucine-aspartate-valine (LDV), glutamate-aspartate-serine (EDS), and arginine-glycine-aspartate (RGD), which benefit cell attachment and proliferation. These motifs are biologically significant because the RGD sequence in particular is recognized by integrin receptors on cell surfaces, which is relevant to keratin's potential applications in tissue engineering and wound healing.

4. Mechanisms of Action

Bioavailability and Digestibility of Supplemental Keratin

Due to its molecular structure, native keratin protein is not metabolized by mammalian enzymes and has a very low nutritive value. The bioavailability of unprocessed keratin for humans is close to zero. Keratin is insoluble in water, weak acids, alkali solutions as well as many organic solvents, and is difficult to digest with mammalian proteolytic enzymes. Hydrolysis is therefore essential for any nutritional application.

The cysteine in keratin is largely in the form of cysteic acid, which is normally considered to be poorly digestible. As keratin in any form is not a common component of the human diet, the safety and health effects of crude or purified keratin in humans was initially unknown, requiring assessment in animal models.

Metabolic Pathways After Ingestion

A key mechanistic study published in Nutrients (PMC4772066) investigated the metabolic fate of dietary keratin in a rat model. The safety and digestibility of a dietary protein produced from keratin (KER) was compared to a cysteine-supplemented casein-based diet in a growing rat model for four weeks. KER proved to be an effective substitute for casein at 50% of the total dietary protein, with no changes in the rats' food intake, weight gain, organ weight, bone mineral density, white blood cell counts, liver glutathione, or blood glutathione.

Inclusion of KER in the diet reduced total protein digestibility from 94% to 86% but significantly increased total dietary cysteine uptake and subsequent liver taurine levels. The KER diet also significantly increased caecum weight and significantly decreased fat digestibility, resulting in a lower proportion of body fat, and induced a significant increase in blood haemoglobin. KER is therefore a safe and suitable protein substitute for casein, and the cysteic acid in keratin is metabolized to maintain normal liver and blood glutathione levels.

Cysteic acid cannot be incorporated directly into glutathione (GSH) but is a direct precursor to taurine. Taurine supplementation has been shown to increase GSH levels in rats, likely by increasing the availability of cysteine and methionine for GSH production.

Potential Antioxidant Mechanism

Cysteine-rich proteins such as keratin may have advantages over the simple amino acid or its derivatives as nutraceuticals to safely and beneficially improve antioxidant status in health and disease. The sulfur-rich amino acids released upon hydrolysis — particularly cysteine — serve as precursors to glutathione, the body's principal intracellular antioxidant. Studies focusing on the structure-activity relationship of peptides highlight proline residues as important contributors to their inhibition of angiotensin-converting enzyme, dipeptidyl peptidase IV, and prolyl oligopeptidase, as well as antioxidant activity.

Structural Contribution to Tissue Integrity

Keratin is among the most abundant structural proteins in humans and animals and is a potential source of a variety of free amino acids and biopeptides. The theoretical basis for oral supplementation rests on the premise that absorbed amino acids and peptides from hydrolyzed keratin are re-incorporated as building blocks for new keratin synthesis in hair follicles, nails, and skin. Amino acids are the basic building blocks of peptides and proteins and play a vital role in cellular signaling in human biology. The benefits of amino acid supplementation have been studied in different animal models, with results showing restoration of skin damage, improvement of skin hydration, wound healing, as well as protein metabolism leading to collagen synthesis.

5. Scientific Evidence by Area of Use

5.1 Hair Loss and Hair Quality

Different kinds of keratins have been studied for their benefits on hair, especially for temporary hair loss (acute telogen effluvium), and for nail brittleness.

Cynatine® HNS Clinical Trial (PMC4214097): A randomized, double-blind, placebo-controlled clinical trial evaluated Cynatine HNS as a supplement for improving various aspects of hair and nails. A total of 50 females were included and randomized into two groups. The active group (n = 25) received 2 capsules containing Cynatine HNS, comprising Cynatine brand keratin (500 mg) plus vitamins and minerals, per day, and the placebo group (n = 25) received 2 identical capsules of maltodextrin per day for 90 days. End points for hair loss, hair growth, hair strength, amino acid composition, and hair luster were measured, as well as nail strength and appearance. The results showed that subjects taking Cynatine HNS had statistically significant improvements in their hair and nails when compared to placebo, and the authors concluded it was an effective supplement for improving hair and nails in 90 days or less. Limitation: The study was funded by Roxlor Global, LLC, which provided all raw materials; the primary investigator, Robert H. Veghte, is the General Manager of Roxlor Global, LLC, and other authors received consulting fees from the company on a per-job basis. The sample size was small (n = 25 per group), and the active arm received a combination product (keratin plus vitamins and minerals), making it impossible to attribute effects to keratin alone.

Kera-Diet® Randomized Controlled Trial (2022): A clinical study was designed to investigate the efficacy of a natural keratin hydrolysate obtained from a non-human source (feathers) on acute telogen effluvium and brittle nail syndrome. The in vivo effects of Kera-Diet® (KD) upon human hair and nails were tested in a randomized, double-blind, parallel-group, benchmark- and placebo-controlled study involving 60 women over 90 days. In all subjects, anagen/telogen hair ratio, hair volume and density, pull test, global photography, hair and nail brightness, and nail plate growth were measured at baseline and after 45 and 90 days.

KeraGEN-IV® Clinical Study: KeraGEN-IV, a keratin ingredient by Keraplast, was associated with skin, hair, and nail benefits in a clinical study published in the journal HealthMed. The double-blind study involved 65 women between the ages of 45 and 60. Compared to placebo, those who took the keratin supplement for 60 days experienced greater improvements in measures of hair loss and strength, enhanced skin elasticity and barrier function, and nail strength.

A 2025 comprehensive review examined five key hair-constituent macromolecules — type I collagen, elastin, keratin, ceramides, and melanin — and their physiological and clinical impacts on hair structure, density, shine, and growth. A structured literature search was conducted in PubMed and Google Scholar through January 2025, selecting in vitro studies, animal experiments, and human clinical trials evaluating each macromolecule's effects on follicular function and hair fiber integrity.

Overall evidence quality for hair: While there are multiple published studies on the use of nutritional supplements for improvement of various skin or hair conditions, most of them evaluate complete formulas including active ingredients but also vitamins and minerals. Moreover, the rational chain of scientific evidence is frequently insufficient, and few studies are designed to the gold standard of a randomized, double-blind, placebo-controlled trial, and as a result lack scientific credibility. Evidence is preliminary to moderate; most trials are small, industry-sponsored, and of short duration. Independent replication is limited.

5.2 Nail Strength and Brittle Nail Syndrome

Brittle nail syndrome (BNS) is a common condition affecting up to 20% of the population, especially women over 50 years of age. Nails affected by BNS appear ragged, thin, and dull. In brittle nail syndrome, trace elements and amino acids (especially L-cystine) have been reported to be useful in ameliorating the nail plate condition. A nutritional approach with nutrients whose composition is close to human keratin has been hypothesized as potentially effective in decreasing hair loss during telogen effluvium and improving brittle nail condition.

The Cynatine HNS trial (described above in Section 5.1) also measured nail strength and nail appearance as secondary endpoints and reported statistically significant improvements in these parameters compared to placebo. The Kera-Diet® 90-day RCT similarly assessed nail plate growth and nail brightness as co-primary endpoints. Parameters measured in the Kera-Diet® skin aging study also included nail hardness and gloss of nails.

Overall evidence quality for nails: Similar to hair evidence — preliminary to moderate, with small, often industry-funded trials and limited independent replication. No large, multi-center RCTs have yet been conducted.

5.3 Skin Aging, Hydration, and Elasticity

The role of keratin supplementation in preventing skin aging has remained broadly understudied.

Kera-Diet® Skin Aging RCT (Tursi et al., 2025, published in Journal of Cosmetic Dermatology): This randomized, double-blind, placebo-controlled study evaluated a novel keratin hydrolysate obtained from poultry feathers. This feather keratin hydrolysate (FKH) results in a characteristic mix of free L-amino acids (≥ 83.5%). FKH was administered as a food supplement to a panel of adult women showing aging physiological signs. Participants were randomly assigned to three groups to receive daily dosages of 500 or 1000 mg of FKH or placebo for 90 days. Conducted between July 2022 and May 2023, this study included 99 Caucasian women aged 35 to 65. These participants showed visible signs of skin aging, such as wrinkles or loss of skin elasticity, as well as weakened hair and nails. Parameters of skin roughness, wrinkle features, deep skin moisturization, skin maximum elongation and elasticity, skin thickness, skin anisotropy, skin density, gloss of skin, hair and nails, and nail hardness were evaluated. Both FKH treatments showed a significant improvement of all parameters compared to day 0 and to placebo, with an exception for fiber anisotropy and fiber density, which showed significant improvement compared to day 0 and a tendency (but not statistical significance) to improve compared to placebo. Limitation: This study was fully funded and supported by BCF Life Sciences, the manufacturer of Kera-Diet®, representing a potential conflict of interest. The study was also limited to women and to Caucasian participants, limiting generalizability.

Overall evidence quality for skin: Preliminary. A single industry-funded RCT of moderate size showed statistically significant improvements across multiple skin parameters at both 500 mg and 1000 mg daily doses over 90 days. Independent replication and trials in broader populations are needed before firm conclusions can be drawn.

5.4 Athletic Performance, Body Composition, and Blood Parameters

The favorable amino acid profile of keratin suggests potential for use as a protein source and ergogenic aid for endurance athletes, following treatment to increase digestibility. However, available data suggest that keratin is not useful as an ergogenic aid for endurance athletes but may be a suitable protein supplement for maximizing increases in lean body mass.

Crum et al. (2018, PMC6161438) — Human Crossover Trial: This study investigated whether 4 weeks of soluble keratin (KER) consumption at 0.8 g/kg bodyweight/day by 15 endurance-trained males would have favorable effects on body composition, blood and cardiorespiratory variables, and cycling performance, compared to casein protein. Supplementation was randomized, blinded, and balanced, with a minimum eight-week washout period between trials.

Prior preclinical evidence had suggested a basis for investigation: following the substitution of 50% of a casein diet with KER in male Sprague–Dawley rats over a four-week period, blood profiles exhibited significant increases (~5%) in haematocrit and haemoglobin content. DEXA analysis suggested an improvement in lean to fat mass ratio and increased femoral bone mineral density in the KER-supplemented rats compared to casein alone. However, in this small-scale controlled trial, measures were made only at the end of the dietary intervention, minimizing statistical power.

Overall evidence quality for athletic performance: Very preliminary. The single published human trial found no ergogenic benefit in endurance cycling performance; a possible lean-mass benefit was noted but not conclusively demonstrated in this small crossover study.

5.5 Wound Healing

It has been confirmed that keratin plays important roles in every stage of wound healing, including hemostasis, inflammation, proliferation, and remodeling, making keratin-based materials good candidates for wound dressings. Keratin derivatives, which can be either soluble or insoluble, are utilized as wound dressings since keratins are dynamically up-regulated and needed in skin wound healing. However, the primary application here is topical use of keratin biomaterials in wound dressings, not oral supplementation. Evidence for oral keratin supplementation improving wound healing in humans is not yet established in the peer-reviewed literature.

5.6 Antioxidant Status and Thiol Metabolism

The cysteic acid in keratin is metabolized to maintain normal liver and blood glutathione levels. There were no significant differences observed between KER + casein and casein control groups in glutathione, GSH, GSSG, or GSH:GSSG ratios in the liver or blood in the rat study, suggesting that dietary keratin maintains — but does not substantially elevate — glutathione status. This work was conducted in rats and has not been replicated with glutathione outcome measures in human trials.

6. Body Systems and Health Areas

• Integumentary system (skin, hair, nails): The primary focus of current supplement research. Keratin hydrolysates are active components used in food supplements to alleviate aging signs on skin, hair, and nails.
• Antioxidant/redox system: Via cysteine release and downstream glutathione and taurine synthesis.
• Musculoskeletal system: Proposed lean mass support in athletes, and preliminary rodent data on bone mineral density.
• Hematological system: Preclinical rodent data showed increased haemoglobin and haematocrit with keratin supplementation; not yet confirmed in humans.
• Wound healing: Well-documented for topical keratin biomaterials; oral supplementation evidence is lacking.
• Gastrointestinal system: Dietary keratin is gaining attention as an alternative protein source with potential applications in nutricosmetics, nutraceuticals, prebiotics, and food packaging, though prebiotic mechanisms remain under investigation.

7. Dosage Forms and Reported Dosages

The following dosages are drawn directly from published clinical and preclinical research sources:

• Cynatine® HNS (hair and nails RCT, PMC4214097): The active group received 2 capsules containing Cynatine HNS, comprising Cynatine brand keratin (500 mg) plus vitamins and minerals, per day, for 90 days.
• Kera-Diet® (skin aging RCT, Tursi et al., 2025): Participants were randomly assigned in three groups to receive daily dosages of 500 or 1000 mg of FKH or placebo for 90 days.
• Soluble keratin for athletic performance (Crum et al., 2018): During each intervention, participants consumed 0.8 g protein per kg bodyweight per day, for 6 days per week, with one non-supplementation day each week. This was based on the recommended dietary intake for protein for males aged 18–70 of 0.84 g/kg/day.
• Rodent safety/metabolism study (PMC4772066): Diets were formulated to contain 17% protein from the desired source (casein, 50:50 casein + keratin, or yellow pea flour).

8. Safety Considerations

General Safety in Animal Models

In a four-week rat model, keratin (KER) proved to be an effective substitute for casein at 50% of the total dietary protein, with no changes in the animals' food intake, weight gain, organ weight, bone mineral density, white blood cell counts, liver glutathione, or blood glutathione. Dietary keratins have been characterized as a nutritive, safe alternative protein when best extracted from wool by microwaving or feathers by acid/alkaline hydrolysis.

Digestibility and Gastrointestinal Effects

Inclusion of keratin in the diet reduced total protein digestibility from 94% to 86%, but significantly increased total dietary cysteine uptake and subsequent liver taurine levels. The KER diet also significantly increased caecum weight and significantly decreased fat digestibility, resulting in a lower proportion of body fat. The increase in caecum weight is consistent with a prebiotic or fiber-like effect and warrants further investigation in humans. Both effects were observed in rats and have not been systematically characterized in human studies.

Amino Acid Imbalances

Most keratins contain only low concentrations of several essential amino acids (histidine, lysine, and tryptophan), which suggests the overall quality of keratin protein is lower than other protein sources. Using keratin as a primary — rather than supplementary — protein source could therefore lead to essential amino acid insufficiency.

Processing-Related Concerns

Hydrolysis of keratin at high pH (>11.0) or temperatures (>70°C) can also irreversibly degrade cystine into potentially toxic by-products. This means that the safety profile of a keratin supplement is partly dependent on the extraction and processing method used to manufacture the product.

Source Purity and Contamination

Keratin extraction from animal by-products requires harsh chemicals and mechanical treatments, which alter the palatability, yield, amino acid composition, and bioavailability of the extracted keratin. Consumers should be aware that the source animal species, the specific extraction protocol, and any chemical residues from industrial processing can all influence the final product's composition and safety.

Limitations of Human Safety Data

As keratin in any form is not a common component of the human diet, and the safety and health effects of crude or purified keratin in humans is not fully established, initial assessments were made in animal models. To date, no large-scale, long-duration human safety trials of oral keratin supplementation have been published in peer-reviewed sources indexed in PubMed. Published human RCTs have used durations of 60–90 days without reporting serious adverse events, but long-term safety data are absent.

Interactions

No pharmacokinetic drug-interaction studies with oral keratin supplements have been identified in the peer-reviewed literature. Because keratin supplementation increases cysteine bioavailability and elevates taurine levels, theoretically there could be additive effects when co-administered with other thiol-donor supplements (e.g., N-acetylcysteine, whey protein) or antioxidant supplements, though this has not been tested in humans.

9. Evidence Summary and Research Gaps

Keratin is naturally water-insoluble and indigestible, necessitating harsh extraction techniques; consequently, the dietary use of keratin remains understudied. The most clinically explored application of oral keratin supplementation is in hair and nail quality, where several small, industry-sponsored RCTs (50–99 participants, 60–90 days) report statistically significant improvements over placebo. A single RCT (Tursi et al., 2025) has extended this to skin aging parameters. A single human crossover trial has assessed athletic performance, finding no ergogenic benefit. All existing human trials are of short duration, conducted predominantly in women, and most have been funded by the ingredient manufacturer, creating a substantial risk of bias. Most published studies evaluate complete formulas including vitamins and minerals alongside keratin, meaning the rational chain of scientific evidence is frequently insufficient to attribute effects to keratin alone. Independent, adequately powered, long-duration RCTs are needed to confirm preliminary findings and to assess safety over longer time horizons.

References

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