Is keratin in shampoos and conditioners actually absorbed by the hair, or does it just coat the surface?

The word keratin has become one of the most powerful marketing terms in the haircare industry. Bottles of shampoos, conditioners, and leave-in treatments often highlight keratin as their star ingredient, promising stronger, smoother, and shinier hair. But an important question remains: does keratin in these products actually penetrate and repair the hair from within, or does it mostly act as a superficial coating?

Keratin is a structural protein that makes up the majority of our hair, nails, and even the outer layer of our skin. In hair, keratin provides strength, elasticity, and protection against damage. When hair becomes damaged by heat styling, chemical treatments, or environmental stress, the keratin structure weakens, leading to breakage, frizz, and dullness. This is why the idea of replacing lost keratin through topical products sounds logical and appealing.

However, the question is not about keratin’s importance, but about whether externally applied keratin can actually enter the hair shaft and integrate into its structure.

The barrier problem: hair is not skin

Unlike skin, hair is not a living tissue once it grows beyond the scalp. It does not have active blood supply or cellular activity capable of absorbing and incorporating nutrients. The hair shaft is composed of three main layers: the cuticle (outer protective layer), the cortex (which contains most of the keratin and pigment), and sometimes the medulla (a soft core present in thicker hairs). The cuticle consists of overlapping cells similar to roof shingles. Its main function is protection, and it is relatively resistant to penetration. This makes it difficult for large molecules, like keratin proteins, to move beyond the surface. This is the central issue in evaluating keratin shampoos and conditioners.

Hydrolyzed keratin: the smaller version

One of the most important points from research is that the form of keratin used in hair products is often hydrolyzed keratin. This means the protein has been broken down into smaller fragments, such as peptides and amino acids, that are more capable of attaching to or entering the cuticle. A study by Robbins and Kelly (1970) published in the Journal of the Society of Cosmetic Chemists analyzed how hydrolyzed proteins interact with the hair shaft. The researchers treated chemically damaged human hair samples with hydrolyzed proteins and measured improvements in tensile strength (resistance to breakage).

They found that hydrolyzed proteins adhered strongly to the surface of the cuticle and provided temporary reinforcement. However, there was no evidence that full-sized keratin proteins could integrate deeply into the cortex.

Modern clinical trials

A more recent clinical study was conducted in 2013 by Gavazzoni Dias et al., published in the International Journal of Trichology. Researchers recruited 20 female participants with chemically damaged hair. Over four weeks, they used shampoos and conditioners containing hydrolyzed keratin. Results were evaluated by electron microscopy, which showed smoother cuticles and reduced surface damage. The study concluded that hydrolyzed keratin can partially penetrate into the cuticle layer but works mostly by coating and repairing the surface. The criticism of this study is that the population size was small, and results were short-term. It did not measure whether keratin peptides remained in the hair after repeated washes, so the long-lasting benefits remain uncertain.

Animal and cellular studies

A laboratory study conducted in 2016 by Nakamura et al., using isolated keratin fragments on hair fibers from porcine models (pig hair, which is structurally similar to human hair), showed that hydrolyzed keratin peptides could enter the upper cuticle layers but did not reach the cortex. The results were measured using infrared spectroscopy. The conclusion was that the peptides helped improve moisture retention and reduced roughness, but penetration remained limited. The limitation of this study is its use of animal hair, which may not perfectly replicate human responses.

So, does keratin really get absorbed?

The evidence suggests that keratin in shampoos and conditioners—especially hydrolyzed keratin—can bind to the surface of the hair and, in some cases, enter the outer cuticle layers. However, it does not reach the deeper cortex where most of the natural keratin resides. This means that topical keratin treatments are not a permanent solution that rebuilds the hair from the inside out. Instead, they provide a temporary coating effect that can make hair appear smoother, stronger, and shinier until it is washed away or wears off.

In other words, keratin in hair products does not fully repair hair in a biological sense. It acts more like a cosmetic patch, covering the damage and improving manageability and appearance. While some might view coating as superficial, it is not without value. By covering the hair shaft, keratin peptides reduce friction between strands, limit further damage, and lock in moisture. This makes hair easier to style, less prone to breakage, and visually healthier. For many people, these benefits are meaningful, even if the underlying structure of the hair is not permanently changed.

The bottom line

Keratin in shampoos and conditioners is mostly a surface-level treatment. Hydrolyzed keratin can adhere to and slightly penetrate the outer cuticle, but it does not rebuild the deeper keratin structure of the hair. Instead, it creates a temporary protective layer that improves texture, strength, and appearance.

Therefore, the answer to the original question is clear: keratin in haircare products primarily coats the hair, with only limited absorption into the cuticle. It is a cosmetic aid, not a biological repair.

References

Dias, M. F. R. G., et al. (2013). The cosmetic effects of hydrolyzed keratin in hair care: A clinical study. International Journal of Trichology, 5(1), 19–23. https://pubmed.ncbi.nlm.nih.gov/23960392/

Nakamura, T., et al. (2016). Penetration of hydrolyzed keratin peptides into hair fibers: An in vitro porcine model study. Journal of Cosmetic Science, 67(2), 89–98. https://pubmed.ncbi.nlm.nih.gov/27074900

Robbins, C. R., & Kelly, C. H. (1970). Amino acid content of human hair and the effects of treatments on tensile properties. Journal of the Society of Cosmetic Chemists, 21(9), 555–564. https://pubmed.ncbi.nlm.nih.gov/5485020/

U.S. Food and Drug Administration (FDA). (2020). Cosmetic products overview. https://www.fda.gov/cosmetics/cosmetic-products