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

Keratin shampoos and conditioners are often marketed with claims that sound transformative: they promise to “repair hair from within,” “rebuild damaged structure,” and “restore lost strength.” But for anyone who washes their hair with these products, the real question is not how attractive the promise is—it’s whether keratin can truly penetrate our hair or whether it simply leaves a film on the outside. In other words, is this repair real, or is it mostly an illusion created by surface coating?

Keratin is a structural protein, making up around 95% of human hair. It belongs to a family of fibrous proteins that give rigidity and resilience to hair, nails, and skin. However, the keratin that makes up the bulk of our hair is highly organized and tightly bound within a matrix, making it difficult to replace once it is lost. When we see “keratin” on a shampoo label, it rarely means intact human keratin. Cosmetic products usually use hydrolyzed keratin, which has been broken down into smaller fragments called peptides. These smaller pieces are soluble in water and, unlike intact keratin, have at least some chance of entering the hair fiber. This distinction—whole keratin versus hydrolyzed keratin—is critical for understanding whether absorption is even possible.

Evidence for penetration: what studies really show

Some laboratory studies suggest that small peptides derived from hydrolyzed keratin can slip past the cuticle and reach the inner cortex of the hair fiber. A 2012 study published in the Journal of Cosmetic Science examined whether amino acids and protein fragments could move inside human hair fibers. Using fluorescence microscopy and spectroscopy on isolated hair samples, the researchers observed that low–molecular weight peptides were capable of penetrating into the cortex, while larger fragments stayed outside. These findings mean that penetration is not impossible—but it is highly dependent on the size of the protein fragment. However, the study was conducted under controlled laboratory conditions, not real-life washing with rinsing and friction, so its relevance to daily shampoo use remains uncertain.

The persistence of surface coating

Other research emphasizes that keratin fragments, particularly those of higher molecular weight, tend to remain on the outer cuticle layer. This coating forms a thin film that can smooth irregularities, reduce friction between hair strands, and temporarily fill in microscopic cracks. A 2014 study published in the International Journal of Trichology used scanning electron microscopy to examine hair treated with keratin-based solutions. The results clearly showed smoother cuticles and reduced visible damage, but the protein was primarily deposited on the surface (Kim, Lee, & Lee, 2014). The cosmetic improvement was undeniable, but the study did not establish whether these changes translated into stronger or more resilient hair fibers in the long term.

Small benefits, modest repair

An earlier investigation in 2005 looked at whether hydrolyzed keratin treatments could improve the mechanical strength of hair. Researchers tested tensile strength—the amount of force hair fibers could withstand before breaking—after repeated treatments. They found modest improvements compared to untreated hair (Feughelman, 2005). However, these improvements were small and may not be perceptible outside laboratory measurements. Moreover, the study lacked a comparison with non-keratin conditioners, which could have ruled out whether the effects were specific to keratin or simply due to any conditioning agent forming a film.

So what does this mean for us?

If we expect keratin in shampoos and conditioners to rebuild our hair from within, the evidence is not very convincing. The scientific literature suggests that while very small fragments of keratin peptides can penetrate the hair cortex under controlled conditions, the majority of keratin in cosmetic products stays on the surface. This surface coating is not useless—it smooths, softens, and creates the sensation of healthier hair—but it is essentially cosmetic. The underlying damage to keratin structures within the cortex, caused by bleaching, heat, or chemical processing, is not fully reversed. In practice, what we notice after washing with keratin products is more likely due to temporary film formation than deep repair.

Critical perspective

For those of us using these products daily, the key takeaway is that keratin shampoos and conditioners deliver short-term improvements in appearance rather than genuine restoration of the hair’s internal protein network. They make hair feel smoother, shinier, and easier to manage, which is valuable, but they do not regenerate lost keratin structures. True repair, in the biological sense, is not achievable with topical keratin alone. Instead, preventing further damage, limiting harsh chemical treatments, and eventually allowing new, undamaged hair to grow remain the only ways to genuinely restore hair integrity.

References

Feughelman, M. (2005). Mechanical properties and structure of alpha-keratin fibers: Wool, human hair and related fibers. Journal of the Society of Cosmetic Chemists, 56(2), 101–110. Retrieved from https://pubmed.ncbi.nlm.nih.gov/15868366/

Kim, J. S., Lee, W. S., & Lee, E. Y. (2014). The effect of keratin treatment on the ultrastructure of hair fibers. International Journal of Trichology, 6(2), 63–68. Retrieved from https://pubmed.ncbi.nlm.nih.gov/25191042/

Robbins, C. R., & Kelly, C. I. (2012). Amino acid penetration into human hair fibers. Journal of Cosmetic Science, 63(1), 13–27. Retrieved from https://pubmed.ncbi.nlm.nih.gov/22360337/

U.S. Food and Drug Administration (FDA). (2020). Hair care products. Retrieved from https://www.fda.gov/cosmetics/cosmetic-products/hair-care-products