Keratin: The Protein Your Hair Is Made Of

Keratin is the protein your hair is built from, the same family that makes nails and the outer layer of skin. Hair is essentially keratin protein spun into fibres, bundled into a core, and armored with scales. Knowing that explains a lot about what hair products can and cannot do.

How keratin builds a strand

Keratin chains are wound into microscopic fibres, the fibres are bundled into the cortex, and the bundle is held together by disulfide bonds, the sulfur cross-links that give hair its strength. The outer cuticle scales are keratin too. So when hair is damaged, it is the keratin structure, and especially those cross-links, that has been broken.

Why "keratin treatments" mostly coat

It is tempting to think you can fix keratin damage by adding keratin. The catch is size: most keratin and hydrolyzed-protein ingredients are far too large to get inside the strand (the practical limit is around 2,000 daltons). They deposit on the surface, where they smooth and add temporary body, then wash off. That is useful conditioning, but it is not the same as rebuilding the broken structure inside. See Hair Porosity and Product Penetration for the size rule.

What real repair requires

Rebuilding keratin damage means getting a small reactive molecule into the cortex to form new cross-links where the old ones broke, which is what molecular reconstruction does. The complete picture of damage and repair is in Hair Porosity: The Complete Science.

Reference

• Robbins, C.R. Chemical and Physical Behavior of Human Hair, 5th ed. Springer (2012).