Human hair is composed primarily of keratin, a fibrous structural protein rich in the amino acid cysteine. These cysteine molecules form disulfide bonds, which create the internal architecture that gives hair its strength, elasticity, and resistance to mechanical stress.

The hair shaft itself has three main layers:

The cortex contains tightly packed keratin filaments organized into macrofibrils. When hair is exposed to chemical processing, UV radiation, heat styling, oxidative stress, or excessive mechanical manipulation, the hydrogen bonds, ionic bonds, and even disulfide linkages within the cortex can weaken. This structural disruption leads to:

Protein treatments work through a mechanism known as substantivity, the ability of hydrolyzed proteins to bind to damaged areas along the cuticle and cortex. Because these proteins are hydrolyzed (broken into smaller molecular weight fragments), they can temporarily adhere to the hair fiber through electrostatic attraction and film formation.

Research shows that certain hydrolyzed proteins, such as keratin and wheat protein can

Importantly, protein does not permanently “repair” hair (as hair is biologically dead once it exits the follicle), but it can reinforce and fortify weakened areas, improving structural performance and resistance to future damage.

Optimal hair health requires a balance between:

Protein (structure + strength)
Moisture (plasticity + flexibility)

When protein reinforcement is paired with adequate hydration and lipid support, the hair fiber maintains resilience without becoming brittle.


Healthy hair begins at the molecular level, and protein integrity is the foundation of strand strength.