Hair is primarily composed of protein, which dictates its strength, texture, and overall appearance. This protein gives the hair fiber its structural integrity. When hair is strong and healthy, its internal protein structure is intact and properly organized. Understanding the specific type of protein and its construction is key to maintaining healthy hair and mitigating damage.
The Core Component of Hair: Identifying Keratin
The main protein component of hair is keratin, a tough, fibrous structural protein. Keratin also makes up fingernails and the outer layer of skin. This protein is synthesized deep within the hair follicle, hardening and compacting as it moves up to form the visible hair shaft.
Keratin is built from long chains of amino acids, including cysteine, which contains sulfur. These sulfur atoms form strong chemical connections called disulfide bonds. These bonds are largely responsible for the hair’s durability and resistance to damage, creating a resilient structure that is insoluble in water. Once the hair emerges from the scalp, the keratin in the visible shaft is biologically dead and cannot self-repair.
How Protein Forms Hair Structure
The keratin protein forms the three distinct layers of the hair strand. The outermost layer is the cuticle, consisting of flat, overlapping protein cells that function like protective shingles. An intact cuticle shields the inner structure and gives the hair a smooth, reflective surface.
Beneath the cuticle is the cortex, which constitutes the main bulk of the hair fiber and is densely packed with keratin proteins. The cortex holds the majority of the hair’s strength, elasticity, and pigment. Protein filaments here are twisted into rope-like structures held together by disulfide, hydrogen, and salt bonds, which determine the hair’s natural shape and tensile strength. The innermost layer is the medulla, a soft, often-absent core typically only present in thicker hair types.
Causes of Protein Loss and Damage
External factors can severely compromise the hair’s protein structure by attacking its chemical bonds.
Chemical Treatments
Chemical treatments like permanent waving, relaxing, and bleaching cause significant damage by directly breaking the strong disulfide bonds within the cortex. Bleaching, for example, oxidizes the hair proteins, making the fiber brittle and highly porous.
Heat Styling
Heat styling, especially with tools exceeding 100°C, degrades the keratin proteins through a process known as denaturation. This intense heat causes the protective cuticle layer to lift and chip, exposing the inner cortex. This leads to accelerated protein loss and increased porosity.
Environmental Factors
Ultraviolet (UV) radiation from the sun causes photochemical degradation of hair proteins. UVB rays specifically lead to protein loss by breaking disulfide bonds on the cuticle’s surface.
Mechanical Stress
Mechanical stress from aggressive brushing, tight hairstyles, or towel-drying can create micro-tears in the cuticle. This weakens the fiber and accelerates the loss of internal protein.
Replenishing Protein for Hair Health
Restoring hair strength often involves using targeted topical treatments. Protein treatments utilize hydrolyzed proteins, which are broken down into small fragments that penetrate the hair shaft. These fragments fill in gaps and weak spots along the cuticle and cortex.
Ingredients like hydrolyzed wheat, soy, or keratin reinforce the hair fiber, resulting in reduced breakage and improved mechanical strength. A careful balance is necessary, as excessive protein application can lead to “protein overload.” This occurs when the hair absorbs too much protein without enough moisture, causing the strands to become stiff, brittle, and prone to snapping.
For internal support, a consistent intake of dietary protein is required to supply the amino acid building blocks needed to synthesize new keratin in the hair follicle. Vitamins, such as B vitamins, also support the metabolic processes necessary for healthy hair growth, ensuring the body produces structurally sound keratin from the root.