Hair is a complex biological fiber, often viewed as a sign of overall health. Understanding its composition reveals that this seemingly simple strand is almost entirely built from a single class of organic molecules. This structure is constantly exposed to environmental and chemical stressors, relying heavily on its core protein content for strength, flexibility, and shape.
The Quantitative Makeup of Hair
The human hair strand is overwhelmingly composed of protein, representing approximately 80 to 90% of its total weight. This high concentration of protein provides the bulk and structural integrity for the entire fiber. The remaining fraction is a mixture of other necessary components.
Water is the next largest component, accounting for about 4 to 13% of the hair’s fresh weight, and this moisture content is vital for elasticity. Lipids (fats and oils) make up a small percentage of the hair’s composition and function to lubricate and protect the surface. Finally, trace elements like zinc, iron, and copper, along with melanin pigments that determine hair color, complete the structural profile of the hair shaft.
Keratin: The Structural Foundation
The dominant protein in hair is keratin, a fibrous structural protein that forms the primary material of the hair cortex and cuticle. Keratin proteins assemble into long polypeptide chains that coil tightly into a spiral shape known as an alpha-helix. Multiple alpha-helices twist together to form robust intermediate filaments, providing the hair with resilience.
The strength of the hair structure depends heavily on disulfide bonds, which are strong covalent cross-links that form between sulfur atoms in the amino acid cysteine. These bonds, often called sulfur bridges, hold the keratin chains in their fixed positions. The quantity and placement of these disulfide bonds directly influence the hair’s texture, determining whether it is naturally straight, wavy, or tightly curled. These strong chemical linkages make the hair insoluble and highly resistant to degradation.
Recognizing and Addressing Protein Damage
When the protein structure of the hair is compromised, the integrity of the keratin network is weakened, leading to noticeable changes in hair quality. Damage from harsh chemical processes, such as bleaching and perming, or from excessive heat styling, directly breaks the stabilizing disulfide bonds. This structural collapse results in a rough, porous texture, increased brittleness, and loss of elasticity, causing strands to snap easily rather than stretch.
To support the internal protein structure, optimizing the dietary intake of building blocks is important. The body requires specific sulfur-containing amino acids, like cysteine and methionine, to manufacture new keratin protein in the hair follicle. Consuming foods rich in protein and sulfur, such as eggs, beans, and lean meats, ensures the necessary raw materials are available for the continuous growth of healthy hair.
Another method for addressing existing damage is the topical application of protein treatments, which provide a temporary external solution. Products containing hydrolyzed proteins have been chemically broken down into small, low-molecular-weight peptides and amino acids. These tiny fragments are small enough to penetrate the outer cuticle layer and temporarily adhere to the damaged keratin structure within the cortex. By filling microscopic gaps in the hair shaft, these treatments can restore lost strength and improve smoothness until the next wash.