What is hair made of chemically? Mostly keratin protein, plus water, lipids, melanin, and trace minerals held together by side bonds. Here's the science.
Chemically, hair is made mostly of keratin, a fibrous protein assembled from long chains of amino acids. The rest of the fiber is water, lipids, melanin pigment, and small amounts of trace minerals. Those ingredients are organized into a tough structure held together by several different types of chemical bonds.
Keratin: The Protein Hair Is Made Of
Keratin is the main protein in hair, and it makes up roughly 80 to 95 percent of the dry weight of a human hair fiber. If you have ever asked what protein hair is made of, keratin is the answer.
Keratin belongs to the structural protein family, which means it is built for strength and shape rather than for chemical reactions. It is also unusually rich in the amino acid cysteine, which contains sulfur.
That sulfur is the key detail. Because cysteine carries sulfur, neighboring keratin chains can link to each other and turn a soft protein into a tough, water-resistant fiber. Hair, nails, and the outer layer of skin all rely on the same chemistry.
Amino Acids and the Polypeptide Chain
Inside each strand, the cortex is made up of millions of polypeptide chains packed side by side. Every polypeptide chain is made up of amino acids joined by peptide bonds, the same backbone found in every protein in the body.
If you have ever wondered what is the monomer of a polypeptide, the answer is the amino acid. Hair uses about 18 different amino acids, and their order determines how the protein folds and how strong the fiber becomes.
In plain terms, what is a polypeptide? It is a chain of amino acids, usually fewer than 100, linked end to end. Hundreds of these chains then twist together into keratin filaments that give hair its shape and elasticity.
A few amino acids dominate the profile of human hair:
- Cysteine, the sulfur-bearing amino acid that forms disulfide bonds
- Serine and glutamic acid, which help attract and hold water
- Glycine and proline, common in the smaller keratin-associated proteins
Side Bonds in Hair: What Holds the Chains Together
Side bonds in hair are the cross-links that connect one polypeptide chain to another. They explain why hair can be curled, straightened, and colored, and why repeated chemical treatments cause lasting damage.
- Disulfide bonds link two cysteine residues. They are the strongest side bonds, and permanent waves and relaxers work by breaking and re-forming them.
- Hydrogen bonds break when hair gets wet and re-form as it dries. They are why wet hair can be reshaped and why humidity causes frizz.
- Salt (ionic) bonds shift with pH, breaking in strongly acidic or alkaline products and re-forming afterward.
- Hydrophobic and van der Waals forces are weaker attractions that still add stability across the fiber.
A simple way to picture it: peptide bonds run along each chain like beads on a string, while side bonds tie separate strings together.
| Bond type | What it connects | Strength | Everyday effect |
|---|---|---|---|
| Peptide bond | Amino acids within one chain | Very strong, covalent | Forms the protein backbone |
| Disulfide bond | Cysteine residues on two chains | Strong, covalent | Permanent waves, relaxers, bleach damage |
| Hydrogen bond | Oxygen and nitrogen groups on nearby chains | Weak | Wet styling, curling, frizz |
| Salt bond | Oppositely charged groups | Weak, pH-sensitive | pH of shampoos and treatments |
| Hydrophobic and van der Waals | Close-packed nonpolar groups | Very weak individually | Overall fiber cohesion |
What Else Is in Hair Besides Protein?
Protein is the bulk of the fiber, but hair is not pure keratin. The remaining components are small in mass yet important for how hair looks and feels.
- Water — usually 10 to 15 percent of the total weight of healthy hair, and higher in humid conditions.
- Lipids — fatty acids and cholesterol-based compounds, including the 18-MEA layer that coats the cuticle and makes hair feel smooth.
- Melanin — eumelanin (brown to black) and pheomelanin (red to yellow), the pigments that set natural hair color.
- Trace minerals — calcium, magnesium, zinc, copper, and iron in very small amounts.
| Component | Approximate share | Main role |
|---|---|---|
| Keratin protein | 80–95% of dry weight | Structure, strength, elasticity |
| Water | 10–15% of total weight | Flexibility and hydrogen bonding |
| Lipids | 1–9% | Cuticle barrier and smooth feel |
| Melanin | Varies widely | Pigment and some UV protection |
| Trace minerals | Under 1% | Enzyme and structural cofactors |
Is Hair Made of Dead Cells?
Yes — the visible hair shaft is made of dead, keratinized cells. Living cells divide in the follicle below the skin surface, but once they fill with keratin and are pushed upward, their metabolic activity stops.
That is why cutting or chemically treating hair causes no pain and why a damaged strand never truly heals. New hair has to grow from the follicle to replace it.
Does Hair Have Protein If You Eat It?
Hair does contain protein, but eating it provides no useful nutrition. Human digestive enzymes cannot efficiently break down keratin because the protein is densely cross-linked and shielded by a tough cuticle.
Swallowing hair is also risky. Indigestible strands can clump into a bezoar in the stomach or intestines, a problem that sometimes requires medical treatment.
People exploring protein and peptide science sometimes review a list of injectable peptides and what they do, but those compounds have nothing to do with the keratin in a strand of hair. If you are concerned about protein intake or hair loss, talk with a healthcare professional instead of self-treating.
Why Hair Chemistry Matters in Real Life
Hair chemistry explains most of what happens in a salon chair. Water breaks hydrogen bonds, which is why wet hair stretches and can be reshaped. Bleach and relaxers break disulfide bonds, which is why repeated processing leaves hair weaker and more porous.
No shampoo, mask, or serum can rebuild disulfide bonds. Products can reduce friction, add temporary bonds, and seal the cuticle, but the structural protein itself has to be replaced by new growth from the follicle.
Sudden shedding, breakage, or texture changes can also point to a medical issue such as thyroid disease, anemia, or a nutritional deficiency. Persistent changes are worth discussing with a doctor or dermatologist rather than guessing at the cause.
Frequently Asked Questions
Is hair made of dead cells?
Yes, the visible hair shaft is made of dead, keratinized cells. Living cells in the follicle divide and build the fiber, but once those cells fill with keratin and move up the shaft, they are no longer metabolically active.
What protein is hair made of?
Hair is made primarily of keratin, specifically a hard alpha-keratin. Keratin is rich in the amino acid cysteine, and the sulfur in cysteine forms disulfide bonds that give hair its strength and shape.
Does hair have protein if you eat it?
Yes, hair contains protein, but human digestive enzymes cannot break down keratin efficiently. Eating hair offers no nutritional benefit, and swallowed strands can clump into an indigestible mass called a bezoar.
This page provides educational research information and does not replace medical advice, diagnosis, or treatment.