Dark hair gets its color from a pigment called melanin, specifically a brown-to-black form called eumelanin. The more eumelanin packed into each strand, the darker your hair appears. Black hair contains roughly 7.2 mg of melanin per gram of hair, compared to about 5.2 mg in dark brown hair and just 2.5 mg in blond hair. The difference comes down to your genes, the cellular machinery inside each hair follicle, and a handful of nutrients that keep the whole system running.
How Melanin Gets Into Your Hair
Hair color is built from the inside out, starting deep in the hair follicle. Specialized pigment-producing cells called melanocytes sit in the upper portion of the hair bulb, right next to the immature cells that will eventually become the visible hair strand. These melanocytes manufacture melanin inside tiny compartments called melanosomes, which mature through four stages before they’re fully loaded with pigment.
The process starts with the amino acid tyrosine. An enzyme called tyrosinase converts tyrosine into a series of intermediate compounds that ultimately polymerize into melanin. This chemistry is surprisingly pH-sensitive: the early steps require acidic conditions, but the final formation of melanin pigment works best at a neutral or slightly alkaline pH. Once melanosomes are saturated with pigment, melanocytes transfer them into the neighboring hair cells. Those hair cells absorb the pigment packages, carry them upward as the hair grows, and form the visible colored strand.
Scientists have proposed several theories for how this transfer happens. Melanocytes extend arm-like projections called dendrites toward the hair cells. The hair cells may actively engulf the tips of these dendrites, or melanosomes may be released into the space between cells and then absorbed. Either way, the result is the same: pigment granules end up embedded throughout the hair cortex, and to a lesser extent in the inner medulla.
Two Types of Melanin, One Key Ratio
Your body produces two forms of melanin. Eumelanin is the brown-to-black pigment responsible for dark hair. Pheomelanin is a red-to-yellow pigment that dominates in red and strawberry blond hair. Every person’s hair contains some mix of both, but the ratio between them is what determines your shade.
Black hair is composed of roughly 99% eumelanin and only about 1% pheomelanin. Spectrophotometric studies confirm this: researchers can measure the eumelanin-to-total-melanin ratio by analyzing how hair absorbs light at different wavelengths. Black and brown hair consistently shows higher ratios than yellow or red hair. So having dark hair isn’t just about producing more pigment overall. It’s about producing the right kind of pigment in overwhelming proportion.
The Genes Behind Dark Hair
Several genes work together to determine how much eumelanin your follicles produce. One of the most influential is MC1R, which encodes a receptor on the surface of melanocytes. When a signaling molecule called alpha-MSH activates this receptor, it triggers a chemical cascade that pushes the cell toward eumelanin production. Certain MC1R variants, common in European populations, reduce this signaling and shift production toward pheomelanin instead, resulting in red or lighter hair.
Other important genes include TYR, which codes for the tyrosinase enzyme that kicks off melanin synthesis, and OCA2, which influences melanin processing inside melanosomes. KITLG plays a role in the development and survival of melanocytes themselves. Variants in KITLG have been linked to lighter pigmentation in Eurasian populations, likely by affecting how many melanocytes are present or how actively they produce pigment. In equatorial populations, where UV radiation is intense, the functional versions of MC1R, TYR, and OCA2 that promote dark pigmentation have been maintained by natural selection over tens of thousands of years.
This is why dark hair is the most common hair color worldwide. The ancestral human condition favors high eumelanin production, and lighter shades result from mutations that partially disrupt the pigmentation pathway.
Nutrients That Fuel Pigment Production
Tyrosinase, the enzyme at the center of melanin synthesis, is a copper-dependent protein. Two copper ions sit at its active site and directly participate in the chemical reactions that convert tyrosine into melanin precursors. Without adequate copper, tyrosinase activity drops and melanin production slows. Copper deficiency has been associated with premature lightening of hair.
The amino acid tyrosine is the raw material for the entire process. Your body can synthesize tyrosine from another amino acid, phenylalanine, but both ultimately come from dietary protein. Iron also plays a supporting role in melanin chemistry, though its contribution is less direct than copper’s. A diet chronically low in protein, copper, or iron can reduce the amount of pigment your follicles are able to produce.
Why Dark Hair Lightens in the Sun
Unlike skin, which can ramp up melanin production in response to UV exposure, hair is made of dead cells. Once a strand has grown out of the follicle, it can’t produce new pigment. Instead, prolonged sun exposure gradually breaks down the melanin already embedded in the strand through a process called photobleaching.
UV radiation generates free radicals that attack the chemical structure of melanin, opening up its ring-shaped molecular components through oxidation. This degrades the pigment and shifts the hair’s color lighter over time. Dark hair resists this effect better than light hair because eumelanin is more photostable than pheomelanin. The higher melanin content in dark hair also absorbs more UV radiation before it can damage the hair’s structural proteins. So while a brunette’s hair may develop reddish or coppery tones after a summer outdoors, the change is slower and less dramatic than what happens to lighter shades.
How Hair Loses Its Darkness With Age
Gray and white hair appear when melanocyte stem cells in the hair follicle become depleted. These stem cells, which sit in a region called the bulge, normally replenish the melanocytes that produce pigment during each hair growth cycle. When the stem cell pool runs low, new hairs grow in without pigment.
Research published in Nature revealed one mechanism behind this depletion. Stress activates the sympathetic nerves that run alongside hair follicles, causing them to release a burst of norepinephrine. This flood of signaling molecules forces melanocyte stem cells out of their resting state and into rapid proliferation. About 50% of affected stem cells entered active cell division within 24 hours in experimental models. But instead of replenishing the pool, these cells differentiate prematurely, migrate away from their niche, and are permanently lost. The damage is irreversible: once the stem cells are gone, that follicle will only produce unpigmented hair.
This isn’t caused by DNA damage or cell death. The stem cells essentially exhaust themselves by overreacting to the stress signal. The same mechanism may contribute to ordinary age-related graying, since patients who have undergone surgical removal of sympathetic nerves on one side of their body develop fewer gray hairs on that side as they age. Melanosome size and shape also change with age. In Japanese males, melanosomes grew measurably larger and rounder over time, with the volume increasing significantly, though what this means for perceived color is still being studied.
Sex Differences in Hair Pigment Structure
Men and women don’t just differ in hair color trends. They differ at the melanosome level. Studies of Japanese hair found that melanosomes in male hair are significantly larger in volume than those in female hair, with a less elongated shape. In younger individuals, female melanosomes tend to be uniformly elongated, while male melanosomes already resemble the rounder shape typically seen in adults. These structural differences were statistically significant and may contribute to subtle differences in how hair color appears between sexes, even when the overall melanin content is similar.