Why Does Hair Turn Gray?

The graying of hair is one of the most obvious signs of aging. It’s also known by a technical term that I had never heard of, “canities.” The first time I saw the word I misread it, wondering what gray hair had to do with dentistry. Evidently gray hair isn’t really white, but the pale yellowish tinge of the constitutive keratin protein, but, like polar bears, it just looks white by the way light reflects off of it. Why is hair pigmented in the first place?

Some suggest it may be for detoxification, as natural toxins like heavy metals bind to the pigment melanin to be excreted from the body through hair outgrowth. That’s why you can estimate the amount of fish consumption from the mercury content of children’s hair clippings, this is why “[a]nalysis of hair mercury may be warranted before pregnancy” in women who eat a lot of fish, and why the levels of mercury in the hair of those eating plant-based diets were found to be up to ten times lower than those who just occasionally ate fish. Within three months of switching to a plant-based diet, the levels of mercury, lead, and cadmium growing out in your hair appear to drop significantly, but build back up when meat and eggs are added back into the diet. Enough with the science trivia. What causes our hair to turn gray, and what can we do about it?

In medical school you learn about the so-called 50-50-50 rule. By age 50 years, the dictum goes, 50 percent of the population has at least 50 percent grey hair. But that was based on a homogenous population of largely white, fair-haired Australians. The global range is more like 6 percent to 23 percent are 50 percent gray by 50. Though not as memorable an axiom, it looks like about 75 percent of people between 45 and 65 years of age have about 25 percent gray hair.

Those of African or Asian descent tend to show less gray hair. White people generally start to gray in their mid-30s, Asians in their late-30s, and Africans in their mid-40s, such that premature graying is defined before age 20 in Caucasians but before age 30 in African Americans. Age 25 has been suggested as a cutoff for India. Premature or not, what causes hair to turn gray in the first place?

The hair follicle is a complex little mini-organ that anchors and grows a single hair. In the base of the follicle bulb as few as 100 pigment-making cells normally stain the emerging hair shaft with melanin pigments that can range in color from black to red. A single hair grows for about 3.5 years before falling out, and a new one starts growing in its place. So, graying may start after about 10 hair cycles when pigment deposition declines as the pigment generating cells, called melanocytes, start to become depleted. What kills them off?

There’s a depigmentation disorder called vitiligo where melanocytes in the skin are killed off by a buildup of hydrogen peroxide, which can decompose into toxic free radicals. So, researchers checked, and indeed the same “massive” levels were found in aging hair follicles. Where does the hydrogen peroxide come from? It’s naturally generated as a by-product of melanin synthesis, but normally countered by an antioxidant enzyme called catalase. The problem, it seems, is that catalase and other antioxidant defenses may decline as we age and leave melanocytes vulnerable to their own hot potato production of pigment.

Our own eyelashes present “an enigma in plain sight.” Eyelashes tend to be the darkest hairs on the human body and undergo graying significantly later than other hairs. The explanation appears to be the presence of a particular antioxidant protein missing from regular hair, consistent with the prevailing “free radical theory of graying.”