What causes our gray hair? And is there anything we can do to prevent or reverse the process? These are questions all of us have considered at some point in our lives, and most of us still don’t have any answers. We either live with it, spend hours, and fortunes, putting color back into our head coverings, or just give up and shave it all off. Some of us lose our hair anyway, which partially solves the problem but it almost never disappears completely and that creates the same dilemma of what to do with what’s left. So let’s try and answer the fundamental questions posed above. What causes or gray hair, and what can we do about it, if anything?

     Our hair gains its color through melanin, a natural pigment that also determines the color of our eyes and the shade of our skin. In young hair, melanocytes (specialized cells that produce melanin) transfer this pigment along the length of each strand as it emerges from its follicle. Combinations of two main types of melanin (one for darker hues and another for lighter ones) make up all the dazzling shades of human hair—from icy blond to fiery red to ebony black. But aging disrupts this process, leaving our hair without pigment, pale and gray.

     As anyone who’s had to grow out a bad perm (usually female), or a too-short cut, knows, hair goes through years-long phases of growth and rest, in which new hair comes in and then falls out. After many such cycles, the stem cell melanocytes in our hair follicles fatigue. “We once thought that these stem cells eventually pooped out and died, but now we think that some are still there,” says Dr. Kord Honda, a dermatologist with University Hospitals Cleveland Medical Center.

     In fact, recent work in mice showed that stem cell melanocytes can switch from mature, pigment-producing melanocytes back to immature cells that can be used in future rounds of hair growth. (This differs from typical non-embryonic stem cells, in which development proceeds in one direction only.) But after multiple cycles, stem cell melanocytes can become trapped, which prevents them from reaching the base of the follicle and developing into melanocytes.

     So you should avoid tearing your hair out, when frustration with increasing grayness strikes, because it might just come in back in gray. “By plucking your hair, you force it to form a new hair. This constant cycling of the hair stresses the hair follicle and may cause that hair to become prematurely gray,” Dr. Honda explains. Theoretically, however, if we can coax our stem cell melanocytes into becoming melanocytes again, we could reverse gray hair.

     Stress is thought to be another route to a head packed with gray hairs. But the evidence in people, while suggestive, is limited by small experimental sample sizes, Dr. Korda says. One 2021 study, for example, looked at hair from 14 individuals and found that changes in color within a single strand can be tied to periods of stress (as reported by study participants). More intriguingly, in 10 out of 14 subjects, the researchers discovered strands with light tips and dark roots, indicating that some reversal of graying hair might be possible.

     In mice, the link between stress and graying is stronger, with experiments that have teased out potential mechanisms. In a 2020 paper, for example, researchers found that injecting mice with norepinephrine—the body’s fight-or-flight hormone—caused melanocyte stem cells to rapidly transform into melanocytes and migrate away from the follicle. The fur that then grew out of that melanocyte-less follicle was gray. So when that off-leash dog (or your boss or partner) starts to growl at you, not only will your heart rate rise and your pupils dilate, but your hair might just start its trek toward gray.

     Some medications can also cause premature graying. However, that is sometimes reversible once patients stop taking it. Various ailments (among them heart diseaseAlzheimer’s disease, and autoimmune conditions like vitiligo and alopecia areata) have been associated with gray hair.   

     Some researchers hope to someday predict an individual’s disease risk by looking at how much gray hair they have. But in many cases, Dr. Korda says, “patients can have a disease where they lose more colored hair than gray hairs,” giving the impression that they’re suddenly more gray.

     Overall, according to Dr. Korda, genetics remains the best predictor of when someone will go gray. So look to your parents for hints of what’s to come, and as someone to blame.

     Unlike hair loss, however, for which there has been great strides in treatment, “there’s almost nothing you can do to modify the rate at which you turn gray, or when it will happen,” says Dr. Korda.

     “Unfortunately, we don’t currently have any mechanisms to reliably bring hair color back. If somebody did, they’d be very well off,” Dr. Korda says with a laugh. They would also destroy a very lucrative hair-coloring industry.

     Hope springs eternal that we can stop the graying process, but I don’t think the hair salon industry should worry too much about losing their livelihoods anytime soon unless, of course, some future fashion dictates we all go bald.

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