Why does hair turn gray?

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If you look at photos of President Obama taken before he ran for president and since he left office, you'll notice a distinct difference: where there used to be only dark brown hair, there is now far more gray than brown. It seems that the stress of running a country would turn any person's hair gray. But is stress really to blame? And why does hair turn gray, even for those of us who don't have jobs quite as stressful as President of the United States?

In humans, most gray hair is not related to stress. In fact, hair doesn't actually "turn" gray at all. Once a hair follicle produces hair, the color is set. If a single strand of hair starts out brown (or red or black or blond), it is never going to change its color (unless you color your hair). Your hair follicles produce less color as they age, so when hair goes through its natural cycle of dying and being regenerated, it's more likely to grow in as gray beginning after age 35. Genetics can play a role in when this starts. 

While being under stress can't change the color of individual strands of hair, stress can trigger a common condition called telogen effluvium, which causes hair to shed about three times faster than normal. The hair grows back, so the condition doesn't cause balding. But if you're middle-aged and your hair is falling out and regenerating more quickly because of stress, it's possible that the hair that grows in will be gray instead of its original color.

That said, a 2020 study of mice demonstrated how stress can lead to their fur may turn gray under stress.  Whether this applies to humans and how often it plays a role in hair graying are unclear.

Illnesses that cause gray hair

The vast majority of people with gray  hair have age-related graying. However, sometimes graying hair indicates an illness, especially if it occurs at a particularly young age. Health problems that may be heralded by gray hair include:

• vitamin B₁₂ deficiency
• neurofibromatosis (also called Von Recklinghausen's disease): this group of inherited diseases causes tumors to grow along nerves and abnormal development of the bones and skin.
• tuberous sclerosis: an uncommon, inherited condition that causes benign tumors in multiple organs (including the brain, heart, kidneys, eyes, lungs, and skin).
• thyroid disease
• vitiligo: this condition causes melanocytes (the cells at the base of hair follicles that produce color) to be lost or destroyed — perhaps because the immune system "misfires" and attacks the scalp rather than an infection.
• alopecia areata: a disorder in which patches of hair may be suddenly lost, especially the colored (non-gray) hairs. This may lead to "overnight" graying because previously present gray or white hairs suddenly become more obvious. When hair growth resumes, it may be white or gray, but colored hair may eventually return.

The bottom line

When and how thoroughly your hair turns gray is influenced mostly by the genes you inherit from your parents. Though stress may play a role in the process, it would be more helpful to look to past generations rather than your current stress levels to help you predict when or if you'll go gray. That's true whether you're the President of the United States, or someone with a less stressful job.