Stress doesn’t actually turn hair grey. In fact, hair doesn’t actually “turn” grey at all. Once a hair follicle produces hair, the colour is set. If a single strand of hair starts out brown (or red or black or blond), it is never going to turn grey. Your hair follicles produce less colour as they age, so when hair goes through its natural cycle of dying and being regenerated, it’s more likely to grow in as grey beginning after age 35. Genetics can play a role in when this starts.

New research suggests that anxiety and fear cause the body to exhaust its supplies of a type of stem cell required for hair colour. The findings could explain why some world leaders, such as Barack Obama, appear to go grey at an accelerated pace as they shoulder the burdens of office, experts said. The study, published in the journal Nature, might also put researchers on the path to a treatment for premature salt and pepper hair.

“Everyone has an anecdote to share about how stress affects their body, particularly in their skin and hair - the only tissues we can see from the outside,” said senior author Ya-Chieh Hsu, an associate Professor of Stem Cell and Regenerative Biology at Harvard University.

“We wanted to understand if this connection is true, and if so, how stress leads to changes in diverse tissues. We were genuinely curious to see if stress indeed leads to hair greying.”

Scientists initially thought the immune system might be responsible, going haywire during extreme stress and attacking the pigment cells. But in mice without immune systems the effect still happened. They then looked at whether the stress hormone cortisol might be causing damage, but after removing the adrenal gland from mice, they found their hair still went grey. After systematically eliminating different possibilities, they honed in on the sympathetic nerve system, which is responsible for the body's fight-or-flight response. Sympathetic nerves branch out into each hair follicle on the skin. The researchers found that stress causes these nerves to release the chemical norepinephrine, which gets taken up by nearby pigment-regenerating stem cells. Researchers found that the norepinephrine from sympathetic nerves causes the stem cells to activate excessively, sending them into overload and depleting the colour reservoir.

“After just a few days, all of the pigment-regenerating stem cells were lost. Once they're gone, you can't regenerate pigment anymore,” said Dr Hsu. “The damage is permanent."

Although fight-or flight is generally a beneficial response, it also shuts down many systems in the body that it does not deem beneficial to survival, such as reproduction. Most of those systems come back online when the threat diminishes, but the new study shows some are damaged permanently if the stress is great enough.

"Acute stress, particularly the fight-or-flight response, has been traditionally viewed to be beneficial for an animal's survival,” said postdoctoral fellow Bing Zhang, the lead author of the study.

“But in this case, acute stress causes permanent depletion of stem cells.”

Researchers hope the findings can help illuminate the broader effects of stress on various organs and tissues and pave the way for drugs that can stop the damaging effects.

"By understanding precisely how stress affects stem cells that regenerate pigment, we've laid the groundwork for understanding how stress affects other tissues and organs in the body," Hsu said.

"Understanding how our tissues change under stress is the first critical step towards eventual treatment that can halt or revert the detrimental impact of stress. We still have a lot to learn in this area."

The new study was published in the journal Nature.

Stress can also 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 grey instead of its original colour.

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