The wheezy old horror story trope about terror turning someone’s hair white is true! Though, stress can’t bleach hair that’s already grown out. Only peroxide does that. But stress can kill off the cells that give hair color.
There are many ways your hair can lose its color, but Ya-Chieh Hsu, a professor of stem cell and regenerative biology at Harvard, and his team have a study out in Nature that identifies one cellular pathway resulting from stress. In a study on rats:
The team began searching for corresponding changes in the physiological pathways that give rise to coat color. They were particularly interested in the behaviors of two types of cells: differentiated melanocytes, which produce pigment in the hair and skin, and melanocyte stem cells, the raw material from which melanocytes develop. popsci.com
Pushed by a flood of stress-induced noradrenaline, the relevant stem cells completely disappeared from hair follicles, never to return. Without stem cells to replenish the color-creating melanocytes, hairs grew out white.
While this is a nifty discovery, I do cringe a bit at how the rats were stressed. The scientists injected the rats with resiniferatoxin, an analogue of the chili pepper compound capsaicin, which causes pain.
I want to understand my hair as much as the next guy, but do we have to specifically cause pain to do it? I would guess there’s no permanent damage, and any stress inducer is something the rats wouldn’t volunteer to endure, but it makes me queasy.