What makes hair turn gray?

Answer

Hair gets less melanin as pigment cells die.

Context

Hair color comes from melanin, made by pigment cells called melanocytes that sit in the hair follicle and pass pigment to the growing hair. A supply of melanocyte stem cells in a region of the follicle called the bulge replaces these cells every time the hair regrows. Graying happens when that supply fails, so new hair grows in with little or no pigment. Gray hair is not dyed white or gray; it is hair that has lost its melanin.

A 2023 study in Nature, funded by the NIH and led by Mayumi Ito at New York University, tracked fluorescent melanocyte stem cells in mice. These cells normally move between two follicle regions, maturing into pigment-producing cells in one and returning to a stem-cell state in the other. With age, more of the cells became stuck in between and could neither renew nor make pigment. The authors described a lost 'chameleon-like' flexibility, and the work is in mice, so its application to people is still an inference.

Why the stem cells fail is not fully known. Genetics strongly influence the age of onset, and graying often starts in the thirties for people of European ancestry. Oxidative stress in the follicle and damage to cells are among proposed contributors. Separate NIH-funded mouse research indicated that stress responses by the nervous system can deplete these stem cells, though that does not show that everyday stress turns people gray.

Hair that has gone gray is not generally thought to revert to color, though rare, documented exceptions exist.