Why does hair turn grey?

As we age, our hair follicles steadily lose their ability to produce colour pigment. That means when new hair grows from those follicles, the strands emerge colourless - typically white or grey - so over time the overall look of your hair changes as more follicles stop producing pigment.

Dr Mohamed Mohamed, a hair loss and restoration surgeon and founder of City Clinics, UK, explains that pigment is produced by specialised cells called melanocytes, located in the hair bulb.

“These cells manufacture melanin and transfer it into the growing hair shaft during the active growth phase,” he says. “Over time, we see a gradual decline in both the number and function of these cells.

“More importantly, it is the depletion of melanocyte stem cells - the reservoir responsible for regenerating pigment-producing cells - that drives permanent greying. Once these are significantly reduced, the follicle loses its ability to produce colour altogether.”

Dr Mohamed Mohamed

Oxidative stress occurs when there's an imbalance between antioxidants and free radicals in your body. Antioxidants are compounds that protect your cells from damage, while free radicals are unstable molecules that can harm cells.

When free radicals outnumber antioxidants, this can contribute to health issues such as heart disease, impaired brain function, and premature ageing. Common contributors to oxidative stress include smoking, a poor diet, and long-term stress.

Mohamed says one of the biggest reasons for greying hair is oxidative stress inside the body.

“Hair follicles naturally generate small amounts of hydrogen peroxide,” he explains. “In a healthy system, this is neutralised by enzymes such as catalase. As we age or remain under prolonged stress, these antioxidant systems become weakened. The result is a build-up of hydrogen peroxide within the follicle.

“This has a dual effect. It inhibits the key enzyme needed for pigment production and simultaneously bleaches the hair from the inside out. In simple terms, the follicle begins to lose its ability to both create and preserve colour.”

A diet lacking in nutrients such as vitamin B12, vitamin D, iron, and copper can contribute to premature greying.

Mohamed says these nutrients help cell renewal, enzymes and melanin production - so if you’re low on them, the pigment-making cells can slow down.

“If identified early, correcting these deficiencies may stabilise the process and, in some cases, lead to partial repigmentation,” he says. “Once the underlying pigment cell population has declined significantly, reversal becomes unlikely.”

Mohamed notes that it can be possible to reverse greying at the follicle, but it isn’t the same for everyone and only works in certain cases.

“We occasionally see repigmentation where greying has been triggered by acute stress, nutritional deficiencies, or certain medical conditions,” he says. “Once those factors are addressed, some follicles can resume pigment production. Age-related greying, driven by stem cell depletion, is largely irreversible with current treatments.”

Grey hair is a natural part of life. While some lifestyle habits can make it happen earlier, it’s generally not something to worry about. However, if you notice sudden or early greying in your 20s or 30s, it’s worth checking in with your doctor.

“We typically assess patients for blood markers such as B12, ferritin, Vitamin D, and thyroid function, review medical history and stress load, and check for any underlying inflammatory or autoimmune conditions,” explains Mohamed. “It is important to distinguish between a reversible trigger and natural ageing of the follicle.”

Mohamed concludes by saying there’s no harm in eating well, managing stress, and taking care of your hair - and if that means fewer grey hairs, consider it a nice bonus.

“Greying is, in many cases, a natural and genetically programmed process,” he says. “Ensure your hair remains strong, dense, and healthy, regardless of its colour.”