The Dark Side of Dietary Timing: What Your Fasting Routine Might Be Doing to Your Hair

A new study suggests that while beneficial for metabolism, intermittent fasting may have an unexpected side effect: slower hair growth.

Research published Friday in the journal Cell found that mice and humans following time-restricted eating patterns experienced reduced hair growth compared to those with unrestricted eating schedules.

"We don't want to scare people away from practicing intermittent fasting because it is associated with a lot of beneficial effects — it's just important to be aware that it might have some unintended effects," said Bing Zhang, senior study author and stem cell biologist at Westlake University in China.

The research team discovered that fasting programs significantly delayed hair regrowth in mice. Mice with unlimited access to food regrew most of their hair within 30 days, while those on restricted feeding schedules showed only partial regrowth after 96 days.

Scientists identified the mechanism behind this effect: intermittent fasting causes the body to use stored fat as fuel, releasing free fatty acids that can damage and kill recently activated hair follicle stem cells (HFSCs), which are essential for hair growth.

Researchers conducted a small clinical trial with 49 healthy young adults to verify these findings in humans. Participants who fasted for 18 hours daily over 10 days showed hair regrowth rates 18 percent slower than those who ate without time restrictions.

Zhang noted that these results might vary among different populations. "The human population is very heterogeneous, so the effects might be different for different people," he said. "Mice also have a very high metabolic rate compared with humans, so fasting and metabolic switching have a more severe effect on mouse HFSCs."

The research team found that applying vitamin E, an antioxidant commonly found in hair growth products, helped protect hair follicle stem cells in mice during fasting periods. When applied twice daily to the skin, vitamin E appeared to help the cells survive the fasting process.

Interestingly, the study revealed that skin stem cells remained unaffected by intermittent fasting. Researchers attributed this resistance to these cells' natural ability to neutralize harmful free radicals.

The findings add to growing research on intermittent fasting, a dietary approach that restricts eating to specific time windows. Previous studies have shown that this eating pattern can improve blood sugar control, reduce inflammation, and enhance the function of stem cells in blood, intestinal, and muscle tissue.

However, this is not the first research to identify potential drawbacks to intermittent fasting. A separate major study from this year found that people who limited their eating to less than eight hours per day faced a higher risk of death from cardiovascular disease compared to those who ate within a 12- to 16-hour window.

The research team plans to expand its investigation through collaborations with local hospitals. Their next steps include studying how fasting affects wound healing and the regeneration of other cells and searching for substances that might protect hair growth during fasting periods.