Intermittent is increasingly emerging as a path to better health. Some studies suggest weight loss, better blood sugar control, and even longer life spans as potential perks. But a recent study suggests there may be a trade-off: slower hair growth.

The findings, published on December 13 in the journal Cell, come from researchers at Westlake University in Zhejiang, China. Their experiments in mice revealed that while fasting regimes benefited metabolic health, they also delayed hair regeneration. A smaller clinical trial in humans hints at a similar, though less dramatic, effect.

“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, a stem cell biologist and senior author of the study.

A Surprising Setback for Hair Growth

The team tested two fasting regimes on mice: time-restricted feeding, where mice ate for 8 hours and fasted for 16, and alternate-day fasting, where you fast every other day but eat whatever you want on the non-fasting days. They then shaved the mice and monitored how quickly their hair grew back.

The results were unexpected. When the scientists embarked on this study, their working hypothesis was that fasting would accelerate hair growth but the opposite was observed.

Mice with constant food access regrew most of their hair within 30 days. But the fasting mice took over three times as long — 96 days — and still showed incomplete hair regrowth.

The culprit, Zhang’s team found, was the response of hair follicle stem cells (HFSCs). These cells are essential for hair regeneration, but during fasting, they struggle with oxidative stress — damage caused by unstable molecules called free radicals. The stress stemmed from the body’s switch from burning glucose to fat during fasting. This switch caused free fatty acids to flood the HFSCs, which led to their programmed cell death, or apoptosis.

“During fasting, adipose tissue starts to release free fatty acids, and these fatty acids enter the HFSCs that were recently activated, but these stem cells don’t have the right machinery to use them,” Zhang explained.

Does this apply to humans?

Other types of stem cells appeared to cope better. Epidermal stem cells, which help maintain the skin, were unaffected. These cells have a higher antioxidant capacity, which helps neutralize free radicals.

The researchers tested whether antioxidants could protect HFSCs from the effects of fasting. Topical application of vitamin E or genetic boosts to antioxidant levels helped the HFSCs survive and regenerate hair more effectively.

To explore if these findings apply to humans, the team conducted a clinical trial with 49 young adults. Participants who fasted 18 hours daily showed hair growth rates that were 18% slower compared to those who ate without restrictions. Zhang notes, however, that this study was short and small. “The human population is very heterogeneous, so the effects might be different for different people,” he said.

Because humans have slower metabolisms than mice, fasting’s impact on human hair growth is likely milder. “We see a milder effect in humans—there are still apoptotic stem cells, but many HFSCs survive. So, there is still hair regrowth; it’s just a little bit slower than usual,” Zhang said.

The Bigger Picture

Fasting has long intrigued scientists because of its potential to boost overall health. Research shows it can enhance the resilience of stem cells in the blood, intestines, and muscles. However, Zhang’s study highlights the need to understand how fasting affects other tissues, like skin and hair.

The team plans to explore this further by investigating how fasting influences wound healing and other stem cells in the body. “We also want to figure out how fasting impacts skin wound healing and identify metabolites that could help the survival of HFSCs and promote hair growth during fasting,” Zhang said.

For now, the message isn’t to abandon intermittent fasting. Rather, it’s a reminder that even healthful habits can come with side effects. Slower hair growth might seem like a small price for metabolic health. But for some, knowing the potential trade-offs could shape how they approach their fasting journey.

The findings appeared in the journal Cell.