Intermittent fasting can be an excellent tool for weight loss, managing blood sugar levels, and improving cardiovascular health. However, a new study published in the journal Cell warns intermittent fasters of one particular disadvantage. “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,” says senior author and stem cell biologist Bing Zhang in a press release. One of those unpleasant side effects includes hair loss. and in the case of hair regrowth, thinner and shorter hair follicles.
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How does intermittent fasting work?
Perhaps we should first provide a quick debrief on intermittent fasting and how it works. Intermittent fasting is a type of diet in which you only eat during certain times of the day. For example, you might restrict yourself to a nine-hour eating window and fast the other 15 hours.
"It involves no pills, no injections, and no hidden costs—it's all about what you eat. Or rather, what you don't eat," explains Michael Mosley in the BBC documentary Eat, Fast, and Live Longer.
As you might have gathered, the key to intermittent fasting is adhering to a strict schedule. "The reason people fail at intermittent fasting is simply falling short of the fasting timing goals. Also, many people think they are fasting because they only 'eat a little bit in the morning.' As I tell my patients, you are either fasting or you aren't. Unfortunately, you cannot be 'nearly' fasting,” Biote chief clinical advisor Cory Rice, DO, previously told Best Life.
Intermittent fasting-related hair loss was first found in mice.
Before exploring the link between intermittent fasting and hair loss in adults, the researchers used a mouse model. Twenty-four shaved adult mice were evenly divided into three groups. One group followed a 16:8 intermittent fasting pattern (eight-hour eating window), while the other did alternate-day fasting (fasting one day, unrestrictive eating the next day). The third group was the control group.
The control group, which had unlimited access to food, saw normal hair regrowth. However, both intermittent fasting groups “exhibited significantly impaired hair follicle regeneration,” said researchers. On day 90, “only partial hair regrowth” was observed, compared with the control group, which regrew all their hair by day 30.
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The science behind it.
“Collectively, these findings indicate that while commonly used intermittent fasting regimens offer metabolic benefits, they exhibit substantial inhibitory effects on hair follicle regeneration,” wrote the researchers. More specifically, they said “the oxidative stress associated with switching from using glucose to fat” is to blame for “inhibited hair growth.”
As explained in a press release, hair follicle stem cells (HFSCs) “go through phases of activity and dormancy, and hair regrowth depends on these cells becoming active. While the control mice’s HFSCs began to become activated around day 20 post-shaving and remained active until their hair had regrown, the intermittent fasting mice’s activated HFSCs underwent apoptosis (programmed cell death) during extended fasting periods.”
The same rules apply to humans who intermittently fast.
Researchers carried out the same experiment with humans and found similar results. Nearly 50 healthy young adults (both men and women) were randomly assigned to one of three groups: the energy restriction diet (consuming 1,200-1,500 kcal/day), 6:18 intermittent fasting (six-hour eating window), and the control group (no restrictions).
In the end, the 6:18 group experienced 18 percent slower hair growth than the control group. “Consistent with our findings in preclinical models, human participants undergoing intermittent fasting showed a significant inhibitory effect on hair growth,” wrote the study authors. Additionally, new hairs were noticeably “shorter and thinner in diameter.”
But more research is needed.
While enlightening, more research is needed to understand the full effect of intermittent fasting and hair loss among adults, especially those who have chronic health issues, including obesity.
“Considering the widespread adoption of intermittent fasting globally, in the future, it will be important to thoroughly assess the effects of various fasting regimens on different stem cell systems. Understanding the complexity of responses across different stem cells and tissues is critical for optimizing these intervention strategies in humans and exploring ways to mitigate any unintended effects on tissue biology while retaining their benefits,” the researchers concluded.
