Intermittent Fasting: Beware of Hair Loss

Data from mice and humans [1] show that a popular weight - loss method can inhibit hair growth. The study found that intermittent fasting, which requires temporary food deprivation, induces a stress response that can inhibit or kill hair follicle stem cells, the cells responsible for hair growth.

This newly published research points out that while short - term fasting is beneficial to health, such as extending the lifespan of mice, not all tissues and cell types can benefit.

"I was shocked when I saw the study results," said Ömer Yilmaz, a stem - cell biologist at the Massachusetts Institute of Technology who was not involved in the study. "We've gradually come to believe that fasting is beneficial to most, if not all, cells and is good for stem cells. This result goes against our expectations, and it seems to be the case in humans too."

"Precision" Fasting

In the past decade, intermittent fasting has become a popular diet. In 2023, about 12% of adults in the United States tried intermittent fasting. A common approach is "time - restricted eating," where people eat only within a limited time frame each day.

Stem cells seem to be particularly vulnerable to dietary changes. Yilmaz and his colleagues reported in August 2024 [2] that stem cells in the mouse gut suddenly become active during the binge - eating phase after a fasting period. This activity helps repair gut damage in mice.

To find out if dieting affects hair regeneration, which is known to be influenced by stress, Bing Zhang, a regenerative biologist at Westlake University in China, and his colleagues shaved mice and then put them on one of two intermittent fasting regimens: time - restricted eating or alternate - day fasting. Alternate - day fasting requires animals to fast for 24 hours and then eat normally for the next 24 hours. The authors found that by the end of the three - month study, the mice on the diet regimens grew less hair back compared with a control group that consumed a similar number of calories.

The research team found that the root cause of this effect lies in the death of hair follicle stem cells. Further experiments showed that intermittent fasting activates communication between the adrenal glands (organs that produce stress hormones) and fat cells in the skin. This "crosstalk" causes skin fat cells to release molecules called fatty acids. These fatty acids interfere with the metabolism of stem cells and cause damage, which in some cases can lead to stem - cell death. Using antioxidants before and during the diet can prevent the inhibition of hair growth, indicating that this effect is not inevitable.

To see if this effect extends to humans, the research team randomly assigned 49 people to an intermittent - fasting group or a control group with no restrictions on eating times. The hair - growth rate of the fasting group was 18% slower than that of the control group. Molecular tests suggest that the mechanism causing this effect may be similar to what was observed in mice.

"Interestingly," this effect doesn't extend to the stem cells responsible for generating and repairing the skin's surface but does extend to the cells that form a key immune barrier, said Elaine Fuchs, a stem - cell researcher at Rockefeller University in New York who was not involved in the study. "At least some of us agree that unless you're a musk ox living in the Alaskan tundra, hair is dispensable," Fuchs said. "During intermittent fasting, the body doesn't know when the next meal will be, so it focuses on protecting vital organs by sacrificing non - essential ones."

Yilmaz and his colleagues also discovered a drawback of intermittent fasting: If genetic changes that can lead to cancer occur in mice during the post - fasting phase, this diet may promote the development of precancerous intestinal hyperplasia [2]. Based on this result, future research should further explore the effects of fasting on brain cells and liver cells, Yilmaz said.