What People Get Wrong About Intermittent Fasting

When Krista Varady, a professor of nutrition at the University of Illinois, Chicago, began studying intermittent fasting 20 years ago, she couldn’t wait for this way of eating to become mainstream.

Then in 2012, Varady got her wish. The popularity of intermittent fasting—a way of eating that involves switching between fasting and eating on a regular schedule—exploded with Michael Mosley’s television documentary Eat Fast, Live Longer, his book The Fast Diet, and Kate Harrison’s book that same year, The 5:2 Diet.

After the initial wave of excitement and sometimes overblown claims that intermittent fasting can reduce inflammation, boost immunity, and prevent chronic disease, the backlash began. In the last few years, professionals in the health and fitness industry began to sound the alarm, saying women should avoid intermittent fasting because it causes fertility issues, disrupts menstrual cycles, and interferes with sleep.

But Varady says the science doesn’t support these claims.

“We’ve done about 30 clinical trials in intermittent fasting, and about 85 percent of our samples are women,” she says. “This diet either has no effect on reproductive hormones, or it actually benefits people with polycystic ovarian syndrome (PCOS).”

While intermittent fasting isn’t the magic bullet for perfect health, as many claimed in the beginning, it also doesn’t appear to damage women’s reproductive hormones, as long as you consume enough calories and maintain a healthy body fat percentage in the process.

A Look at the Science

Much of the concern about intermittent fasting’s effect on women’s fertility stems from a 2013 study published in PLoS One on three- to four-month-old rats. Intermittent fasting did negatively affect the reproductive health of these young rats, but the same result has not been replicated in adult human women. Additionally, a three-month-old rat is equivalent to a nine-year-old human, an age group that shouldn’t be fasting. Doctors recommend against fasting for children and teens because they experience periods of rapid growth and could potentially develop disordered eating habits.

A 2022 comprehensive review of human trials published in Nutrients found that intermittent fasting lowers androgen levels and increases sex hormone-binding globulin (SHGB) in premenopausal women with obesity, both of which aid fertility. A 2022 study in the journal Obesity on premenopausal and postmenopausal women found that following a time-restricted eating plan resulted in mild weight loss and had little effect on participants’ hormones.

That study did find that dehydroepiandrosterone (DHEA), a hormone produced in the adrenal glands, decreased about 15 percent in each group. Low levels of DHEA can lead to a lower cancer risk in premenopausal women, but it is also connected to higher rates of vaginal dryness and diminished skin tone. Varady says those levels aren’t clinically significant, and the participants didn’t report any adverse effects, but she says it’s something her team will keep in mind for future studies.

“We’ve studied thousands of people of childbearing age, including women who are doing CrossFit and resistance training,” Varady says. “They do show slight decreases in testosterone but no changes in muscle mass or training capacity.”

James Nodler, MD, a reproductive endocrinologist at CCRM Fertility, says he thinks intermittent fasting is a fine option for women who are experiencing fertility issues as a result of obesity. However, in general, he does not recommend it for people trying to conceive.“Studies in humans have shown that intermittent fasting is not effective for long-term weight loss,” he says.

The current research suggests that this diet doesn’t have a negative effect on women’s hormones. However, more studies need to be done in order to confirm this. Researchers agree the lack of studies conducted on intermittent fasting make it difficult to draw any solid conclusions.

Part of this murkiness lies in the problems with nutrition science as a whole that often leads to conflicting or inconclusive results. Unlike other fields of research, nutrition science relies on observational studies that aren’t controlled in a lab setting and use self-reported food surveys, which are unreliable. Additionally, people often react very differently to the same diet, making it difficult to reach consistent conclusions.

The other problem with much of the discussion around intermittent fasting is many conflate it with severe calorie restriction, since the diet doesn’t offer guidelines about how much, or what, to eat.

Who Should Avoid Intermittent Fasting

Carrie Forrest, a blogger and cookbook author who has a master’s in public health, is always on the lookout for ways to improve her health through nutrition. When intermittent fasting first became popular around 2012, Forrest, then in her late 30s, tried it out. She followed the Fast Five diet, which has a five-hour eating window. Even though her hunger woke her up in the middle of the night, Forrest, who struggled with disordered eating as a child, kept following the diet, restricting herself more and more.

“I got really scared of food, and then the fasting just seemed to reinforce that,” she says. “I thought that was the healthiest way to do it because I kept hearing how healthy fasting was,” Forrest says.

After a few years of intermittent fasting, Forrest began showing signs of under-eating. She developed peach fuzz on her back, a sign her body was undernourished and struggling to keep warm, and lost her period.

Her experience underscores that intermittent fasting is not for everyone. A 2022 review published in Nature explains that intermittent fasting (and calorie restriction in general) is not recommended for people with a BMI below 18.5, those with a history of eating disorders, children, adolescents, women who are pregnant or lactating, or individuals over the age of 70. Women who don’t eat enough calories during their eating window or whose body fat drops below 16 percent can experience fertility and hormone issues.

Many people find intermittent fasting an easy eating plan to follow because it doesn’t require special foods or calorie counting, but it’s not for everyone. As always, the key is listening to your body’s cues and following a balanced nutritional approach that works best for you.

Hilary Achauer is a health, fitness, and wellness writer based in San Diego. Her work has appeared in The New York Times, The Washington Post, Slate, and Eating Well, among other publications. She writes marketing content for health and wellness companies and nonprofits and spends her free time surfing, doing CrossFit, and working her way through the massive pile of novels on her nightstand.