Oct. 3, 2024 – Ethan Weiss, MD, was already sold on intermittent fasting by the time he began studying it.
For 7 years, the cardiologist practiced a type of fasting known as time-restricted eating. He ate whatever he wanted each day from noon to 8 p.m., and he fasted the other 16 hours.
But once he started researching the topic, his thinking changed.
The University of California San Francisco researcher and his colleagues published a 12-week study that found no significant difference in weight loss between people who practiced time-restricted eating and those who ate a traditional breakfast, lunch, and dinner.
By then, Weiss was no longer following the diet or encouraging others to use it.
“It worked OK for me at first,” he said. But his family, he added, “was very happy I stopped. I am nicer now.”
Over the past quarter-century as obesity rates rose, intermittent fasting has emerged as one of the most popular diets in the U.S., thanks in part to research touting its extraordinary weight loss and health benefits. Just this week, a study grabbed headlines with the finding that time-restricted eating coupled with nutrition counseling led to greater reductions in body fat and better blood sugar control than nutrition counseling alone in adults with metabolic risk factors like belly fat and high blood pressure. In a recent survey, about 13% of Americans on a diet reported using intermittent fasting.
The appeal is clear. You don’t have to count calories, cut carbs, or eat like someone from a bygone era. In fact, you can eat whatever you want, as long as you consume it within a limited daily window.
“People like it because it’s so easy to incorporate into your lifestyle,” said Krista Varady, PhD, a professor of nutrition at the University of Illinois Chicago who’s studied intermittent fasting since 2005.
Her research shows that people who practice intermittent fasting usually eat less overall. In her 2024 study, people with obesity cut about 200 to 550 calories per day – without counting a single one.
That can lead to some serious weight loss, which often leads to meaningful health improvements.
But other recent research shows mixed results.
One study from August found a heightened risk of cancer cell growth in mice when they were refed after a fast.
Earlier in the year, a poster presentation at a scientific conference reported higher rates of diseases of the heart and blood vessels among people whose eating habits resembled intermittent fasting.
Does this mean fasting diets are dangerous? That’s a hard no, and we’ll explain why in a moment. But despite what enthusiasts have long argued, they may not be uniquely effective for weight loss and disease prevention, either.
Fasting and Weight Loss
Fasting’s weight loss and health-promoting reputation was built on studies of mice.
That’s also where Varady’s research began. But she and her colleagues quickly moved on to studying intermittent fasting in humans.
“Mice are just not good models for fasting,” she said. “They have really, really high metabolic rates. If you fast a mouse for a day, that’s like fasting a human for a week.”
At first, Varady saw intermittent fasting as an improvement over more traditional approaches to dieting. “What I noticed was that people just really hated tracking calories all the time,” she said. “They also didn’t like having to restrict themselves every single day.”
Varady and her colleagues tested the effects of alternate-day fasting – eating about 25% of their normal diet one day and whatever they wanted the next.
“But then we slowly realized people also hate that diet,” she said. That’s when her research pivoted to the 16:8 approach – 16 hours of fasting and 8 hours of eating. "It seemed like a natural progression to study the one people actually do.”
The results of those studies, which by now have included thousands of people, show a consistent pattern: Improvements in health markers and reductions in disease risk are driven by people eating less and losing weight.
“If you don’t lose weight, you don’t see any health benefits,” she said.
Potential Health Risks
So what about those recent reports connecting intermittent fasting to health risks?
The first was a poster presentation at a scientific conference sponsored by the American Heart Association. (In other words, it was not a published study in a peer-reviewed journal.) It looked at self-reported nutrition data from a large sample of American adults and identified a very small group – just 414 out of more than 20,000 – who said they completed all their meals within an 8-hour window.
Compared to people who reported eating windows of 12 to 16 hours, they were 91% more likely to die of heart disease during at least 8 years of follow-up.
No one could say if they accurately reported what and when they ate, whether they were demographically different from other people in the data set, or even if their eating pattern was a deliberate choice.
Despite those limitations, the American Heart Association promoted the presentation’s findings in a news release, leading to extensive news coverage and a scathing response from 34 nutrition scientists, including Varady.
The study linking intermittent fasting to a higher cancer risk was very different.
Scientists at the Massachusetts Institute of Technology found that, in mice, many of the health benefits don’t actually come from fasting– but rather from the refeeding after a fast. When mice ate again after a fast, their bodies activated stem cells to repair and regenerate damaged tissues. That repair process, which includes the removal of damaged or dysfunctional cells, is considered a major benefit of intermittent fasting.
But there is a potential downside: Cancer cells were more likely to multiply during the refeeding period – at least among mice that were genetically engineered to be more cancer-prone.
How does that apply to a human who practices time-restricted eating or alternate-day fasting?
It doesn’t.
“You can find links to almost anything good and anything bad” with any diet, Weiss said. But he has yet to see any evidence that intermittent fasting is harmful.
On the contrary, previous animal experiments have found a lower cancer risk linked to fasting.
And preliminary research in humans suggests improvements in metabolic health and brain function with 5:2 intermittent fasting – eating normally for 5 days, and then restricting yourself to a single meal on the other 2 days.
In that study, people in the fasting group were compared to a matched group who ate what the authors described as a “healthy living diet” 7 days a week for 8 weeks. Both groups achieved the same metabolic benefits, while the 5:2 fasting group did slightly better in tests of thinking skills.
Fasting Myths
One challenge with any diet is separating myth from fact.
With fasting, for example, advocates will often say that going long periods without food was a natural part of human evolutionary history. That’s true, and it’s why our bodies today are metabolically flexible – we can function on just about any kind of food, or with no food at all.
But it doesn’t follow that humans, ancient or modern, routinely chose to go without food, beyond specific religious practices.
Duke University anthropology professor Herman Pontzer, PhD, has spent a lot of time with the Hadza, a traditional hunter-gatherer community in Tanzania.
“My observations don't align with the idea that extended fasts are a normal part of life,” Pontzer said. “It's very rare that there's not something to eat.”
On the other side of that coin is the notion that fasting is so unnatural, it causes harm. Varady is sometimes asked to comment on false narratives.
“The main one that really bothers me, and the one that gets a lot of traction on the internet, is that intermittent fasting messes up people’s reproductive hormones, particularly in women,” she said. “I don’t know how that got started, but there’s literally no research in humans showing any clinically significant changes in any hormone like estrogen.”
It’s part of a bigger challenge faced by nutrition scientists as well as doctors, nutritionists, and other practitioners.
“The only thing people really listen to is random influencers on social media and what they think about intermittent fasting,” Varady said.
Another myth: Fasting gives people license to eat unhealthy foods during their feeding periods.
The idea seems intuitive, especially for more extreme versions of intermittent fasting. For example, if someone has just fasted for 24 hours, or if their feeding window is just 4 hours a day, you’d expect them to race to the drive-thru the minute that window opens.
“Interestingly, they don’t,” Varady said. “People don’t want to switch out all the foods in their pantries. So they eat the same foods, but less of them. We don’t see any increases in sugar or saturated fat intake.”
How to Make Intermittent Fasting Work for You
If you decide to try it, Varady recommends 16:8 intermittent fasting.
“The 8-hour window is a good place to start,” she said. “It results in some of the nicest weight loss we’ve seen, and it’s one of the easiest to incorporate into someone’s lifestyle.”
It also helps to go into it with realistic expectations.
“Intermittent fasting results in mild weight loss,” Varady said – basically what you’d expect from any diet. “It just helps people eat less. It’s not magical.”
