A new study raises concerns about intermittent fasting and a higher risk of death. Here's why experts aren't worried.

Intermittent fasting has been touted as being good for heart health and longevity, but a new study raises some questions.
Intermittent fasting has been touted as being good for heart health and longevity, but a new study raises some questions. (Getty Images)

People who routinely eat only within an eight-hour window — a form of intermittent fasting — have a 91% higher risk of dying from cardiovascular disease, suggests a new study presented on Monday at an American Heart Association meeting in Chicago. Intermittent fasting has soared in popularity in recent years, thanks to some research linking it not only to weight loss, but also to improvements in blood pressure and blood sugar levels, along with longevity.

The new study calls those touted benefits into question and suggests, for the first time in a major study, that intermittent fasting may be harmful for some in the long term. However, some experts say that people don't necessarily have to pause intermittent fasting until more long-term research is done, pointing out limitations to the new study and the questions it leaves unanswered. Here’s what to know.

What is intermittent fasting and is it healthy?

Intermittent fasting is an alternative to traditional dieting. Rather than cutting out certain foods or restricting calories, people who follow intermittent fasting diets restrict the times that they eat. There are a number of ways to do this, but popular ones include 5:2 fasting, in which people eat whenever and whatever they like for five days a week, but two days a week, they eat only 500 to 600 calories a day instead of the 2,000 calories a day for women and 2,500 for men recommended by the National Institutes of Health (NIH). Another popular style is 16:8 fasting, the type researched in this new study. Following this plan, people fast for 16 hours of the day and eat all of their meals during an eight-hour period.

Research on intermittent fasting has yielded mixed results, and many of the studies were done on animals rather than humans. However, some studies have found it works as well or better for weight loss than calorie restriction, without all the calorie counting. It’s also been linked to everything from a longer life span to clearer thinking. There has also been some evidence that intermittent fasting might improve insulin resistance to help prevent or counteract diabetes. Intermittent fasting has also been linked to improvements in blood pressure, triglycerides and cholesterol, suggesting its benefits for heart health.

Penny Kris-Etherton, registered dietitian, American Heart Association nutrition committee member and emeritus professor of nutritional sciences at Penn State University, tells Yahoo Life that while we don’t know for sure exactly why intermittent fasting seems to have positive effects in the short-term, the principle is not very different from health benefits of other diets: For people who need to lose weight, eating fewer calories and moving more reduces inflammation and improves overall health.

“The important thing to point out is that there are a lot of beneficial effects that have been seen [in research], but we haven’t seen studies that show adverse effects” of intermittent fasting, says Kris-Etherton. At least, not until now. She adds: “This is really a whammy, isn’t it, because it just flies in the face of the benefits that we thought time-restricted eating had.”

What did the new study find?

Scientists at the Shanghai Jiao Tong University of Medicine in China collected information from more than 20,000 American adults whose average age was 49 years old. Study participants were followed for up to 17 years.

Compared to people who ate on a relatively normal schedule — having their meals and snacks over 12 to 16 hours of the day — those who ate during just eight hours of the day were 91% more likely to die of cardiovascular disease. The study also found that, in people who already had cardiovascular disease, fasting for 16 to 14 hours of the day (and eating over an eight- to 10-hour period) was linked to a 66% higher risk of death from heart disease or stroke. Risks of dying of cancer were higher for those who fasted, too.

Is intermittent fasting dangerous?

It's important to point out that this doesn't mean intermittent fasting is a cause of death, study author, Victor Wenze Zhong, chair of the department of epidemiology and biostatistics at the Shanghai Jiao Tong University School of Medicine, noted in a press release.

Still, the associated risks are concerning. There also isn’t much research on the long-term effects of intermittent fasting and time-restricted eating. Short-term fasting can trigger the same fat-burning process as keto diets, which has been linked to health risks when used long term. Some research suggests 24-hour fasting may lead to muscle loss, Courtney Peterson, an intermittent fasting researcher and associate professor of nutrition sciences at the University of Alabama Birmingham, tells Yahoo Life.

But Peterson is among the experts who tell Yahoo Life that this study alone is not enough to say that intermittent fasting is dangerous. "I’m quite skeptical at the moment because it flies in the face of all other epidemiological and other studies that I’m aware of," Peterson says.

She also flagged concerns about the new study. For instance, more than 27% of the people eating time-restricted diets were smokers, compared to only about 18% of the overall study group. The proportion of Black Americans in the time-restricted eating group was also nearly four times greater than in the overall study population, and Black Americans are 30% more likely to die of heart disease than non-Hispanic white Americans. “There are way more smokers in that group and way more Black Americans, and either of those can easily raise the death rates,” regardless of their eating schedules, Peterson says.

Participants only had to self-report what they ate over a two-day period, so it’s hard to say how consistently or for how long they were following an intermittent fasting schedule. “It’s just a randomized snapshot,” says Peterson.

Both Peterson and Kris-Etherton say more research is needed on the long-term impact of intermittent fasting. “I think we’ve got to be careful here,” Kris-Etherton says. “Should we put a hold on intermittent fasting for now? I don’t know, but this is calling it into question and leaving us wondering what to recommend.”

However, Peterson doesn't suggest making significant changes for now. “Let us kick the tires on this study, but I wouldn’t change what anyone’s doing yet,” she says. “That’s the beautiful thing about science: If this is a true result, it should stand the test of time.”

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