What to Know About Intermittent Fasting

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March 20, 2024Updated 12:43 p.m. ET

You may have seen the headlines: “Intermittent fasting linked to 91 percent increase in risk of death from heart disease”; “The intermittent fasting trend may pose risks to your heart.”

The news came from an abstract presented Monday at an American Heart Association conference in Chicago. The study has not yet been published in a peer-reviewed journal, and experts cautioned that it had many limitations. Here’s what we know.

Intermittent fasting involves cycling between eating and fasting for specific periods of time. A common approach, for example, is to eat only within an eight-hour window each day, said Krista Varady, a professor of nutrition at the University of Illinois Chicago.

Several short-term trials have suggested that this eating style can lead to some weight loss and may lower blood pressure and improve blood sugar control in certain people, she said.

But the longest intermittent fasting trial lasted only one year, said Victor Wenze Zhong, lead author of the new study and an epidemiologist at Shanghai Jiao Tong University School of Medicine in China. His aim, he said, was to look at longer term health.

The new study included more than 20,000 adults from the United States. The participants completed two questionnaires, less than two weeks apart, about what time they ate on the previous day. The researchers then calculated the participants’ average eating windows and assumed it was their typical schedule for the rest of the study, Dr. Zhong said. The participants were followed for an average of eight years.

During that time, the participants who limited their eating to eight hours in a day had a 91 percent greater chance of dying from cardiovascular disease than those who ate over a 12- to 16-hour time frame, the researchers reported.

But there were just 414 people in the eight-hour eating group, Dr. Zhong said. And they tended to be younger and less educated; have lower income and less access to food; and be more likely to smoke than the other participants.

The researchers accounted for these factors in their analysis, Dr. Zhong said. But the study did not show that this style of eating caused deaths from cardiovascular disease, only that the two were linked.

Since the study has not been published or peer-reviewed, it’s challenging to fully evaluate it, Dr. Varady said.

A “major limitation” is that they used just two diet questionnaires to accurately represent people’s typical eating pattern, Dr. Varady said; and the study did not seem to evaluate what kinds of foods people ate.

Dr. Dariush Mozaffarian, a cardiologist and professor of medicine at Tufts University, called the study “very problematic.” The eight-hour eating group may have included many people who were very busy, or faced other challenges that forced them to miss meals or eat erratically, he said.

The group also could have included people who were already in poor health — those with eating disorders or illness that reduced their appetite, for instance, which may have resulted in them eating during a shorter window, said Satchidananda Panda, a professor at the Salk Institute for Biological Studies in San Diego.

And if intermittent fasting is really harmful, it’s not clear why that might be. Dr. Zhong said that his study was not designed to answer that question.

More research is needed to evaluate the long term health effects of intermittent fasting, Dr. Zhong said.

Intermittent fasting isn’t a good fit for everyone, said Dr. Pam Taub, a cardiologist at the University of California, San Diego. But many of her patients have enjoyed its benefits, like reduced cholesterol levels.

Now, her patients are “confused and scared” by the headlines they’re reading, Dr. Taub said. But she won’t recommend that they change anything based on this study, she said, adding that people should always talk with their doctor before shifting their diet or lifestyle.

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