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Can intermittent fasting have negative effects? Study finds higher risk of cardiovascular death

A new study finds that intermittent fasting made increase risk of cardiovascular disease when practiced over several years.
A new study finds one major drawback for intermittent fasting: a possible increased risk of death when practiced over several years. But experts say to interpret the findings with caution.
A new study finds one major drawback for intermittent fasting: a possible increased risk of death when practiced over several years. But experts say to interpret the findings with caution.Getty Images
/ Source: TODAY

Restricting eating to eight or fewer hours a day, a type of intermittent fasting, may raise the risk of dying prematurely from heart disease over the long term, a new study suggests.

An analysis of data from more than 20,000 adults revealed that over several years participants who consumed all food in eight hours or less a day were almost twice as likely to die from cardiovascular disease compared to those who consumed food over the standard 12 to 16 hours a day, according to the report presented on Monday, March 18, at an American Heart Association conference in Chicago.

Time-restricted eating, aka intermittent fasting, has become increasingly popular, with mounting evidence from human and animal studies that associate the eating pattern with better health. Specifically, studies have associated intermittent fasting with weight loss and improved regulation of blood sugar along with decreased blood pressure, cholesterol, resting heart rate and inflammation.

There are a few different types of intermittent fasting with 16:8 — when you fast for 16 hours and eat over the course of eight — being one of the most popular. Another type is 5:2, when you eat only 500 calories two days a week and follow a normal diet the other days.

So what does the new study mean if you practice intermittent fasting or are interested in trying it out? Here's what to know.

What did the study find?

To take a closer look at the long term health impacts of time-restricted eating, the researchers turned to data from the National Health and Nutrition Examination Survey (NHANES) and linked it with information from the National Death Index.

The researchers focused on NHANES participants who were at least 20 years old at enrollment. Between 2003 and 2018, the 20,078 men and women twice filled out a questionnaire that asked about what food they consumed and when during the previous 24 hours. They were followed for a median of eight years. During the course of the study, there were 2,797 deaths overall, 840 of which were due to heart disease and 643 to cancer.

When the researchers analyzed the data, they found that people who consumed all their food in eight hours or less a day were 91% more likely to die from heart disease than those who spread their eating out over 12 to 16 hours.

The findings — which are not peer-reviewed or published in an academic journal — came as a surprise to the researchers.

“We had expected that long-term adoption of eight-hour restricted eating would be associated with lower risk of cardiovascular death and even all-cause death, because short-term randomized controlled trials, generally conducted within a one-month to one-year period, have reported that time-restricted eating reduces weight and improves cardiometabolic health,” senior author of the study Victor Wenze Zhong, Ph.D., professor and chair of epidemiology and biostatistics at Shanghai Jiao Tong University School of Medicine, tells TODAY.com via email.

He says possible explanation for the finding is that the intermittent-fasting group had less lean muscle mass than the other, and low lean muscle mass has been associated with a higher risk of cardiovascular death in previous research.

But Zhong stresses it's "too early to give a specific recommendation on time-restricted eating based on our study alone."

In fact, “practicing intermittent fasting for a short period, such as three months or six months, may likely lead to benefits on reducing weight and improving cardiometabolic health based on what we know so far," he says.

So, in light of the new research, he simply recommends caution for people who've been doing 16:8 intermittent fasting for years. 

What do experts say?

TODAY.com nutrition editor and registered dietitian Natalie Rizzo says the results seem to align with how intermittent fast can affect your cravings.

"Since intermittent fasting is based on eating in a certain window of time, there is no limit to the types of foods you can eat," Rizzo explains. "Restricting eating to eight hours of the day can cause extreme hunger, which may result in binging on food or making unhealthy food choices. Plus, restricting food for 16 hours per day can increase cravings, which can also lead to unhealthy choices."

Mark Mattson, Ph.D, a leading researcher on intermittent fasting and adjunct professor of neuroscience at Johns Hopkins University School of Medicine, says the study has "a number of problems."

First, the findings are based on just two days of dietary recall, Mattson, author of “The Intermittent Fasting Revolution: The Science of Optimizing Health and Enhancing Performance,” tells TODAY.com. Moreover, the researchers don’t appear to have accounted for the number of calories participants consumed. It’s possible the people who ate only during eight hours were consuming high-calorie, less nutritious foods.

“If they binge eat during those eight hours, it’s not going to be good for their cardiovascular system,” agrees Dr. Deepak Bhatt, director of the Mount Sinai Fuster Heart Hospital in New York City.

But most importantly, the study doesn’t prove that the greater risk of death was due to people cramming all their eating into eight hours or less, Mattson explains. It’s possible that the 8-hour fasting group had something in common besides their pattern of eating.

While most clinical trials designed to look at the impact of restricted eating have lasted only several months to a year, “the changes in health indicators — insulin resistance, belly fat, blood lipid (fat) profiles, for example — are consistently going in the right direction,” Mattson says. 

Also, the data for the new study were collected at a time when many people weren’t purposely practicing restricted eating, says Dr. Sean P. Heffron, a preventive cardiologist and director of fitness-focused cardiology at the Center for the Prevention of Cardiovascular Disease at NYU Langone Heart. So, we don’t know why they were eating all their food in eight hours or less, Heffron tells TODAY.com.

“There is nothing here that would suggest I make any clinical recommendations one way or the other,” he says.

The new results are “surprising,” adds Bhatt. But the biggest problem with the study, he says, is that it depends upon dietary recall, “which is imperfect. ... People often don’t recall what they had for breakfast. For this sort of research, you need detailed dietary information gathered in real time.”