Is Intermittent Fasting Bad for Your Heart?
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The results are certainly attention-grabbing. But it’s important to note that this is an observational study so it didn’t find that time-restricted eating causes cardiovascular death, but that there is an association.
There are additional caveats that put the results of the study into question, says Christopher D. Gardner, PhD, professor of medicine at Stanford University in California, and a leading nutrition researcher who was not involved in this study.
For instance, Dr. Gardner wonders: What kinds of foods did people in the study eat? Because the analysis hasn’t yet been published in a peer-reviewed medical journal, important details aren’t available yet.
“I find the concept of time-restricted dieting problematic in general because the focus remains on when foods are consumed rather than the quality of what is being consumed,” Gardner says. “As a nutrition scientist, I am more concerned with the quality of what people eat.”
Earlier Research Showed Time-Restricted Eating Can Improve Measures of Heart Health in the Short Term
Time-restricted eating involves consuming calories only during a specific range of hours each day, generally ranging from a 4- to 12-hour time window.
People may follow a 16:8 schedule, fasting for 16 hours a day and getting all their calories in an eight-hour window — from 7 a.m. to 3 p.m., say, or 11 a.m. to 7 p.m.
The New Study Followed 20,000 Adults for a Median of 8 Years
The new study used information collected for years — eight years was the median length of follow-up, with a maximum of 17 years — from approximately 20,000 U.S. adults taking part in the annual 2003–2018 National Health and Nutrition Examination Surveys.
Soon after enrollment, participants completed two questionnaires that asked them to recall what they’d eaten within the previous 24 hours. Researchers placed people into one of five categories according to the length of their eating window, ranging from less than 8 hours to 16 hours.
The scientists then circled back with the subjects for years to track their health.
After controlling for numerous factors, including age, race, and ethnicity, and history of disease, the researchers concluded that:
- People who restricted their eating window to less than eight hours per day had a 91 percent higher risk of death due to cardiovascular disease (mainly heart disease and stroke) compared with people with a window of 12 to 16 hours.
- Among people with existing heart disease, an eating window of 8 to 10 hours per day was associated with a 66 percent higher risk of death from cardiovascular disease.
- Time-restricted eating did not reduce the overall risk of death from any cause.
Study Has Limitations That May Call the Findings Into Question
Nutrition experts asked to comment on the new research point to limitations in the study setup that may call the results into question.
“I think the conclusions are extremely overstated considering that the investigators only have two days of dietary intake data over a 20-year period — what were the subjects eating on the other 7,300 days of the study?” says Krista Varady, PhD, professor of kinesiology and nutrition at the University of Illinois in Chicago. Dr. Varady, who was not involved in the study, has done extensive research on intermittent fasting for weight loss and metabolic disease reduction in people with obesity.
Gardner also finds it problematic to group people into eating window groups based on just two days of data. “Better than one day, but it is still likely that many people in the study were misclassified. Better would have been multiple days in the first year, and then additional diet data from subsequent years,” he says.
Dr. Zhong acknowledges this as an important limitation of his study, but notes that the analysis excluded people who reported an atypical diet on either of the two days.
Factors Such as Stress Level and Access to Food Could Play a Role
There are many factors that could have influenced the risk for heart disease among subjects in the study, though these may be addressed once the full analysis is published, says Gardner.
“For example, what if those eating in a shorter time period had less access to food, worked more work shifts, and experienced more life stress compared with those in the 12 to 16 hour category?” Gardner says. That would mean the findings about cardiovascular deaths could be caused by something other than the smaller eating window.
Researchers Agree: More Studies on Time-Restricted Eating Are Needed
“It’s too early to give a specific recommendation on time-restricted eating based on our study alone,” says Zhong. Still, he says his findings suggest that eating this way for years should be approached with caution, he says.
Rather than following a time-restricted diet, he believes it’s better to follow a diet with proven health benefits, such as the DASH diet and Mediterranean diet. “Based on the evidence as of now, focusing on what people eat appears to be more important than focusing on the time when they eat,” Zhong says.
Gardner shares that opinion. But he does believe that time-restricted eating may benefit some individuals who find it helps them psychologically.
He notes that many people find it hard to make mostly healthy choices in the current food environment in the United States where low-quality foods are inexpensive, highly available, highly palatable, and highly convenient, he says.
“I’m convinced that for some people a time-restricted diet proves helpful — from a behavioral psychology perspective,” says Gardner.
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