People who limit their eating to an eight-hour window may be putting extra strain on their hearts, raising their risk of death related to cardiovascular health.

That’s a new preliminary finding on a particular type of intermittent fasting called time-restricted eating. The research, which has not been published and comes with some caveats, is being presented this week at the American Heart Association’s Epidemiology and Prevention/Lifestyle and Cardiometabolic Scientific Sessions 2024 in Chicago.

In time-restricted eating, people don’t count calories, but rather only eat during a certain number of hours daily, which can range from four to 12 hours. Per the association in a study announcement: “Many people who follow a time-restricted eating diet follow a 16:8 eating schedule, where they eat all their foods in an eight-hour window and fast for the remaining 16 hours each day. Previous research has found that time-restricted eating improves several cardiometabolic health measures, such as blood pressure, blood glucose and cholesterol levels.”

The study, led by researchers from the Shanghai Jiao Tong University School of Medicine in Shanghai, China, is based on findings from examining dietary patterns of participants in the National Health and Nutrition Examinations Surveys, collected from 2003-2018. The research team ran that data against the Centers for Disease Control and Prevention National Death Index database.

Among the findings:

  • Those limiting food intake to less than eight hours a day had a 91% higher risk of death from heart disease.
  • The increased risk was also true for those living with heart disease and cancer.
  • For those who already had known heart disease, eating during an eight- to 10-hour schedule was linked to 66% greater risk of death from heart disease or stroke.
  • They found no drop in deaths from other causes related to time-restricted eating.
  • Those who ate in a window of at least 16 hours a day had less risk of cancer death among those who already were diagnosed with cancer.

“We were surprised to find that people who followed an eight-hour, time-restricted eating schedule were more likely to die from cardiovascular disease. Even though this type of diet has been popular due to its potential short-term benefits, our research clearly shows that, compared with a typical eating time range of 12-16 hours per day, a shorter eating duration was not associated with living longer,” said Victor Wenze Zhong, the senior study author, a professor and chair of the department of epidemiology and biostatistics at the Shanghai Jiao Tong University.

Questioning the findings

But the findings come with caveats and also prompted some pushback. The release on the study said its limitations include relying on people’s self-reports of what they did and didn’t eat. And “factors that may also play a role in health outside of daily duration of eating and cause of death were not included.”

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Matthew Herper, senior medical writer for StatNews, had his own list of concerns, starting with the fact that the study is based on observational research, which is not the most reliable. He notes people are not always honest and “we often misremember.” But the bigger problem, per Herper, “is that the people who choose to be on a diet or those who stay on it might be fundamentally different from those who don’t in ways that we cannot measure.”

Nor is it clear from the abstract, he said, whether researchers did more than look for people “who only ate for a short period of time during the day based on two reports to the survey of what they ate.”

Herper would like to see more research, preferably in randomized, controlled studies.

A 2022 study on intermittent fasting in the New England Journal of Medicine concluded that “among patients with obesity, a regimen of time-restricted eating was not more beneficial with regard to reduction in body weight, body fat or metabolic risk factors than daily calorie restriction.” But it didn’t try to measure the impact on cardiovascular-related death. As The Washington Post article said of that study, “the findings suggest that any benefits of time-restricted eating likely result from eating fewer calories.”

What to eat or when to eat?

Zhong believes the new study shows it’s more important to focus on what one eats than to focus on the time that they eat, he told The Washington Post. But he, too, noted that the findings are suggestive, rather than definitive.

The researchers didn’t explain why intermittent fasting might be hard on the heart. But they did note those folks had less lean muscle mass when they were compared to people who eat in longer windows of time during the day. Per the Post, “That lines up with a previous clinical trial published in JAMA Internal Medicine, which found that people assigned to follow a time-restricted diet for three months lost more muscle than a control group that was not assigned to do intermittent fasting.”

Less muscle mass can lead to falls and also impacts metabolism.

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