Does intermittent fasting raise your risk of death from heart disease? That’s what you might think from headlines about early research presented at a recent American Heart Association conference – drawing skepticism from experts and cautions from the researchers themselves.
Here’s what you should know.
The American Heart Association issued a news release headlined: “8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death.”
Media outlets piled on with headlines saying some forms of intermittent fasting – a diet plan where food intake is limited on certain days or in certain hours of the day – “may pose risks to your heart” or “could lead to much higher risk” of death, contradicting research showing time-restricted eating can improve heart health factors such as insulin sensitivity, inflammation, obesity, and cholesterol levels.
Among other study findings, according to the AHA news release:
- Those with heart disease or cancer also saw an increased risk of cardiovascular death.
- Among people with heart disease, eating in a window that’s no less than 8 but less than 10 hours a day was linked to a 66% higher risk of death from heart disease or stroke.
- Fasting did not reduce the risk of death from any cause.
Those conclusions are premature and misleading, says Christopher Gardner, PhD, a professor of medicine at Stanford University and director of nutrition studies at the school’s Prevention Research Center, who commented on an abstract of the study for the AHA news release before study results were presented in Chicago.
Gardner tells WebMD that people in the study group who consumed all their food in a daily window of 8 hours or fewer had a higher percentage of men, African Americans, and smokers, and they had a higher BMI than those who ate over longer time spans – any of which could’ve raised the group’s heart disease risk. Also, investigators lacked data on shift work, stress, and other variables, including the important element of the quality of nutrients in their diets, which alone might have provided another explanation, he says.
As with all experts in this story, including the study’s co-authors, Gardner pointed out this research provides no reason to stop intermittent fasting if you currently see benefits.
Gardner, who isn’t a proponent of intermittent fasting, summarized in an email his thoughts on what he feels is the overstatement of the research:
“This particular finding is PRELIMINARY and should be treated with HEALTHY SKEPTICISM, and should await PEER-REVIEW before it receives any additional media coverage.”
In response to questions about the study and the presentation of findings, the AHA said its intention is always “to promote ideas and supporting research – in context – that stimulate and provoke discovery.”
The abstract, news release, and news article were reviewed by scientific experts, the AHA says, and the release included context and background indicating a link, not causality, and it said readers should always consult their doctors before changing their diet.
“We understand and regret that some news stories did not properly include this important context and did not report on this study for what it is – a single study contributing to the larger body of evidence. We will continue our efforts to educate and counsel journalists in this regard,” the statement says.
Questions remain, says Jason Fung, MD, a nephrologist who has written articles and books on intermittent fasting, including The Obesity Code.
With their headlines, Fung feels, the AHA and media made correlation tantamount to causation, a mistake that would get any first-year medical student a failing grade, he says.
“The whole thing is just outrageous.”
Just because there’s a link between shorter eating windows and bad health outcomes in a particular population doesn’t mean the eating window caused the outcome, Fung says.
For example, he says, research shows you’re more likely to drown if you’ve recently eaten ice cream. It would be easy to conclude that eating ice cream leads to drowning. Yet a closer look shows people eat more ice cream in warmer weather, when they’re more likely to swim and drown. Thus, ice cream correlates with drowning but doesn’t cause drowning.
Another issue, Fung says, is that the study data was taken from a health and nutrition survey done by the CDC between 2003 and 2018, when intermittent fasting was largely unknown as a way to manage health. Most people skipping meals before 2018 weren’t trying to improve their health. They were ignoring what was then standard dietary guidance, he says. It could be that people in this group were more likely to have poor eating habits and diet.
In addition, study authors used just 2 days of self-reported eating activity to estimate 16 years of dietary habits, says Krista Varady, PhD, a kinesiology and nutrition professor at the University of Illinois, Chicago, and co-author of several fasting studies.
“I think the conclusions are extremely overstated,” she says. “Two days of diet record data is NOT at all reflective of an individual’s regular eating pattern – this is a major limitation to the study.”
“The science is very, very sloppy. You expect better,” Fung says.
Study co-author JoAnn Manson, MD, MPH, DrPH, a Harvard University professor of medicine, said in a statement, “Correlation doesn’t prove causation, and we’ll need more research to understand whether the observed associations are cause and effect.”
Randomized clinical trials are necessary to test whether the timing of meals or duration of fasting changes health outcomes. Until those trials, she says, the links “shouldn’t lead to alarm or to changes in one’s preferred and long-term dietary habits.”
Another co-author, Victor Wenze Zhong, PhD, a professor and chair of the Department of Epidemiology and Biostatistics at the Shanghai Jiao Tong University School of Medicine in China, acknowledged that despite controlling for many demographics and health factors, “This is only an observational study that is subject to many limitations.”
The findings do not mean a shorter eating window causes cardiovascular death, he says, but given the lack of long-term data on time-restricted eating, patients should be “extremely cautious” before following the diet for years. Zhong insists in the news release, “Our research clearly shows … a shorter eating duration was not associated with living longer.”
It’s not clear why, Zhong tells WebMD, but those who restricted eating to 8 hours or fewer per day had less lean muscle mass than those with longer eating windows, which “has been linked to higher risk of cardiovascular mortality.”
He, too, calls for randomized clinical trials but notes that a study demanding people stick to eating schedules as investigators follow their progress for years “is challenging to conduct if not impossible.”
“This study unfortunately is not able to well answer the underlying mechanisms driving the observed association between 8-hour (time-restricted eating) and cardiovascular death.”
