Intermittent fasting could double risk of cardiovascular death, study shows

In recent years, intermittent fasting has been promoted as a great way to lose weight and boost lifespan. However, a recent study is warning that the diet may be dangerous for heart health. So, what’s the full story? Read on to find out!

Intermittent fasting is a general term that refers to restricting eating over periods of time like days or weeks. However, most of the scientific buzz has been focused on limiting feeding windows each day. For instance, that could look like eating just eight hours per day (like noon until 8 p.m.) and fasting for the other 16. This is what the study in question looked at.

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Indeed, people who limit their eating to fewer than eight hours a day are 91% more likely to die from cardiovascular disease compared to people who eat 12-16 hours per day, according to the groundbreaking study presented to the American Heart Association in March 2024.

Like many big, nutrition-centered studies, this one followed 20,000 adults in the US for an average length of eight years to test the hypothesis that restricting feeding times to eight hours or less a day is associated with a lower risk of mortality. The study was conducted by researchers from Shanghai Jiao Tong University, Northwestern University, Harvard University, University of Massachusetts Lowell and Wuhan University.

“Restricting daily eating time to a short period, such as 8 hours per day, has gained popularity in recent years as a way to lose weight and improve heart health,” said senior study author Victor Wenze Zhong, Ph.D. in the study's press release. “However, the long-term health effects of time-restricted eating, including risk of death from any cause or cardiovascular disease, are unknown.”

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Shorter-term studies have shown that intermittent fasting can improve health indicators related to heart health and longevity, which caused Zhong to say he was “surprised” to see it linked to a spike in cardiovascular deaths. “It’s crucial for patients, particularly those with existing heart conditions or cancer, to be aware of the association between an 8-hour eating window and increased risk of cardiovascular death.”

“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,” continued Zhong. Indeed, he found it was linked with increased mortality.

This study has made major waves in the nutrition community and hit headlines in major media outlets like the Washington Post and New York Times. However, it has also come under intense scrutiny.

One important fact to note is that there were just 414 in the eight-or-less-hour eating group. Zhong said, according to the New York Times, that although those people were also generally younger, less educated, more likely to smoke, and with less access to food — all of those factors were controlled for.

The study’s press release, abstract, and a poster with its findings were released on the American Heart Association website, which is a reputable place. However, experts still haven’t had their chance to analyze the details of the study to be able to really judge it adequately.

Another gripe with the study is that researchers relied on two diet questionnaires to capture when those in the study were eating. While the details are not available yet, it seems it also did not even look at what foods participants were eating.

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Correlation is not causation. Cardiologist Dr. Dariush Mozaffarian told the New York Times that the study was “very problematic” because many of the people in the eight-hour eating group could have restricted feeding windows due to being very busy or having hectic lives. Other experts suggested it may have more people with disordered eating, illnesses that reduce appetite or food insecurity.

According to the International Food Information Council (IFIC), intermittent fasting is more popular than a ketogenic or high-fat diet. The IFIC survey showed that about 10% of Americans aged between 18-80 use intermittent fasting as a diet.

Previously, some of the shorter-term studies around limiting eating around certain hours have been positive. These studies have gotten some experts and influencers alike really excited about focusing not as much on what you eat, as much as when.

A study looking at all the past research, published in the New England Journal of Medicine, found that eating in a six-hour period increased stress resistance, increased longevity, and a decreased incidence of diseases, including cancer and obesity. Another 2019 study found an average weight loss of 2.5% to 9.9% of body weight.

Another study by a team of researchers at King's College London made more bold claims about its health benefits, saying the diet improves hunger, energy levels and boosts mood. That tightly controlled, but short term study, had 37,000 people log information about their eating habits, mood, energy and. The research was presented in Nov. 2023.

Despite some studies, intermittent fasting is notoriously tricky to nail down, as diets within the eating window can vary widely. However, one randomized clinical trial — the gold standard of research — looked at 116 overweight people and found no significant difference in weight loss between people who restricted eating from 8 p.m. and noon the next day and those who did not. Other good studies also began to doubt any extra weight loss from solely restricting times of eating.

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What’s clear is that more research is needed. Cardiologist Dr. Pam Taub told the New York Times that many of her patients have seen benefits from the eating strategy like reduced cholesterol. She said that while she will not recommend her patients change anything based on this study, she said these diets aren’t right for everyone and should be discussed with your doctor.