“It Just Goes Against Everything Else That We’re Seeing:” Is Intermittent Fasting Linked to Bad Heart Health?

Intermittent fasting — where you only eat during time-restricted periods of the day — has been a popular fad diet for at least the past decade. As its popularity continues to grow, researchers have been studying the diet’s effects on our health and numerous research studies have found correlations linking this type of eating to better health. But new findings, presented on March 18 at the American Heart Association’s Scientific Sessions in Chicago, seem to challenge time-restricted eating’s health halo. However, at least one nutrition expert says to take this study with a large grain of salt.

What do we know about intermittent fasting?

A body of research depicts this method’s benefits when it comes to managing weight, blood pressure, and blood sugar in people. One randomized clinical trial whose results were published in the journal JAMA Internal Medicine in 2022 demonstrated that adults who ate only between 7 a.m. and 3 p.m. for 14 weeks effectively helped them lose weight and improve their blood pressure compared to adults who could eat all throughout the day. A non-randomized trial in obese women from 2021 published in the Journal of Translational Medicine found that this practice reduced weight, percentage body fat, and risk of cardiovascular disease.

What did the new intermittent fasting study reveal?

Researchers from Shanghai Jiao Tong University School of Medicine and School of Public Health as well as Wuhan University, Northwestern, Harvard, and UMass Lowell analyzed data on more than 20,000 people collected in the CDC’s National Health and Nutrition Examination Survey between 2003 and 2018. Follow-ups with participants between 4.2 and 11.8 years later showed 2,797 all-cause deaths, including 840 cardiovascular deaths. After adjusting for variables such as age, sex, and race, the researchers found an association between an 8-hour eating window and a 91 percent increased risk of cardiovascular death in the long term.

Alarming as this verdict seems, there’s reason to probe their data. The information analyzed came from participants who “completed two valid 24-hour dietary recalls.” In other words, dietary information on each participant came from just two days of self-reported eating patterns.

Drawing conclusions from this dietary data alone is questionable, says Courtney Peterson, associate professor of nutrition sciences at the University of Alabama at Birmingham who was not involved in the research. She says that this method means the researchers deduced, from just two days, how participants typically ate.

“They didn’t ask participants if they had a lifelong practice,” she says. “It's not super accurate to take two random days of someone's life and extrapolate out” their dietary habits. For example, participants may not practice intentional time-restricted eating but rather skip breakfast, which some studies show is associated with worse health. In that case, it’s not that time-restricted eating is associated with death, but ditching a nutritious breakfast.

Peterson also points out that time-restricted eating isn’t always a conscious choice. For example, eating for fewer hours might be a consequence of poor health rather than a cause. It could be the case that some of these participants were already in declining health, eating less as their condition worsened.

Peterson calls this type of conclusion “reverse causation,” in which researchers conclude that one factor may cause a result when it’s actually the opposite. In this case, Peterson says the researchers may have found that time-restricted eating causes cardiovascular death when really, poor health makes it more likely someone will only eat during restricted time windows.

In all, Peterson urges readers to regard this finding with a grain of salt. “There are probably other factors here causing the association,” she says. “It just goes against everything else that we’re seeing.”