Intermittent fasting — a dieting strategy that limits when someone can eat each day — has been linked to a 91% higher risk of heart-related death in a large study. This risk was tied to eating in an eight-hour or shorter window in the 24-hour day, compared with a more typical 12- to 16-hour window.
The new, preliminary research was presented March 18 at the American Heart Association (AHA) EPI Lifestyle Scientific Sessions 2024. It looked at deaths from cardiovascular diseases in more than 20,000 U.S. adults, who were followed for an average of eight years.
Experts told Live Science that the study highlights a need to exercise caution around the use of intermittent fasting. However, one noted that, until all the data from the study is published, it's difficult to say whether the time window or types of food consumed are more relevant to a person's risk of death.
"It's quite possible to eat a really low-quality diet while time-restricted eating," Christopher Gardner, professor of medicine at Stanford University, told Live Science. It may also be that some participants with restricted eating windows were facing food insecurity and not eating well or enough, he added. "We don't know everything yet. I'll wait for more," he said of the new research.
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'There is so much that isn't known'
Intermittent fasting involves eating only during a specific window of time each day, often between four and 12 hours out of 24. Previous research suggests that intermittent fasting improves metrics tied to cardiovascular health in the short term, over a few months, including measures of insulin resistance.
Of the 20,000 people in the new study, 414 reported eating in time windows of eight hours or less each day. Participants were not assigned diets to follow, but rather, their typical diets were assessed through two surveys, in which participants recalled everything they'd eaten in two 24-hour time frames.
The study uncovered a link between eight-hour eating windows and cardiovascular death, but due to its design, it cannot say whether this eating schedule caused the deaths — it shows only a correlation.
"We were surprised to find that compared to people whose eating duration was 12-16 hours … those who restricted their eating time within 8 hours per day had higher risk of cardiovascular death and did not live longer," lead study author Victor Wenze Zhong, professor of epidemiology and biostatistics at the Shanghai Jiao Tong University School of Medicine, told Live Science in an email.
Restricting eating to this short time window was tied to a higher cardiovascular death risk in the overall group and in people with either cancer or heart disease, who were singled out in separate analyses. In addition, participants with existing cardiovascular disease who ate during an eight- to 10-hour window also had a 66% higher risk of dying from heart disease or stroke, compared to those with longer eating windows.
Dr. Wendy Bennett, an associate professor of medicine at Johns Hopkins University School of Medicine, noted that this study suggests a need to scrutinize the "fad diet push that people should be doing time-restricted eating." Bennett and others published research in January 2023 that also raised doubts about the purported benefits of intermittent fasting, suggesting that it's not a successful weight-loss strategy in a six-year time window.
Zhong agreed that the new study calls for more caution regarding intermittent fasting, particularly for people with heart conditions or cancer. Overall, though, "it's too early to give a specific recommendation on [time-restricted eating] based on our study alone." He highlighted the need to explore exactly how eight-hour intermittent fasting might affect heart health and to assess additional populations around the world.
The study does have several limitations. For instance, participants provided dietary data through self-reporting, and some may have incorrectly recalled what they'd eaten. Only two diet surveys were used, so it's unknown whether these accurately represented people's long-term eating habits. The study also didn't consider the nutrient quality of participants' diets or reasons they practiced intermittent fasting.
"It looks tantalizing, but there is so much that isn't known," Gardner said, adding that there is a lot of interest in the study's full results.
Setting the new research aside, intermittent fasting can have other negative consequences, Cynthia Bulik, a psychiatry professor who studies eating disorders at the University of North Carolina School of Medicine, told Live Science in an email. For example, despite it being "hailed as the next great strategy for weight control," intermittent fasting can affect a person's familial and social relationships when their eating schedule doesn't align with that of their loved ones, she noted.
"In addition, for individuals who are genetically predisposed to some eating disorders, prolonged fasting periods could theoretically flip them into negative energy balance (expending more energy than they are consuming)," she said, "and trigger the onset of an eating disorder."
This article is for informational purposes only and is not meant to offer medical advice.
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