Health Talk: Eating in 8-hour window may raise heart risk by 135%, study says

Using data from over 19,000 adults in the US NHANES, researchers studied long-term health outcomes linked to intermittent fasting

While intermittent fasting (IF) and time-restricted eating (TRE) have captured global attention as effective tools for weight loss and metabolic health, a new study has, for the first time, reported an increased risk of heart disease associated with it.

The so-called 16:8 intermittent fasting, where people eat all meals within an eight-hour window and then fast for 16 hours, is hugely popular. (Representative photo)

The study—Association of Eating Duration Less Than 8 Hours With All-Cause, Cardiovascular, and Cancer Mortality—published in Diabetes & Metabolic Syndrome: Clinical Research and Reviews last Friday, found the risk to be as high as 135% for those who consume all meals in less than eight hours.

The so-called 16:8 IF, where people eat all meals within an eight-hour window and then fast for 16 hours, is hugely popular.

Drawing on data from more than 19,000 adults in the US National Health and Nutrition Examination Survey (NHANES), the researchers examined the long-term health outcomes of individuals with varying daily eating durations.

During a median follow-up of 8.1 years, the researchers compared those who consumed meals in a 12 to 14-hour window with those who consumed them within eight hours. They found that the second group was robustly associated with higher cardiovascular mortality, though not with all-cause or cancer mortality.

“…participants who ate for less than 8 hours per day had a 135% higher risk of cardiovascular mortality (death due to heart and blood vessel diseases) compared to those with a typical 12–14-hour eating period. Although the association with overall mortality was weak and inconsistent, the cardiovascular mortality risk persisted across multiple demographic and lifestyle subgroups and in 14 different sensitivity analyses (multiple advanced testing),” the authors in the paper said.

Senior author of the paper Victor Wenze Zhong said in a statement, “Our study provides the first evidence that individuals adhering to a less than 8-hour eating window were more likely to die from cardiovascular disease compared to people who followed a typical eating window of 12–14 hours. Although our observational results are subject to residual confounding, people should be extremely cautious to adopt a short eating window for a long time (e.g., over years) to pursue cardiovascular health or longevity, which has no evidence support from human studies to date.”

The study authors stressed that these findings do not prove causation.

In an accompanying editorial, “Intermittent Fasting: Evidence for Benefit, Lack of Effect, or Potential Cardiometabolic Risk?,” Anoop Misra, chairman of Fortis-C-DOC Centre of Excellence for Diabetes, Metabolic Diseases and Endocrinology, highlighted the pros and cons of intermittent fasting research.

On the benefits side, multiple randomised controlled trials and meta-analyses show that IF can lead to weight loss, improved insulin sensitivity, reduced blood pressure, and better lipid profiles, with some evidence of anti-inflammatory effects. IF may also help certain individuals achieve better blood sugar control without strict calorie counting. It aligns well with cultural or religious fasting practices and is simple to follow.

However, the potential downsides include nutrient deficiencies, excessive hunger, irritability, headaches, and reduced adherence over time. In people with diabetes, unmonitored IF can trigger hypoglycemia (low blood sugar), and in older adults or those with chronic illness, prolonged fasting may worsen frailty or cause muscle loss. The study now adds possible cardiovascular risks—at least in some subgroups—to this list of concerns, Misra said in the article.

“Intermittent fasting is a promising tool in our dietary arsenal—low-cost and simple too—but enthusiasm should be tempered with careful risk assessment. Until more long-term data are available, especially on hard outcomes like cardiovascular events (e.g., heart attacks), intermittent fasting should be individualized and ideally supervised, particularly for people with pre-existing health conditions, and applied for short term only,” Misra added.

Another article in the same journal reinforces a more optimistic narrative, reporting metabolic and weight-related benefits in specific populations. “This juxtaposition illustrates that IF is not a one-size-fits-all solution and that its benefits and risks may vary significantly depending on the population, the specific fasting protocol, and the context in which it is applied,” it said.

More long-term randomised trials are needed to clarify safety and guide tailored recommendations, the authors added.

The results are still emerging, and experts caution that medical supervision is necessary before following any such drastic measures.

“There is emerging evidence to caution people that intermittent or any other type of fasting where you aren’t eating for long hours can also be harmful to your body. It may have some benefits, but there’s also associated harm that one needs to be aware of. There is a need for risk-benefit analysis, and such things anyway shouldn’t be followed without proper medical supervision,” former head of cardiology department in Delhi’s All India Institute of Medical Sciences VK Bahl said.