Is an 8-Hour Eating Window Bad for Your Heart?

In March 2024 a headline went around that made a lot of people set down their fasting apps: eating all your food inside an 8-hour window was tied to a 91% higher risk of dying from heart disease. If you fast, that number lodges in your head and stays there. It’s worth walking through what the study actually did, because what it measured, and what it couldn’t, changes the story a lot.

The short version

The 91% figure came from an observational study of US survey data, first shown as a conference abstract in 2024 and published in a peer-reviewed journal in 2025, where the estimate grew to a 135% higher risk of cardiovascular death. Observational data can spot a pattern but can’t prove that a short eating window causes heart deaths. The “eating window” was worked out from two days of food recalls, the short-window group was small, and those people differed from everyone else in ways that raise heart risk on their own. The authors themselves say they can’t yet tell whether the short window or something behind it explains the link. If you have a heart condition, that’s a conversation for your doctor, not a headline.

Where the number came from

The data came from NHANES, a large US health survey. The researchers took about 20,000 adults, average age around 49, used their diet records from 2003 to 2018, and matched them against death records through 2019. Over a median of about eight years, they compared people whose recalls showed eating within less than eight hours a day with people who ate across a more typical 12 to 16 hours. The lead researcher was Victor Wenze Zhong at Shanghai Jiao Tong University, and the American Heart Association’s press release carried the 91% figure from a poster at its March 2024 meeting.

That’s where most of the coverage stopped. The full paper came out the following year in Diabetes & Metabolic Syndrome: Clinical Research & Reviews. With 19,831 adults, a median follow-up of 8.1 years, and 12 to 14 hours as the comparison, an eating duration under eight hours was linked to a hazard ratio of 2.35 for cardiovascular death. There was no link to dying of any cause, or of cancer.

So the claim didn’t vanish under peer review. But the parts that made people skeptical in 2024 are still there.

Why researchers pushed back

It’s observational. Nobody was assigned to fast. The researchers looked at what people were already doing and who died later. That design is good at raising questions and bad at answering them, because the groups differ in a hundred ways besides when they ate.

Two days stood in for eight years. The eating window came from two 24-hour dietary recalls, collected early on, and was treated as a stand-in for how someone ate over the next decade. Christopher Gardner of Stanford, commenting in the AHA release, stressed that the grouping rested “on the basis of just two days of dietary intake.”

The short-window group was tiny and different. In the 2024 analysis only about 414 people, roughly 2% of the sample, ate within eight hours, with 31 cardiovascular deaths among them, per TCTMD’s report. That group was more likely to smoke and had a higher BMI. Donald Lloyd-Jones of Northwestern noted it had three to four times as many Black participants, who carry higher baseline cardiovascular risk in US data for reasons unrelated to meal timing. His summary: “It was what they brought to the table.”

Cause may run the other way. A short eating window often isn’t a wellness choice. It can be a marker of shift work, juggling two jobs, illness or a poor appetite. People who are already unwell sometimes eat less and over a narrower span, which would make a short window look risky when the real driver is the health problem.

To be fair to the published version: it reports that the link held across subgroups split by race and ethnicity, socioeconomic factors and smoking, and through 14 sensitivity analyses. Statistical adjustment can only correct for what was measured, though, and the paper’s own conclusion calls for research into whether the risk comes from “the short eating duration itself or residual confounding.” Zhong said much the same in 2024: “this does not mean that time-restricted eating caused cardiovascular death.”

And the media pile-on bothered people as much as the data. Pam Taub of UC San Diego told TCTMD, “The fact that an abstract at a meeting got this much press is just absurd.”

What the trials show

Flip it around. When researchers have actually assigned people to an eating window, heart-related markers have tended to move the right way, if only slightly.

A small 2020 pilot in Cell Metabolism put 19 adults with metabolic syndrome, most on statins or blood pressure drugs, on a 10-hour window for 12 weeks. Weight, blood pressure and atherogenic lipids came down. It had no control group, so read it as a hint. The follow-up was a proper randomized trial: in Annals of Internal Medicine in 2024, 108 adults with metabolic syndrome added an 8- to 10-hour window to standard care for three months, and HbA1c improved by 0.1 percentage points more than in the control group. That’s modest, and it says little about heart deaths, but the trial reported no major adverse events.

Neither design can settle the 2025 finding. Trials run for weeks or months; the cohort followed people for years. What you can say is that no trial has shown a short window harming the heart, and the observational signal comes with confounding problems nobody has ruled out. That’s the same habit of reading a study for what it shows that we used on the weight-loss evidence for fasting, in both directions.

How much to trust this

Treat it as an open question, not a verdict. A peer-reviewed association with a large hazard ratio deserves attention, and “it’s only observational” isn’t a reason to wave it off. But a short eating window based on two days of recall, in a group of a few hundred people who differ from everyone else, is a weak base for telling anyone how to eat. A long randomized trial would settle it, and none exists.

What to do with this

• If you have heart disease or take medication for blood pressure or blood sugar, don’t start, stop or tighten a fasting pattern because of a headline in either direction. Bring it to your doctor, who knows your numbers.
• You don’t need a tight window to fast. A 10- or 12-hour window is easier to keep, and in a head-to-head trial a stricter window didn’t beat a looser one. If a gentler schedule fits your life, use it.
• Look at the rest of the picture. Smoking, blood pressure, diet quality and activity matter far more to your heart than whether you eat across eight hours or ten.

Atomic Fast lets you set whatever window suits you, and the free timer shows the hours elapsed and the phase you’re in, so a 12-hour fast gets the same attention as a 16-hour one.

Frequently asked questions

Does intermittent fasting cause heart disease? No study has shown that it does. The 2024 headline and the 2025 paper found an association in survey data, not a cause, in a small group that also smoked more and had a higher BMI. Randomized trials, which are short, have shown neutral to slightly positive effects on heart and blood sugar markers.

Was the 91% heart-risk study peer reviewed? Not at first. The 91% figure came from a conference abstract in March 2024. The full analysis was published in a peer-reviewed journal in 2025, reporting a hazard ratio of 2.35 (about 135% higher risk) for cardiovascular death, with the authors noting that residual confounding may explain it.

Is an 8-hour eating window dangerous? There’s no trial evidence that a chosen 8-hour window harms a healthy person’s heart, and the observational link can’t separate the window from the people who happen to eat that way. If you have a heart condition or take medication, check with your doctor before changing how you eat.

Should I stop fasting because of this study? Not on the strength of one observational study. If you want to hedge, a 10- or 12-hour window is a reasonable middle ground. Any real concern about your heart belongs with your doctor.

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Atomic Fast is free on the App Store and Google Play, with a timer that works for whatever window you settle on.

This article is for general education and isn’t medical advice. If you’re pregnant, managing a health condition (especially a heart or cardiovascular one), taking medication, have a history of disordered eating, or are considering fasts longer than 24 hours, talk to a healthcare professional first.