Intermittent Fasting Under 8 Hours May Raise Heart Death Risk by 135 Percent: Study Finds

"Eating duration of less than eight hours was linked to 135% higher cardiovascular mortality, but was not associated with all-cause and cancer mortality," the study stated.

The findings challenge the widely held notion that intermittent fasting and extreme time-restricted eating offer uniform health benefits. While such dietary practices have shown short-term advantages for weight loss and metabolic health in earlier trials, their long-term safety remains uncertain.

Senior author of the paper, Victor Wenze Zhong, states.

“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 to support from human studies to date”.

Researchers analyzed data from 19,831 adults enrolled in the National Health and Nutrition Examination Survey (NHANES) between 2003 and 2018, with mortality follow-up until December 2019. Participants reported usual intake based on two valid 24-hour dietary recalls. Eating duration was grouped into categories: less than 8 hours, 8–10 hours, 10–12 hours, 12–14 hours (reference), more than 14–16 hours, and more than 16 hours.

The study led to the following findings:

• After a median follow-up of 8.1 years, an eating duration of less than eight hours was linked to a 135% higher risk of cardiovascular mortality (HR 2.35) compared to a 12–14 hour eating window.
• The association persisted across various subgroups and 14 sensitivity analyses.
• No strong association was observed between eating for less than eight hours and all-cause or cancer-related mortality.
• Signals for all-cause and cancer mortality largely disappeared after adjustments.

The authors emphasized the need for further research to determine whether the elevated cardiovascular risk is directly attributable to shorter eating durations or confounded by related factors such as diet quality, lifestyle behaviors, and underlying health conditions.

Commenting on the findings, Dr. Anoop Misra, Chairman of Fortis-C-DOC and Editor-in-Chief of the journal, stressed caution in adopting extreme fasting patterns. “This study serves as an important safety signal against the routine use of very short eating windows. Although intermittent fasting can provide modest metabolic benefits, an eating window of less than eight hours should not be considered universally safe,” Dr. Misra noted.

He explained possible biological mechanisms behind the association, including increases in LDL cholesterol, sympathetic activation, and elevated cortisol levels, which may collectively worsen cardiovascular risk. Behavioral factors such as skipping breakfast and poor diet quality could further contribute to harm.

According to Dr. Misra, clinicians should recommend a balanced and individualized approach. He suggested avoiding eating windows shorter than 8 hours, especially for those with heart disease, uncontrolled hypertension, diabetes, pregnancy, frailty, or eating disorders. If time-restricted eating is considered, a 10–12-hour window earlier in the day is preferable. Emphasizing diet quality, adequate protein, fiber-rich foods, healthy fats, and avoiding highly processed foods are also crucial. Regular monitoring of blood pressure, lipid profile, glucose, weight, and any new symptoms is advised.

He also cautioned that in regions like India, where cardiovascular risk is already high and fasting is commonly practiced without medical guidance, professional supervision is crucial. Dr. Misra called for randomized controlled trials comparing different eating windows, as well as studies evaluating effects in diverse populations, particularly in South Asia. Until then, time-restricted eating can be considered for weight and metabolic benefits, but only under medical guidance and without extreme restrictions.

"Intermittent fasting is a promising tool in our dietary arsenal and 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.," Dr. Misra concluded.

Reference:Chen, M., Xu, L., Van Horn, L., Manson, J. E., Tucker, K. L., Du, X., Feng, N., Rong, S., & Zhong, V. W. (2025). Association of eating duration less than 8 h with all-cause, cardiovascular, and cancer mortality. Diabetes & Metabolic Syndrome: Clinical Research & Reviews, 103278. https://doi.org/10.1016/j.dsx.2025.103278