Intermittent fasting, one of the most popular diet trends of the past decade, is facing renewed scrutiny after a large-scale study suggested it may be linked to higher cardiovascular mortality.
The eating plan, embraced by tech entrepreneurs, celebrities and even former UK prime minister Rishi Sunak, involves restricting food intake to a narrow daily window – often eight hours – while fasting for the remaining 16. Other variations, such as the 5:2 diet, reduce calories on specific days rather than hours.
Until now, research has largely highlighted potential benefits, including improved metabolism, enhanced cellular repair, and even longer life expectancy. Nutritionists, however, have consistently warned that intermittent fasting is not a cure-all and may pose risks, especially for people with underlying health conditions.
The new study, published in Diabetes & Metabolic Syndrome: Clinical Research and Reviews, analysed data from more than 19,000 US adults tracked over eight years. Researchers found that participants who confined eating to less than eight hours a day had a 135% higher risk of dying from cardiovascular disease than those who spread meals over 12 to 14 hours.
The association remained significant across age, gender, lifestyle, and socioeconomic groups, and was strongest among smokers and people with diabetes or pre-existing heart conditions. Notably, while the study found only a weak and inconsistent link with overall mortality, the cardiovascular risk persisted even after controlling for diet quality, frequency of meals and snacks, and other lifestyle factors.
Lead author Professor Victor Wenze Zhong, an epidemiologist at Shanghai Jiao Tong University School of Medicine, said the results challenge assumptions drawn from short-term trials. “The unexpected finding is that sticking to a short eating window less than eight hours over years was linked to increased death risk from cardiovascular disease,” he explained.
An accompanying editorial by endocrinologist Professor Anoop Misra weighed both benefits and drawbacks of intermittent fasting. He noted that evidence supports weight loss, improved insulin sensitivity, lower blood pressure and cholesterol, and even anti-inflammatory effects. Yet he cautioned that risks include nutrient deficiencies, heightened cholesterol, hunger, irritability, headaches and reduced long-term adherence.
For people with diabetes, fasting without supervision can trigger dangerous drops in blood sugar, while older adults may face worsening frailty or muscle loss.
Intermittent fasting has been questioned before. A 2020 study in JAMA Internal Medicine found that participants lost minimal weight, much of it from muscle, while others have reported side effects including hunger, dehydration, and difficulty concentrating.
Professor Zhong urged caution, particularly for those with existing heart disease or diabetes, recommending more personalised dietary advice. “Based on the evidence as of now, focusing on what people eat appears to be more important than focusing on the time when they eat,” he said. “At least, people may consider not to adopt an eight-hour eating window for a long time either for the purpose of preventing cardiovascular disease or for improving longevity.”
For now, experts say the safest approach is not to abandon intermittent fasting entirely but to tailor it carefully to individual health risks – and to focus less on the clock and more on what goes on the plate.
Source: BBC
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