benefits or risks? Study raises questions about heart health

Intermittent fasting has become the diet trend of the decade.

It promises to hack biology without the drudgery of counting calories or cutting carbs: simply change when you eat, not necessarily what you eat. Tech moguls swear by it, Hollywood stars insist it keeps them trim. Britain’s former prime minister Rishi Sunak once spoke of starting his week with a 36-hour fast.

So far the science has seemed supportive. Research suggests that extending the overnight fast may improve metabolism, aid cellular repair and perhaps even prolong life. Nutritionists, however, have long warned that skipping meals is no magic bullet – and may be risky for those with underlying conditions.

Intermittent fasting compresses eating into a short daily window, often eight hours, leaving a 16-hour gap without food. Other time-restricted diets, like the 5:2 plan, limit calories on certain days rather than hours.

Now, the first large-scale study of its kind raises a more serious red flag. Researchers, analysing data from more than 19,000 adults, found that those who confined their eating to less than eight hours a day faced a 135% higher risk of dying from cardiovascular disease – issues with the heart and blood vessel – than people who ate over 12-14 hours.

An elevated cardiovascular risk means that, based on a person’s health, lifestyle and medical data, they are more likely than others in the study to develop heart-related problems such as heart attack or stroke.

The link to overall mortality – deaths from any cause – was weaker and inconsistent, but the cardiovascular risk persisted across age, sex and lifestyle groups even after rigorous testing.

In other words, the study found only a weak and inconsistent link between time-restricted eating and overall deaths. But the risk of dying from cardiovascular disease was sharply higher.

The authors stress that the study doesn’t prove cause and effect. But the signal is striking enough to challenge the narrative of fasting as a risk-free path to better health.

Researchers tracked American adults over eight years. To understand their eating habits, participants were asked on two separate days – about two weeks apart – to recall everything they ate and drank. From these “dietary recalls”, scientists estimated each person’s average eating window and treated it as representative of their long-term routine.

Those who ate within an eight-hour window faced a higher risk of dying from cardiovascular disease than those who spread meals over 12-14 hours, the study found.

They found the elevated cardiovascular risk was consistent across socioeconomic groups, and strongest among smokers and people with diabetes or existing heart disease – suggesting they should be especially cautious about long-term, narrow eating windows. The link held even after adjusting for diet quality, meal and snack frequency, and other lifestyle factors, researchers found.

I asked the researchers how we should read the finding that heart-related deaths go up so dramatically, but overall deaths don’t – is it biology, or bias in the data?

Diet is a major driver of diabetes and heart disease, so an association with higher cardiovascular mortality is not unexpected, said Victor Wenze Zhong, the lead author of the peer-reviewed study in Diabetes & Metabolic Syndrome: Clinical Research and Reviews.

“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,” says Prof Zhong, an epidemiologist at Shanghai Jiao Tong University School of Medicine in China.

That runs counter to the popular belief – supported by short-term studies lasting only a few months to a year – that time-restricted eating improves heart and metabolic health.

In an accompanying editorial in the same journal, Anoop Misra, a leading endocrinologist, weighs the promise and pitfalls of intermittent fasting.

On the upside, he says, multiple trials and analyses suggest it can promote weight loss, improve insulin sensitivity, lower blood pressure and enhance lipid profiles, with some evidence of anti-inflammatory benefits.

It may also help people manage blood sugar without rigid calorie counting, fits easily with cultural or religious fasting practices, and is simple to follow.

“However, the potential downsides include nutrient deficiencies, increased cholesterol, excessive hunger, irritability, headaches and reduced adherence over time,” says Prof Misra.

“For people with diabetes, unmonitored fasting risks dangerous drops in blood sugar and promotes junk food intake during eating window. For older adults or those with chronic conditions, prolonged fasting may worsen frailty or accelerate muscle loss.”

This is not the first time intermittent fasting has faced scrutiny.

A rigorous three-month study, published in JAMA Internal Medicine in 2020, found that participants lost only a small amount of weight, much of which may have come from muscle. Another study indicated that intermittent fasting may produce side effects such as weakness, hunger, dehydration, headaches and difficulty concentrating.

The new study, Prof Misra says, now adds a more troubling caveat – a possible link to higher cardiovascular risk, at least in certain groups.

I asked Prof Zhong what he would advise clinicians and the public to take away from the latest findings.

He said people with heart disease or diabetes should be cautious about adopting an eight-hour eating window. The findings point to the need for “personalised” dietary advice, grounded in health status and evolving evidence.

“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. At least, people may consider not to adopt eight-hour eating window for a long time either for the purpose of preventing cardiovascular disease or for improving longevity.”

Clearly, for now, the message is less about abandoning fasting altogether and more about tailoring it to an individual’s risk profile. Until the evidence is clearer, the safest bet may be to focus less on the clock and more on the plate.