Can intermittent fasting worsen heart health? What a new study says (2024)

Experts said while the study’s data are not conclusive, they do flag the risks of intermittent fasting, and such diets should not be followed without consulting a doctor.

What did the study look at?

Intermittent fasting is essentially a time-restricted diet, where the day’s calorie requirements are consumed over a period of eight to twelve hours, with participants fasting for the rest of the day.

Studies that tracked participants on such diets for short periods — three months to a year — have shown benefits such as weight-loss, increased insulin sensitivity, and better control of diabetes. The data presented recently tried to look at the long-term impact of intermittent fasting, following 20,000 adults from a US database for eight to 17 years.

It found that people who ate all of their food in less than 8 hours per day had a 91% higher risk of death due to cardiovascular disease. Among people already living with heart diseases, intermittent fasting increased the risk of death due to heart disease and stroke by 66%.

Why can intermittent fasting be harmful?

Anoop Misra, Chairman of Fortis C-Doc Hospital for Diabetes and Allied Sciences, New Delhi, offered some explanations.

One, people might gorge on calorie-dense foods during their eating window.

“One of the main reasons the diet is popular is because people believe they can eat whatever they like during the eight-hour window. Consuming pizzas and burgers for only eight hours a day is no healthier than eating it throughout the day. Also, we have seen that diets that restrict calories in every meal lead to just as much weight loss,” said Dr Misra.

According to a review of intermittent fasting by Harvard, 12 clinical trials that compared the fasting group with the continuous calorie restriction group found no significant difference in weight loss outcomes.

Two, Dr Misra said intermittent fasting in the long run might lead to a see-saw effect not ideal for a healthy heart. “Every time you consume a meal, there is a spike in sugar levels. People with a flatter line for insulin release have better cardiovascular outcomes than those with a variable line,” he said.

What do experts say about the study?

First, the data were presented at a conference and are yet to be peer-reviewed.

Second, not a lot is known about the methodology followed to reach the conclusions.

Dr VK Bahl, principal director of cardiac sciences at Max Hospitals and former head of the department of cardiology at AIIMS-New Delhi, said: “The methodology is not clear. Plus, it seems, the researchers are depending on a questionnaire to determine dietary patterns. The problem with that is food recall is pretty bad, most people can’t tell you what they ate or when just a few days ago.”

Third, and most importantly, the figures presented don’t take into account if the participant had diabetes, whether they were on drugs for managing it, whether they were on cardio-protective medicines, or whether they were following a good diet and exercise schedule, Dr Misra pointed out.

What do existing studies say about intermittent fasting?

There have been studies over the years that show intermittent fasting helps in burning fat, triggers essential cellular functions that can improve metabolism, improves insulin sensitivity, reduces blood glucose levels, and improves gut health.

A study published last year said that intermittent fasting improves autophagy — a process by which the body removes damaged cell components — helping with conditions such as cancer. Another early animal study pointed to the benefit of intermittent fasting for dementias and Alzheimer’s disease.

On the other hand, a Cochrane review of studies on the diet found that people may lose more weight by intermittent fasting when compared with normal eating over three months, but not when compared with energy restriction diets. The diet also did not lead to more weight loss over longer periods of up to 12 months.

Also, the Harvard review said the dropout rate for those following intermittent fasting was similar to those on calorie-restrictive diets, showing that intermittent fasting was not easier to follow than regular diets.

There are no long-term studies to show the impact of intermittent fasting on ailments like diabetes, hypertension, and cardiovascular diseases.

Who should do it and how?

Experts say intermittent fasting is not for all. Youngsters under the age of 25 and pregnant women, who have a higher calorie requirement, should not follow intermittent fasting. Neither should long-term diabetics dependent on high doses of medication and insulin. Dr Bahl said people with heart disease should not follow it either.

The other essential thing is to stick to a circadian clock — the eating period should not be delayed to later in the day so that people can continue eating till late at night.