Can intermittent fasting really be bad for your heart?

Fasting for health and psychological benefits is nothing new: Buddhists, Muslims and Christians have done so for centuries. Intermittent fasting (IF) however – cycling between set periods of eating and abstaining – has become wildly popular in recent years. Elon Musk swears by it, as does our very own Prime Minister Rishi Sunak, while Cliff Richard (83 and still performing) has practised it for decades. Now one in 10 Americans are estimated to use this eating pattern, while, in the UK, there are on average 134,000 monthly searches online for IF, making it one of our most popular diet plans.

What exactly is intermittent fasting?

The benefits of restricting your eating to a certain window – commonly six, eight or 10 hours – can lead to better weight management and overall health, claim many fans.

A 10-hour window means limiting your daily eating schedule to 10 hours and fasting for 14 hours. For example, if you eat breakfast at 9am, your final mouthful of the day would be by 7pm. A study by King’s College London last year found 57 per cent of fasters saw their mood improve, and 47 per cent felt less hunger. Other positive outcomes included better sleep and more energy.

So the latest findings may come as a blow to all those following the vastly popular diet pattern. 

Why the concern over fasting now?

Researchers from the Shanghai Jiao Tong University School of Medicine in China surveyed 20,000 Americans over a period of eight years, and found that those who did intermittent fasting – consuming their food within an eight-hour window – had in fact a 91 per cent higher risk of dying from cardiovascular disease (CVD) over an eight-year period, compared to people who ate across 12 to 16 hours. This report was published yesterday.

Could the study be misleading?

Perhaps, according to academics. Before you ditch a diet plan that you feel is working for you, Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics at King’s College London, says that while the study is “interesting” it’s not conclusive.  

“What is not clear is whether those who reported restricted time eating at baseline [the start of the study] continued with this practice throughout the study,” says Prof Sanders. “It is also unclear whether the individuals made a voluntary choice to adopt this pattern. It is likely that those reporting time-restricted eating may be working unsocial hours, such as truck drivers, security personnel, health professionals, night workers.”

There is evidence that shift work, says Prof Sanders, is associated with increased risk of Type 2 diabetes and CVD. The study also doesn’t clearly state whether confounding factors, such as smoking, physical activity, alcohol intake and social class, were taken into account.

Prof Keith Frayn, Emeritus Professor of Human Metabolism at the University of Oxford, highlights other concerns. “We don’t know the body weights of the participants or their medical state. We don’t know, for instance, their blood cholesterol and blood pressure. Or whether those following time-restricted eating might have done so because a physician had told them they need to lose weight because they are at high risk of cardiovascular disease.”

What the heart expert says

“While when to eat is still being researched, we do know that what we eat can make a difference to our risk factors for future cardiovascular disease,” says Dr Sonya Babu-Narayan, associate medical director at the British Heart Foundation (BHF) and consultant cardiologist at Royal Brompton Hospital.

She says that eating a well-balanced diet is something we can all do to improve our health. “Some people already use time-restricted eating to help them reduce their weight, eat healthily or improve sleep and wellbeing, but it’s not for everyone. When it comes to reducing weight there is no one-size-fits-all approach. If you have a heart condition, or are living with other medical conditions, it’s best to seek advice from your GP.

“People who enjoy intermittent fasting shouldn’t stop doing so because of this preliminary data presented at a conference,” Dr Babu-Narayan continues. “For example, we cannot know if the dietary intake recalled by the participants over the two 24-hour periods was typical of their dietary pattern, nor can this type of observational study tell us about cause and effect.”

Regarding cardiovascular risk factors, Prof Sanders suggests it is probably better to spread food intake out throughout the day – aiming for small but often – rather than consume large meals over a shorter period, as the intermittent-fasting method favours. 

“This is because large increases in blood fats and glucose result after big meals,” he says. “These postprandial (happening after a meal) increases in blood fats and glucose can impair endothelial function (inner lining of the blood vessels) and increase the level of blood clotting factors.” 

In other words, these can make the incidence of a heart attack more likely. 

“Prolonged fasting can contribute to raised blood cholesterol because it promotes the release of fatty acids from adipose tissue (your body’s fat stores) that stimulate the synthesis of very low density lipoproteins in the liver that give rise to low density lipoproteins in blood,” he adds. 

Which effectively means that prolonged fasting, says Prof Sanders, can stimulate your body to make bad cholesterol. 

How to fast safely – a dietitian’s tips 

“For some people intermittent fasting can be a useful tool for improving their health because what it does is impose some discipline, so you don’t snack constantly in the evening,” says registered dietitian Marcela Fiuza. “Most of us eat too much and too often.” 

Fiuza advises the following to anyone wanting to try IF:

• Start slowly, with just introducing an overnight fast initially, say 12 hours, so you finish your evening meal at 7 and don’t eat again until breakfast the following morning. See how that feels before you try to narrow the window.
• The quality of the food you ingest is much more important than the timing of it, if you restrict your eating window but eat unhealthy food it wouldn’t be beneficial for your health.
• Aim to eat a wide variety of fruit and veg, plant foods (such as beans and chickpeas), lean proteins (chicken, turkey and fish), and healthy fats (such as olive oil, avocados, seeds and nuts).
• Stay well hydrated – water and herbal teas can and should be freely consumed in your fasting hours.
• Go easy on caffeine, too much will leave you dehydrated and tired
• Alcohol isn’t a strict no-no if you are IF. However there are no health benefits to drinking it, so it’s always safest to limit intake – whether you’re fasting or not.
• Training for a sporting event or have aspirations to be an athlete? Fasting is probably not wise as you’re putting your body under extra stress and it needs a more regular supply of energy (calories) to support you as you exercise. But light exercise, such as walking or yoga, is perfectly possible when restricting your eating hours.
• There are many people who wouldn’t be advised to do IF, including those with diabetes who are insulin dependent and pregnant women are obvious ones. Anyone with a health condition should ask their health practitioner first.

In conclusion… does fasting really work?

The precise biological mechanisms behind IF are still little understood. Previous research has linked it to a reduction in inflammation, fatty compounds in the blood and DNA damage and an improvement in digestive health. Some have claimed it even offers protection against cancer and neurodegenerative disorders.

But Prof Tom Sanders isn’t fully convinced. “Although time-restricted eating has become popular in the media, there is lack of evidence to show it has any benefit in terms of weight loss or weight maintenance,” he says.

In terms of lasting effects, and whether IF is safe to use over decades, Dr Sonya Babu-Narayan is hesitant to answer definitively as more data needs to be collected. “More research is needed to draw firm conclusions about time restricted windows and long-term cardiovascular health,” she says. 

Marcela Fiuza says: “if you’re eating a wide variety of foods, getting enough fresh fruit and vegetables, have enough energy to function in the day and are sleeping well at night then you probably don’t need to adopt a new lifestyle for the sake of it.

“But if you’re a fan of IF and are enjoying benefits then I wouldn’t suggest you stop because of one report which only highlights an association (not cause and effect) between IF and heart disease.”

As often is the case in the diet and health industry, trends come and go, and different eating patterns suit some better than others. The healthiest way of managing your weight is to find a routine that you can put up with in the long term – and to focus on what you eat, instead of when.