What is intermittent fasting? Study finds it raises cardiovascular risk

The discipline of fasting has long been advocated in several religions and cultures around the world, practised by millions to stay in touch with their spirituality. But it has also gained further popularity over the last decade as a means of losing weight, dieting and keeping fit.

However, a new study warns that the popular diet may be linked to an increased risk of cardiovascular problems, some of which could be fatal.

In one of the first studies of its kind, researchers analysed the long-term outcomes of intermittent fasting by tracking 20,000 adults in the US. They found that those who only ate within an eight-hour window everyday and fasted the rest of the time were at nearly double the risk of dying from heart attacks or strokes.

This is compared to adults who said they spread their meals and snacks across 12 to 16 hours each day.

While the researchers acknowledged that their study had limitations - including limited information on the participants' eating habits and the assumption that volunteers followed the same schedule for a number of years - they were "surprised" by the results.

Victor Wenze Zhong, senior author of the study from the Shanghai Jiao Tong University School of Medicine, was quoted by The Times as saying: "We had expected that long-term adaptaion of eight-hour time-restricted eating would be associated with lower risk of cardiovascular death.

"We were surprised to find that compared to people whose eating duration was 12 to 16 hours, those who restricted their eating time to within eight hours per day had higher risk of cardiovascular death and did not live longer."

Experts have called for further follow-up studies to examine the effects of intermittent fasting in the long term.

The benefits of intermittent fasting have long been touted by dieting and weight loss experts, and a number of well-known figures are also fans. Recently, it was reported that prime minister Rishi Sunak fasts for a full 36 hours at the start of each week.

Details of Sunak's fasting regime came as a recent study suggested that regularly fasting for 24 hours between meals has the potential to reduce the risk of developing diseases like diabetes and Alzheimer’s.

University of Cambridge researchers examined how fasting can help reduce inflammation, which is linked to diseases like obesity, type 2 diabetes and heart disease. A high-calorie Western diet has also been shown to increase the risk of developing such illnesses, they said.

The team, led by Professor Clare Bryant from Cambridge’s Department of Medicine, studied blood samples from volunteers who ate a 500-calorie meal, then fasted for 24 hours before consuming another 500-calorie meal.

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They found that restricting calorie intake increased levels of a lipid known as arachidonic acid. Lipids are molecules that store energy and transmit information between cells, and arachidonic acid can help decrease inflammation, particularly an inflammasome called NLRP3.

Prof Bryant said that the NLRP3 inflammasome is "very important in a number of major diseases such as obesity and atherosclerosis, but also in diseases like Alzheimer’s and Parkinson’s disease, many of the diseases of older age people, particularly in the Western world".

She added that the study’s findings provide a "potential explanation for how changing our diet – in particular by fasting – protects us from inflammation, especially the damaging form that underpins many diseases related to a Western high-calorie diet".

However, she cautioned that it is "too early to say whether fasting protects against diseases like Alzheimer’s and Parkinson’s", and called for further research into the potential benefits of fasting.

Intermittent fasting: Fad or fact?

While researchers are still uncovering the science behind fasting, there have been concerns that intermittent fasting (IF) – which involves eating within a specific window of time and avoiding eating for extended periods of time – could be seen as a fad diet promoted through social media.

For instance, the hashtag #intermittentfasting has billions of views on TikTok, with numerous videos of people showing a large amount of weight loss over relatively short periods of time.

While the practice does have its benefits, experts warn that it shouldn’t be mistaken as being essential for a balanced diet and to be aware of the negative effects so you can adjust your diet accordingly.

Personal trainer and fitness expert Isabelle Murray tells Yahoo UK that, while Sunak says he fasts to maintain a "balanced lifestyle", it’s not something that everyone needs – and should – do.

"A balanced diet is focused on having a balance of food groups, including diversity in your meals like having five fruits and vegetables a day, [consuming] enough fibre and fluids… There is no need to fast."

However, fasting is undoubtedly an effective way to lose weight, particularly when combined with a calorie deficit. According to the NHS, the recommended daily calorie intake is 2,000 calories a day for women and 2,500 calories a day for men. A calorie deficit means reducing the amount of calories you usually consume in a day to achieve weight loss.

Murray points out that other factors can affect weight loss, so it isn’t as simple as just fasting and eating less. Not getting enough sleep can contribute to weight gain, as do hormone fluctuations and a diet imbalance, so it’s important to keep these in mind if you are considering any kind of fasting regime.

"Whatever your goal is, taking [the diet] too far can affect your mental health [and] your relationship with food and your body, which may cause long-term damage and may have more extreme results if someone already has underlying mental health issues," she adds.

Benefits of intermittent fasting

When practised according to expert advice and with healthy goals in mind, intermittent fasting can result in a number of health benefits.

Dr Elise Dallas, GP at The London General Practice, tells Yahoo UK she practises it herself because there are several benefits that work for her.

"Firstly, it provides a sense of discipline and structure to my eating habits, which I need when I am a busy working mum! By having designated fasting periods (Dr Dallas does a 16-hour fast), I’m more mindful of what and when I eat, which helps me make healthier choices overall," she says.

"For instance, I will start eating at about midday and then have to complete my meals within eight hours, so by 8pm or 9pm. This suits me as it stops me from snacking in the evening and I am more alert in the morning without eating."

She also finds that IF helps with weight management. "Research has shown it also promotes the burning of stored fat for energy during fasting periods, which can be particularly beneficial for those looking to shed excess pounds."

Another benefit of intermittent fasting is its potential to improve insulin sensitivity. A 2018 study by researchers from the University of Alabama examined the differences between obese men with prediabetes who ate for eight hours and fasted for 16 hours, versus those who spread out their meals over 12 hours.

The study found that, while both groups maintained their weight, the group that fasted for 16 hours had dramatically lower insulin levels and significantly improved insulin sensitivity, as well as lower blood pressure.

Dr Dallas adds that other benefits of the diet can include reduced inflammation, improved brain function and enhanced cellular repair processes.

However, she cautions: "It’s important to note that IF isn't suitable for everyone, especially those with certain medical conditions or nutritional needs.

"So, it's always best to consult with a healthcare professional, like myself, before starting any new dietary approach to ensure it aligns with your individual health goals and needs."

What you need to know before starting IF

Dr Suhail Hussain, a private home-visiting GP who also practices intermittent fasting, believes that it is more than a fad diet, but warns that when done incorrectly or without guidance, IF can be dangerous for some people.

"It can become unhealthy if it leads to nutrient deficiencies, exacerbates eating disorders, or causes severe stress or anxiety about food and eating," he tells Yahoo UK.

"Those with certain medical conditions, like diabetes, or individuals who are pregnant, breastfeeding, or have a history of eating disorders should consult a healthcare provider before starting IF."

If you are thinking of starting IF, keep in mind that your calorie intake and fasting duration depends on individual factors like age, sex, activity levels and health goals.

Dr Hussain recommends maintaining a "balanced intake that meets your nutritional needs on eating days".

"Generally the period of fasting can vary between 12-16 hours, and gradually increasing as tolerated is advisable. It's crucial to focus on nutrient-dense foods, particularly protein to ensure there is no muscle mass loss when not fasting to ensure adequate nutrition," he adds.

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Read more about intermittent fasting:

• Davina McCall on intermittent fasting: What is it and can it help gut health? (Yahoo Life UK, 6-min read)

• Woman sees nine stone weight loss using intermittent fasting like Chris Moyles (Yahoo Life UK, 5-min read)

• 5 intermittent fasting methods - and what they entail (Yahoo Life UK, 6-min read)