Fasting diets were in their heyday when I first embarked on the 5:2 back in March 2014.

By then I was officially "overweight", with a body mass index of 26, and I was finally determined to do something about it.

Having recently become a homeowner after years of sharing rented flats with friends, I was also living on my own for the first time - able to exact absolute control over what foods filled the fridge and cupboards.

Back in September 2012, I had watched the episode of Horizon where Dr Michael Mosley famously investigated the possibility of losing weight and reversing diabetes through intermittent fasting - eating normally five days a week while cutting down to just 500 calories (600 for men) on the other two.


Like many others, I was intrigued. And successful.

By October 2014, when I jetted to Mexico on holiday, I had shed more than two stone - going from around 11st 6Ibs to 9st 4Ibs.

I was a size 10 again, for the first time since I was a teenager.

On fast days I ate a high-protein yoghurt for breakfast, a salad box for lunch, and dinner was usually scrambled egg on toast or stuffed mushrooms.

On the other days I stuck to around 1600 calories - fewer than the 2000 recommended for women - but it seemed like loads compared to a fast day.

Ten years on, my weight still hovers around 9st 7.

I don't stick to the 5:2 religiously any longer - more like 6:1 - but I do go to the gym regularly and keep a close eye on my calories most of the time.

Before the 5:2; on holiday, after the 5:2; and todayBefore the 5:2; on holiday, after the 5:2; and today (Image: Helen McArdle)

The tragic death earlier this month of Dr Mosley, apparently due to heat exhaustion while walking in temperatures of 40°C on the Greek island of Symi, has prompted a renewed interest in the fasting diets he popularised and questions about whether the science really stacks up.

After the 5:2, he went on to promote Fast 800 - a weight loss regime which claims to reduce the risk of Type 2 diabetes, heart disease and cancer through a combination of 800-calorie days and a Mediterranean diet - while alternative approaches such as time-restricted eating (for example, eating all meals within an eight-hour window) have become more fashionable in recent years.

But is any of this evidence-based - or is it just another fad?

"For some people, [fasting diets] provide a good option to help them lose weight," said Naveed Sattar, a professor of metabolic medicine at Glasgow University.

"Some people really like it, because it allows them to eat normally on the other five days, knowing that there will be two days where they have to be really restrictive.

"But the key thing is that they help people cut total calorie intake for the week.

"How long people can keep that up for, I don't know, but I've had colleagues of mine who do it - two days a week they will eat very little and the rest of the time eat what they want, and it did work for them.

"But is there any evidence that it leads to greater weight loss beyond the calorie restriction? No.

"Is there any evidence that it has some magical effect beyond the amount of weight loss achieved? Not really.

"I think it's just another option that some people find easier to follow."

Professor Naveed Sattar said the benefits of fasting diets were ultimately down to cutting overall calorie intakesProfessor Naveed Sattar said the benefits of fasting diets were ultimately down to cutting overall calorie intakes (Image: Gordon Terris/Herald&Times)

It is a view echoed by Dr Maria Chondronikola, the lead for human nutrition at the Institute of Metabolic Science at Cambridge University.

She said: "There are different dimensions in the area of nutrition: one is when we eat, the other is how we eat, and then how much we eat. So it's quantity, quality, and timing.

"At this point, the science in different types of fasting diets is that do they seem to induce some weight loss, and weight loss - especially in people with excess weight - leads to metabolic improvement.

"But whether they are more beneficial than simply cutting down calories, is mostly not very well explored."

In other words, diets like the 5:2 work because they lead to people consuming fewer calories in total over the course of a week than their body needs.

Some people may find intermittent fasting easier to cope with, but there is nothing to indicate that it leads to faster or greater weight loss overall.

Similarly, some of the benefits associated with weight loss in terms of metabolic health - such as reversing diabetes - are not due to the effects of fasting per se, but simply what can happen when you transition from overweight or obese to a healthy weight.


But what about fat burning?

It is commonly claimed that fasting stimulates the body to burn through its fat stores more quickly.

A much-hyped study in mice found that rodents placed on an intermittent fasting plan for 16 weeks used up more fat and ended up with faster metabolisms compared to a control group of mice fed an identical amount of calories.

Dr Chondronikola cautions that we should be careful when it comes to extrapolating those results onto humans.

She said: "There are studies in rodents and studies in people, and rodents have a very different metabolism.

"When those experiments are performed in rodents, they can see that the metabolism speeds up much faster than it does from just doing caloric restriction.

"So there are better metabolic outcomes in rodents when matching them for how much food they eat.

"In people, we don't quite see that - but sometimes we're not able to do as precise metabolic assessments as can be done in mice.

"As long as people cut down calories, as they do on these fasting diets, there is usually weight loss - but that is probably simply associated with the impact of eating less food.

"It is possible that there may be other metabolic processes going on that may increase the oxidation of fat, but this remains to be determined."

Dr Maria Chondronikola said results in mice should not necessarily be extrapolated to humansDr Maria Chondronikola said results in mice should not necessarily be extrapolated to humans (Image: University of Cambridge)

As for time-restricted eating, such as the 16:8 diet, there is nothing to suggest that it comes with any additional benefits beyond traditional calorie cutting, said Prof Sattar. 

He pointed to a recent Chinese trial which concluded that patients placed on time-restricted eating did not lose any extra weight compared to patients consuming the same quantities over a longer period throughout the day. 

Prof Sattar said: "Time-restricted eating is another way where people impose different rules that they themselves can follow to cut their calorie intake - that's all it is.

"So it does help people lose weight, but there's nothing magical about it - it's just that, for some people, if they decide they're only going to eat between 10 and 6 that is a simple rule for them to follow that means they eat less calories than they would otherwise."

Prof Sattar was appointed last year as the chair of the UK Government's Obesity Mission, a £20 million drive to identify and evaluate innovations that could help to treat and reduce obesity at a population level, as well as relieving pressure on the NHS through associated conditions. 

He stresses that when it comes to tackling obesity as a national public health emergency, that has to come down to changing the obesogenic environment we live in - from the foods promoted in supermarkets to the size of portions served by takeaways and restaurants. 

Emergency teams on the island of Symi, Greece, after Dr Mosley's body was discovered on June 9Emergency teams on the island of Symi, Greece, after Dr Mosley's body was discovered on June 9 (Image: PA)

Prof Sattar is also excited by the potential for wider use of weight loss drugs like Ozempic and Wegovy on the NHS, but stresses that his number one piece of advice to individuals is simply to eat more fibre. 

He said: "If people could be encouraged to make small, sustainable changes in their diet, like getting used to eating more fibre-rich foods, that's the main thing I would suggest.

"Replacing some of the confectionary, cakes, crisps, biscuits that they have normally with fruit and veg, and salad.

"Eating more fibre definitely helps with weight loss - there's lots of trials showing that - and I think that's probably more sustainable in the long-term than fasting."