The 5:2 Diet Explained

It's the healthy eating regime of the moment. Celebs can't get enough of it - Beyoncé, J-Lo and Miranda Kerr are rumoured to be fans - and there's a very good chance at least one of your friends or random Facebook acquaintances has succumbed to the hype.

Super-easy to follow, the 5:2 diet involves eating normally on five days of the week and restricting calorie intake on the remaining two. Proponents of the diet swear by its remarkable weight loss and health-boosting benefits.

Though intermittent fasting has been practised for centuries, a 2012 BBC Horizon episode called Eat, Fast and Live Longer extolling the virtues of sporadic calorie-restriction kicked off the current trend.

Medical journalist and broadcaster Dr Michael Mosley, who presented the episode, went on to collaborate with lifestyle writer Mimi Spencer on the original 5:2 Fast Diet Book, an instant bestseller following its launch in January 2013. Hot on its heels, journalist Kate Harrison's hit 5:2 Diet Book was published a month later.

What is this wonder diet all about then?

The 5:2 diet couldn't be simpler and works for many people, hence its runaway success. You eat normally on five “feast” days, then restrict your calorie intake on two non-consecutive “fast” days to 500 if you're a woman, or 600 if you're a man.

That's it. There are no fancy meal plans to follow slavishly or bizarre foods to seek out. Based on its basic 5:2 premise, devotees of the diet claim you can shift up to 12lb in as little as six weeks – yes, that's almost a stone.

Is there any hard evidence the 5:2 diet actually works?

In addition to weight loss benefits, several scientific studies indicate that the 5:2 diet may switch on cell repair genes, reduce a person's risk of developing a number of age-related diseases, boost cardiovascular health and help lower cholesterol levels.

A study conducted in 2010 by Dr Michelle Harvie and her research team at Manchester University found that women following a 5:2 diet achieved similar levels of weight loss as women on a regular calorie-controlled diet, and experienced reductions in biomarkers linked to diabetes, heart disease and other diseases of age. Dr Harvie's findings are backed up by a 2011 medical review.

A 2012 study, also carried out by Dr Harvie and her team, reported that people who follow a 5:2 diet may lower their risk of developing certain obesity-related cancers.

While peer-reviewed scientific studies are still pretty thin on the ground, anecdotal success stories are plentiful. “I lost a total of 10lbs in just 11 weeks to start off with,” says Kate, a 33-year-old account manager from London, “then it gradually levelled off to a healthy weight and BMI I'm really happy with. I've got so much more energy and I can eat whatever I fancy on feast days, it's great.”

Does the 5:2 diet work for everyone?

Unfortunately not. In complete contrast to Kate's experience, author and journalist Lucy Cavendish, writing in the Daily Telegraph, had a particularly bad time following the diet and coping with the pangs of hunger. She lost weight, but the on/off fasting made her feel tired and moody rather than energised and upbeat. “Instead of experiencing a primal rush,” she writes, “my energy levels dropped – and dipped lower as the weeks went by.”

It's also important to point out that certain groups should avoid intermittent fasting completely. The diet isn't recommended for children, pregnant women, women who are breastfeeding, Type 1 diabetics or anyone suffering from an eating disorder. The very elderly and athletes in training may want to give the plan a miss as well.

Could intermittent fasting actually be bad for you?

Along with the potential weight loss and long-term health benefits, the NHS website lists several potential short-term side effects that may result from intermittent fasting, but states there is no solid empirical evidence that going without food for brief-ish periods is harmful. The side effects, which are based on anecdotal accounts and may only affect a minority of dieters, range from sleep difficulties and irritability, to anxiety and low energy levels. A risk of dehydration is also possible.

A recent study by researchers at the University of Bath however suggests that long-term intermittent fasting may dampen down the body's immune response and leave dieters more prone to infection. “Many studies have documented benefits of diet restriction and antioxidants consumption, but there is a lack of data on levels of illness in people administered these anti-ageing treatments,” says Dr Nick Priest, a lecturer in biology & biochemistry at the University of Bath who worked on the study.

“We know that certain stresses such as starvation or exposure to pathogens can extend life and increase fertility, but we have found that ironically this has a trade-off in terms of immune function.”

Knowing the ins and outs of intermittent fasting, would you give the 5:2 diet a go? As with any drastic change in your eating habits, If you have any underlying health issues, or you're not sure the diet's for you, it's always a good idea to check your GP before you take the plunge.