Although the diet may help shed the pounds in the first few weeks, there's no evidence to suggest that the weight stays off. People who follow the diet for up to a year will lose the same amount of weight—an average of 5 kg—as those who follow standard calorie-controlled diets.
Despite the fact the diet's advocates—most notably the BBC's health reporter Dr Michael Mosley—claim it is scientifically proven, very little research into the long-term effects has been carried out, say researchers from the University of Glasgow.
What little there is doesn't suggest the 5:2 diet is any more effective than just reducing the amount of food you eat, the researchers say. They took a look at five studies that had reviewed various types of 'intermittent energy restricting' diets, as they're known, and which includes the 5:2. The diets had been followed for at least three months, and some for up to a year.
Average weight loss was around 5 kgs (11 lbs), but the same was being achieved by people who were following non-fasting diets, the researchers discovered.
The results are a far cry from the claims made by Dr Mosley in his best-selling book, The Fast Diet, who claimed there was scientific evidence that the diet could "significantly" reduce waist size by up to three inches, and typical weight loss was around 13 lbs within three months.