It’s the diet which trimmed down the waistlines of countless celebrities and politicians, with former chancellor George Osborne and Sherlock actor Benedict Cumberbatch among those who have used it to slim down.
But the 5:2 diet always came at a price: the willpower dieters required to limit themselves to just 500 calories a day, rising to 600 for men, for two “fasting days” each week.
Now the diet’s creator, Dr Michael Mosley, has loosened the rules. But he insists his new version of the “Fast Diet” - which increases the limit to 800 calories on fasting days, is just as effective.
“You don't need to stick to 600 calories. Cutting down to 800 calories a day seems to be almost as effective and for some people much more ‘doable’.”
“It's low calorie, without being superlow-calorie. And if you want to have lunch, you can slip those extra 200 calories in there.”
However, he said those thinking the regime would allow them to enjoy an unhealhty snack on fast days should think again.
Dr Mosley said “empty calories” should remain strictly off-limits on fasting days.
He told the Mail on Sunday: “If you are going to have lunch, have something like a nice bowl of hearty soup, or grilled fish and a decent pile of vegetables or salad.”
The original version of the 5:2 said some of the best results came from fasting for about 12 hours from breakfast to dinner.
But Dr Mosley said some people ended up getting “a bit irritable”.
The new regime sets out plans for three meals a day. And it suggests a 24 hour period of “fasting” - on 800 calories a day, could start with an early dinner, followed by a late breakfast, to allow around 13 hours without food overnight.
A number of studies have suggested that short periods of fasting can help control blood sugar levels - reducing the risk of type 2 diabetes.
“What I'd recommend is that you have a slightly earlier supper on the evening before, and then a slighter later breakfast on the fasting day. That way, you'll be fasting for 13 hours or more overnight,” he said.
Adherents of the diet are thought to include singer Beyoncé while a number of politicians, including then chancellor George Osborne have used it to shed pounds.
Alex Salmond, former Scottish First Minister, has said he had dropped about two stone on the regime, while TV celebrity Philip Schofield said he had been left hungry after there weeks on the diet, in which he lost nine pounds.
Dr Mosley, presenter of a number of BBC programmes including Trust Me, I'm a Doctor, became interested in “intermittent fasting” while researching the subject for a documentary on the BBC's Horizon series in 2012.
When he tried the 5:2 diet, he managed to lose 20lb and reversed his newly diagnosed type 2 diabetes.
The Fast Diet, which Dr Mosley penned with journalist Mimi Spencer, setting out the 5:2 principles, is an international bestseller.