The hottest book in the U.S. right now is not the latest page-turner from Dan Brown or Jeffrey Archer, but a wildly popular diet book that’s based on simple principles of feast and famine.

It’s called “The Fast Diet: The Secret of Intermittent Fasting - Lose Weight, Stay Healthy, Live Longer” and has been the bestseller on Amazon’s U.K. site nearly every day since its publication in January.

The diet, which has already spawned a number of copycats, has become known as “the 5:2 diet”: five days of eating and drinking whatever you want, interspersed with two days of fasting.

The book’s author, Dr. Michael Mosley, wrote the book after the success of a BBC documentary he hosted last August called “Eat, Fast and Live Longer.” He says the idea for a weight-loss diet based on fasting sprang from his own health worries.

“About a year ago, I needed to do something about my health,” Mosley told CTV’s Canada AM Wednesday from London.

At 84 kg (186 lbs) on a 5’11 (1.8m) frame, Mosley wasn’t overweight at the time, but his blood tests revealed he wasn’t exactly healthy either. His cholesterol was sky-high, he was carrying too much visceral fat around his abdomen, and his glucose levels were so off, Mosley’s doctor told him he was essentially pre-diabetic.

“So rather than go on drugs, I decided to go look around the world for possible solutions,” he says. “And I came across intermittent fasting. I went to visit centres in the U.S. and U.K. and on the basis of that, I tried intermittent fasting.”

The BBC journalist and physician -- whom many call ‘the Dr. Sanjay Gupta of Britain’ -- experimented with different fasting models and decided that what worked best was a 5:2 model.

For five days during the week, Mosley eats normal-sized portions. On Mondays and Thursdays, he fasts.

But Mosley’s idea of “fasting” is not the hot-water-and-lemon meals that might immediately spring to mind. Instead, it’s simply a day of calorie restriction.

So a Monday morning breakfast might consist of a couple of scrambled eggs and ham with tea.

Mosley will then drink plenty of water, black tea and coffee throughout the day, while skipping lunch. At dinner, he’ll eat “an enormous pile “of vegetables with a portion of chicken and salmon.

That adds up to 600 calories of food for a fast day. (Women are advised to eat a little less: about 500 calories on a fast day.)

“You’re not exactly going without food. You’re just eating protein-rich, vegetable-rich and nutritionally-rich meals,” Mosley says.

Just a few weeks on this new eating plan and Mosley says he lost 20 pounds and his blood levels went entirely back to normal.

What makes his diet superior to other diets, Mosley says, is it allows for more fat loss.

“On a standard diet, you lose 75 per cent fat, 25 per cent muscle. On this, you lose somewhere between 85 and 95 per cent fat. That’s another reason I called it ‘The Fast Diet’: You do lose weight fast but it’s fat, not fat and muscle,” he says.

Evidence gaps

While the book is a commercial success, not everyone is excited. Britain’s health agency, the National Health Service, put out a statement on its website within weeks of the book’s publication, noting that “there is a great deal of uncertainty about I.F. (intermittent fasting) with significant gaps in the evidence.”

It advised that those who try fasting are likely to be very hungry and have less energy on their fasting days and this could affect their ability to function.

“Also, I.F. may not be suitable for pregnant women and people with specific health conditions, such as diabetes, or a history of eating disorders,” the agency warned.

Mosley says he’s heard a lot of the criticism, but says there are lots of studies to back up the merits of fasting.

“I think all of that is based on out-of-date knowledge, if you like. This book is based on 20 years of research in animals and at least 11 human trials,” he said.

The argument that fasting will cause the metabolism to slow so that the body goes into “starvation mode” is based on old research from the 1950s, Mosley insists.

“Actually, that only happens if you don’t eat anything for a week of longer,” he said.

“This is just cutting back on your calories two days a week. It’s not that radical, honestly. But the effects are very impressive.”

The book argues that by fasting for just a few hours a few times a week, the body begins to “turn off” its fat-storing mechanisms. It’s also similar to the feast-or-famine existence our ancestors lived, he says, when we would gorge after hunting down a big animal and then not eat much again until the next kill.

The NHS says there hasn’t been any research that looks directly at the 5:2 diet or at how it compares to other diets. It says while there is some evidence about the potential benefits of intermittent fasting, all the research has been limited in some way.

Mosley disagrees and says there have been good, long-term studies.

“A lot of studies have been done and all of them point in the same direction: that if you do this pattern of intermittent fasting, you will be able to stay on the diet better than on a standard diet,” he says. “You will lose more fat and you will also see greater improvements of your biomarkers such as your insulin levels.”