LONDON — Visitors to England right now, be warned. The big topic on people’s minds — from cabdrivers to corporate executives — is not Kate Middleton’s increasingly visible baby bump (though the craze does involve the size of one’s waistline), but rather a best-selling diet book that has sent the British into a fasting frenzy.
“The Fast Diet,” published in mid-January in Britain, could do the same in the United States if Americans eat it up. The United States edition arrived last week.
The book has held the No. 1 slot on Amazon’s British site nearly every day since its publication in January, according to Rebecca Nicolson, a founder of Short Books, the independent publishing company behind the sensation. “It is selling,” she said, “like hot cakes,” which coincidentally are something one can actually eat on this revolutionary diet.
With an alluring cover line that reads, “Lose Weight, Stay Healthy, Live Longer,” the premise of this latest weight-loss regimen — or “slimming” as the British call “dieting” — is intermittent fasting, or what has become known here as the 5:2 diet: five days of eating and drinking whatever you want, dispersed with two days of fasting.
A typical fasting day consists of two meals of roughly 250 to 300 calories each, depending on the person’s sex (500 calories for women, 600 for men). Think two eggs and a slice of ham for breakfast, and a plate of steamed fish and vegetables for dinner.
It is not much sustenance, but the secret to weight loss, according to the book, is that even after just a few hours of fasting, the body begins to turn off the fat-storing mechanisms and turn on the fat-burning systems.
“I’ve always been into self-experimentation,” said Dr. Michael Mosley, one of the book’s two authors and a well-known medical journalist on the BBC who is often called the Sanjay Gupta of Britain.
He researched the science of the diet and its health benefits by putting himself through intermittent fasting and filming it for a BBC documentary last August called “Eat, Fast and Live Longer.” (The broadcast gained high ratings, three million viewers, despite running during the London Olympics. PBS plans to air it in April.)
“This started because I was not feeling well last year,” Dr. Mosley said recently over a cup of tea and half a cookie (it was not one of his fasting days). “It turns out I was suffering from high blood sugar, high cholesterol and had a kind of visceral fat inside my gut.”
Though hardly obese at the time, at 5 feet 11 inches and 187 pounds, Dr. Mosley, 55, had a body mass index and body fat percentage that were a few points higher than the recommended amount for men. “Given that my father had died at age 73 of complications from diabetes, and I was now looking prediabetic, I knew something had to change,” he added.
The result was a documentary, almost the opposite of “Super Size Me,” in which Dr. Mosley not only fasted, but also interviewed scientific researchers, mostly in the United States, about the positive results of various forms of intermittent fasting, tested primarily on rats but in some cases human volunteers. The prominent benefits, he discovered, were weight loss, a lower risk of cancer and heart disease, and increased energy.
“The body goes into a repair-and-recover mode when it no longer has the work of storing the food being consumed,” he said.
Though Dr. Mosley quickly gave up on the most extreme forms of fasting (he ate little more than one cup of low-calorie soup every 24 hours for four consecutive days in his first trial), he finally settled on the 5:2 ratio as a more sustainable, less painful option that could realistically be followed without annihilating his social life or work.
“Our earliest antecedents,” Dr. Mosley argued, “lived a feast-or-famine existence, gorging themselves after a big hunt and then not eating until they scored the next one.” Similarly, he explained, temporary fasting is a ritual of religions like Islam and Judaism — as demonstrated by Ramadan and Yom Kippur. “We shouldn’t have a fear of hunger if it is just temporary,” he said.
What Dr. Mosley found most astounding, however, were his personal results. Not only did he lose 20 pounds (he currently weighs 168 pounds) in nine weeks, but his glucose and cholesterol levels went down, as did his body fat. “What’s more, I have a whole new level of energy,” he said.
The documentary became an instant hit, which in turn led Mimi Spencer, a food and fashion writer, to propose that they collaborate on a book. “I could see this was not a faddish diet but one that was sustainable with long-term health results, beyond the obvious weight-loss benefit,” said Ms. Spencer, 45, who has lost 20 pounds on the diet within four months and lowered her B.M.I. by 2 points.
The result is a 200-page paperback: the first half written by Dr. Mosley outlining the scientific findings of intermittent fasting; the second by Ms. Spencer, with encouraging text on how to get through the first days of fasting, from keeping busy so you don’t hear your rumbling belly, to waiting 15 minutes for your meal or snack.
She also provides fasting recipes with tantalizing photos like feta niçoise salad and Mexican pizza, and a calorie counter at the back. (Who knew a quarter of a cup of balsamic vinegar added up to a whopping 209 calories?)
In London, the diet has taken off with the help of well-known British celebrity chefs and food writers like Hugh Fearnley-Whittingstall, who raved about it in The Guardian after his sixth day of fasting, having already lost eight pounds. (“I feel lean and sharper,” he wrote, “and find the whole thing rather exhilarating.”)
The diet is also particularly popular among men, according to Dr. Mosley, who has heard from many of his converts via e-mail and Twitter, where he has around 24,000 followers. “They find it easy to work into their schedules because dieting for a day here and there doesn’t feel torturous,” he said, adding that couples also particularly like doing it together.
But not everyone is singing the diet’s praises. The National Health System, Britain’s publicly funded medical establishment, put out a statement on its Web site shortly after the book came out: “Despite its increasing popularity, there is a great deal of uncertainty about I.F. (intermittent fasting) with significant gaps in the evidence.”
The health agency also listed some side effects, including bad breath, anxiety, dehydration and irritability. Yet people in London do not seem too concerned. A slew of fasting diet books have come out in recent weeks, notably the “The 5:2 Diet Book” and “The Feast and Fast Diet.”
There is also a crop of new cookbooks featuring fasting-friendly recipes. Let’s just say, the British are hungry for them.