Visitors to England, 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 U.S. edition arrived last month.

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 in Britain 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 co-author Dr. Michael Mosley, a BBC medical journalist known as 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 called “Eat, Fast and Live Longer.” (PBS plans to air it in April.)

“This started because I was not feeling well last year,” 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, Mosley, 55, had a body mass index and body fat percentage a few points higher than the recommended. “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 said.

The result was the documentary, almost the opposite of “Super Size Me,” in which Mosley fasted and 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 Mosley quickly gave up on extreme 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,” Mosley said, “lived a feast-or-famine existence, gorging themselves after a big hunt and then not eating until they scored the next one.” Similarly, temporary fasting is practiced in Islam and Judaism during Ramadan and Yom Kippur. “We shouldn’t have a fear of hunger if it is just temporary,” he said.

What Mosley found most astounding were his personal results. He lost 20 pounds in nine weeks (he currently weighs 168), his glucose and cholesterol levels went down, and so 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 Spencer, 45, who has lost 20 pounds on the diet within four months and lowered her BMI two points.

The result is a 200-page paperback: the first half written by Mosley outlining the scientific findings of intermittent fasting; the second by 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 nicoise salad and Mexican pizza, and a calorie counter at the back. (Who knew a quarter cup of balsamic vinegar has a whopping 209 calories?)

Not everyone is singing the diet’s praises, however. The National Health System, Britain’s publicly funded medical establishment, put out a statement on its website 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.

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,” plus cookbooks featuring fasting-friendly recipes. Let’s just say the British are hungry for them.