Suzanne Zuckerman
March 25, 2013 12:00 PM

The thought alone of eating just 500 calories a day is enough to make any woman feel faint. But for two days a week that’s exactly what the latest blockbuster diet from Britain recommends. The FastDiet, coauthored by London-based doctor Michael Mosley and writer Mimi Spencer, outlines a weight-loss plan that limits a woman’s daily intake to 500 calories twice a week (men get 600 calories) but allows dieters to eat whatever they want on the remaining five days. (See box.) “It’s not really fasting. It’s just a break from your normal routine,” says Mosley, 56, who dropped 20 lbs. in three months on the plan. “It’s not like an ordinary diet where you think about it all the time. The joy is that you get on with your ordinary life.”

The pitfall, critics say, is the notion that we can have our cake—but only after restricting ourselves to roughly a quarter of our daily recommended calories. “Five hundred calories a day is potentially dangerous,” says Dr. David L. Katz, director of Yale University’s Prevention Research Center. “There is some risk of slowing your metabolic rate. You’ll probably have a headache and feel distracted.” Other experts worry about cravings. “If you eat very little on Monday, by Tuesday you may say, I am going to have that brownie because yesterday I ate nothing,” says Karen Ansel, spokeswoman for the Academy of Nutrition and Dietetics. “I can see more junk working its way into this diet, and over time you could end up with serious nutrient deficiencies.” Dr. Katz goes further: “There’s a potential with this to push people into full-blown binge-eating disorders, where they have extreme restraint and don’t eat anything, and then when they’re eating again, they go completely bonkers.”

But the founders of the FastDiet (also called the 5:2 Diet) insist that their feast-and-famine approach helps people make healthier choices. On fast days, “you cannot have a jelly doughnut and get through the day,” says Spencer, 45, who lost 20 lbs. in four months. “The hope is that you learn to like vegetables and lean proteins and end up incorporating them into your nonfasting days,” adds Mosley, who also cites studies done on rats and mice that say intermittent fasting extends life expectancy. As for the rest of the week, “don’t use this as an opportunity to pig out.”

A doctor by training and BBC producer by trade, Mosley hit upon the latest fad—which promises losing up to 2 lbs. a week—by experimenting with various forms of fasting himself, once living on as little as 25 calories a day (that’s just one cup of low-calorie soup!) for four consecutive days. Now using himself as an example of the program’s success (he’s in maintenance mode where he fasts only once a week), Mosley has sparked a feeding frenzy in the U.S., where his book is already in its third printing since its February release. The authors, however, warn it’s not for everyone. “I’m not going to lie; in the beginning it is quite challenging,” says Spencer. “Hunger will come. Embrace it and recognize it’s short-lived.” Says Mosley: “It’s a powerful thing to do, and some people find they just can’t take it.”

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