By Suzanne Zuckerman with Catherine Kast
March 17, 2013 08:50 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 Fast Diet, 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 aweek (men get 600 calories) but allows dieters to eat whatever they want on the remaining five days.

“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.”