TUESDAY, April 5 (HealthDay News) -- No doubt timed to coincide with the collective angst about the upcoming swimsuit season, a best-selling book detailing a trendy new diet made famous by the French is due out in the United States later this month.

Already, The Dukan Diet has sold more than 3 million copies worldwide, according to the Web site of Random House, which is publishing the book here.

Created by French physician Dr. Pierre Dukan, the diet is reportedly popular with high-profile French citizens. And word has it that Carole Middleton, the mother of soon-to-be British royal bride Kate Middleton, has adopted the diet in preparation for her daughter's April 29 wedding.

The promise on the book's jacket cover sounds simple: "2 Steps to Lose the Weight, 2 Steps to Keep It Off Forever."

But some nutritionists in the United States have some concerns about the eating plan.

In a nutshell, the diet's four steps include the "attack phase," the "cruise phase," the "consolidation phase" and the "stabilization phase."

During the attack phase, you determine how much you want to lose. Then, for two to seven days (depending on how much weight you want to lose), you eat only lean, unlimited protein (reminiscent of the Atkins diet) and daily oat bran.

Then it's on to the cruise phase, where you alternate days of pure protein foods with days of protein fare and healthy vegetables. This continues until you reach your target weight.

The consolidation phase allows unlimited protein and vegetables, with bread and other carbohydrates re-introduced.

The final phase -- the stabilization phase, which lasts as long as you do -- is the maintenance part of the plan: You're allowed to eat whatever you like on all days of the week but one. Once a week, you revert back to the protein-only menu.

Exercise is also encouraged. At the least, Dr. Dukan advises a 20-minute walk daily and avoiding all elevators.

U.S. nutrition experts who reviewed the diet for HealthDay expressed some health concerns, however.

Karen Congro, a dietitian and director of the Wellness for Life Program at Brooklyn Hospital Center in New York City, called it ''a recipe for disaster."

People who follow the diet for a few days will probably be OK, Congro said, but long-term, it's unhealthy. "It is a high-fat diet, it does not restrict salt," she said, among other criticisms.

And, she added, "It really has no evidence'' of effectiveness. "Over the long term, it really isn't good for your heart," she said, citing the fat and salt content.

In France, the National Agency for Food, Environmental and Work Health Safety has criticized the diet as unhealthy. And the British Dietetic Association calls it the "Do-can't Diet" and cautions against it.

Connie Diekman, director of university nutrition at Washington University in St. Louis, calls the Dukan diet "Atkins reincarnated."

A diet with phases injects some fun, she said, "but the very limited nutrient intake makes this a poor diet to choose for healthy weight loss."

Diekman said the limits on carbohydrates aren't supported by science.

And, what about Kate Middleton's mom? "As a registered dietitian, and as a recent mother-of-the-bride, I would caution the royal bride-to-be's mom that she is going to need all the energy she can to thoroughly enjoy this very special day. Carbohydrates are the food that provide us with that energy," Diekman said.

"A much better approach," Diekman added, "would be to choose smaller portions of whole grains, vegetables and fruits with lean protein, and low-fat dairy while limiting the amount of calories from added fats and sugars."

More information

To learn more about how to eat healthfully, visit the American Dietetic Association.