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As the world watched the long-awaited royal wedding of Prince William and now Princess Catherine, most people couldn’t help but admire her beauty and elegant style. Also a point of admiration—the bride’s slim figure.

Being a lifelong athlete and coming from a family with seemingly svelte genes, Catherine has always looked fit. But as she walked down the aisle of Westminster Abbey Friday morning, her waist looked smaller than ever in her white lace gown.

Could the rumors of the Middleton women being on the Dukan Diet be true?

It’s supposed to be the French version of the Atkins Diet and it’s making big waves – both in Europe and now in America, but one New York City-based dietitian said the Dukan Diet can be a dangerous choice – despite the fact that Princess Catherine and her mother allegedly used the diet to shed pounds before Middleton’s big day.

“The Dukan Diet is not based on any scientific data, and seems to be a modified version of the Atkins Diet,” Tanya Zuckerbort, a Fox News contributor said.” This diet is protein-centric and highly restrictive in the initial stages, without much research to back it up.”

The Dukan Diet has been popular in France for years, and now Dr. Pierre Dukan is bringing his weight-loss plan overseas to North America. It consists of high-protein, low-fat meals—which is nothing new—but adds very large amounts of water and oat bran. The diet is cut into week-long cycles with some including vegetables. There is no fruit allowed, but wine and dessert are allowed for a certain number of days—thus appealing to the French.

And what about exercise? The requirement is very light, about 20 minutes a day, with no elevators allowed.

Zuckerbrot was concerned about the restrictions on fruits and vegetables in the plan.

“A diet consisting of high fiber carbohydrates, lean proteins, fruits, vegetables and a small amount of heart healthy fat is the key to looking and feeling your best,” she said.

Dukan’s book has already sold millions of copies worldwide and has been translated into 14 languages—but is it all just hype?

According to Zuckerbrot, the Dukan Diet already has a bad reputation in the world of dietitians.

“France’s National Agency for Food, Environmental and Work Health Safety pointed out the Dukan Diet as one of 15 imbalanced and potentially risky diets. The British Dietetic Association, has also listed the Dukan Diet as one of the five worst diets of 2011,” she said.

By eliminating key foods from your diet, Zuckerbrot said Dukan’s plan can create a nutrition deficiency as well as high cholesterol and even kidney problems.

The bottom line with this diet, is it will more than likely come and go with the rest — there is just not enough variety, Zuckerbrot added.

“You will initially see weight loss on the Dukan Diet, but this eating plan is not sustainable. Also a diet high in protein and low in carbohydrates is expensive. This diet is just that, a diet, not a healthy maintainable lifestyle,” she said.