Clash of the French Diet Titans: Rival Nutritionist Says Dukan Diet Founder Is Hurting Himself With Libel Suit

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The clash of the French diet titans continues to rage, and there seems to be no end to the he-said, he-said fight.

Best-selling diet author and French television personality Dr. Jean-Michel Cohen told ABC News that Dr. Pierre Dukan's libel suit against him will probably fail and could ruin Dukan's reputation.

"This is the issue for all these types of diets," Cohen said. "Don't write you have the solution to a very big problem."

Dukan, the creator of the wildly popular high-protein and oat-bran diet, is suing Cohen for $20,000 in damages, the limit in French court, over claims he made that the Dukan Diet was unsafe.

"I'm very disappointed because this was not my objective. I didn't want to go to court," Cohen said. "He decided to sue. I didn't want this because what I wanted to do was to make prevention and not tell Dr. Dukan, 'you're a bad man,' but at the moment, when I'm attacked I defend myself."

Advocating a 1,600-calorie a day diet in his own book, "Savoir Maigrir" ("Know How to Lose Weight"), Cohen also claimed that the Dukan Diet, which uses a repetitive four-food phase system, just doesn't work.

"I think they are not understanding what they are doing, because when you do this type of diet, you regain more weight than you've lost," he said. "If [dieters] listen to him, they don't lose weight."

In a statement to ABC News, Dukan said he was "not the least bit concerned" with Cohen's claims.

"What Mr. Cohen is saying doesn't make any sense," Dukan said in a statement. "Either he didn't actually read my books, or he is just looking to draw attention to himself. I will let the diet speak for itself."

The Dukan diet is divided into four stages. The first phase, called the attack phase, is supposed to encourage speedy weight loss with nothing but lean protein and Dukan's special ingredient of oat bran culled from the husks.

The cruise phase allows the addition of vegetables, but not starchy potatoes or fatty avocados. Dukan said in a statement this phase allows the dieter to "lose weight at the rate of approximately 2.2 pounds per week."

The third, or consolidation phase, is the most critical. Dieters are allowed to add one serving of fruit and two slices of whole wheat bread per day, as well as cheese and some starches.

The final phase, called stabilization, lasts the rest of one's life. Dieters can eat whatever they want as long as they return to eating only pure protein for one day a week -- Dukan suggests Thursdays.

"The fourth phase, or Stabilization phase, guarantees that the pounds stay off as long as three conditions are respected as has been done by millions of people throughout the world," Dukan said in a statement.

There are other requirements, including that dieters must walk for 20 minutes and eat at least three tablespoons of the oat bran a day.

Dr. David Katz, a Yale University nutritionist, cautioned that "yo-yo" diets can be harmful and said one of the problems of a high-protein diet is loss of water in the body.

"That can increase the risk in kidney stone formation, gall stone formation, and can contribute to bone thinning as well," he said. "Those are acute harms."

A review of Dukan's book posted on the American Dietetic Association website does not recommend the diet, not because it is dangerous, but because the side effects include bad breath and constipation.

Dr. Pierre Dukan Response to Critics of His Diet

In response to critics who say his diet could cause kidney problems, Dukan told ABC News last month that dieters must drink a minimum six glasses of water each day while on the Dukan Diet.

"If you have normal kidneys ... if you drink minimum six glass of water, and you don't practice this diet too long, months and months, you have no kidney problems," he said.

To support his argument that the Dukan Diet doesn't work, Cohen pointed out an investigation by French women's magazine Journal des Femmes, which found that 75 percent of dieters said they regained the weight they had lost on Dukan within two years.

But Dukan called the survey's credibility into question, saying it was a promotional quiz, not a survey conducted by an independent research firm.

However, a recent Brown University study that analyzed several other diets, but not Dukan, estimated as many as 80 percent of overweight dieters fail to maintain their weight loss for more than a year.

Dukan's widely popular book was released in the United States in April and has sold millions of copies after spending six weeks on the bestseller list. In an interview with ABC News last month, Dukan said his protein-heavy diet would not harm a dieter's cholesterol.

"No. Cholesterol, our statistics, and there's a study in France, about that," he said at the time. "Cholesterol is ameliorated in 98 percent of the case."

The diet's popularity started in France and then spread across the United States after rumors circulated that Catherine Middleton, the Duchess of Cambridge and newlywed to Prince William, reportedly went on the diet to lose weight before the royal wedding.

ABC News' Jessica Hopper and Lauren Effron contributed to this report.