June 1, 2011 — -- The inventor of the popular Dukan Diet is defending his weight loss plan in a Paris courtroom, suing for libel against claims the diet is dangerous.
Dr. Pierre Dukan, the French creator of the high-protein and oat-bran diet, is suing another bestselling diet author, rival Jean-Michel Cohen.
Cohen told French health magazine "Meilleure Sante" that Dukan's low-carb diet is dangerous. He accused pro-Dukan doctors, pharmaceuticals and the media of "climbing on the bandwagon of this fantasy." Cohen claims the diet can lead to long-term health problems.
"This is a very dangerous diet because you have too much protein so you eat too much fat ... you have no fiber and you have constipation, breast problems and also sometimes a very frequent heart disease," Cohen said of the diet.
Dukan, who began promoting his diet three decades ago, is suing Cohen for $20,000 in damages, the limit in French court. Some experts say Dukan has a shot at winning his case. French libel laws have a much lower burden of proof than in the United States, experts said.
"There is no truth to the unsubstantiated allegations," Dukan said in a statement. "When followed correctly, the Dukan Diet is both a safe and effective method of weight loss."
Dukan's book was released in the United States in April and boasts an Atkins diet-like mix that promises initial quick weight loss without going hungry. The diet is not a quick-fix plan, Dukan told ABC News earlier this year.
"It's quick to lose weight, but it's very long to consolidate," Dukan said.
The Dukan diet works in four stages. The first phase, called the attack phase, encourages speedy weight loss with nothing but lean protein and Dukan's special ingredient of oat bran culled from the husks.
"Oat bran is very important ... it makes [you] feel full and satisfied. It helps flush out the calories," Dukan said.
The cruise phase allows the addition of vegetables, but not starchy potatoes or fatty avocados.
The third, or consolidation phase, is the most critical. Dieters are allows to add one serving of fruit and two slices of whole wheat bread per day.
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.
There are other requirements. Dieters must drink six glasses of water each day, walk for 20 minutes and eat at least 1 1/2 tablespoons of the oat bran.
The diet's popularity among celebrities such as Jennifer Lopez, as well as Carole Middleton, mom of Duchess Kate Middleton, has spread throughout the United States.
For Marshall Brain, the diet has been the answer to a 10-year struggle with his weight. "I lost 50 pounds in about six months on the Dukan Diet," Brain said.
Brain said he goes through pounds and pounds of chicken per week.
ABC News' Cathy Becker and Amanda Keegan contributed to this report.