dr. dukan was so offended by the criticism, he fought back and last week sued dr.ohen for libel in a paris courtroom. a move dr. cohen thinks will backfire. do you think this lawsuit will hurt him? >> probably yes, because this is the issue for all these type of diets. -- the soul lucian to a very big problem and a very difficult problem. >> reporter: i sat down with dr. dukan last month when his wildly popular book began a six week run on the best-seller's list. there are some critics who say it can't be a healthy diet. perhaps if you're only eating protein, that could damage the kidneys. >> if you have normal kidneys and if you drink minimum six glass of water and you don't practice the diet too long, months and months, you have no kidneys problem. >> reporter: dr. dukan's diet has four stages. phase one is a speedy weight loss, nothing but lean protein and his special ingredient, oat bran. phases two and three add veggies and fruit and two slices of weight bread. in the final phase, he claims you can reasonably eat whatever you want but once a week pure prou tee

when followed correctly it is safe and effective method of weet i sat down with dr. dukan. it is not a quick fix. >> quick to lose weight but long to keep it. >> dukan diet works in four faces. encouraging speedy weight loss with lean protein and his special ingredient. oat brain. not rolled oats. but oat bran from the husk. >> it makes you feel full. >> full and fortified. >>> and also slushes out calories and helps out the plumbing. >> two slices of wheat bread. carbs. which is why dukan said it is a balanced approach to life long weight loss. >> final phase claim to eat what you want as long as you only eat pure protein one day a week. >> carroll middleton was a devoteee. but the growing popularit with hollywood starlets and every day folk as well >> we go through pound and pounds of chicken a week. >> marshall had been trugling to lose weight and nothing seems to work until he tried the dukan. >> marshalls lost 50 pounds on the dukan diet. i have done it. it will drive you nuts. it might work. but i need my carbs. >> there you go. if you need carbs it is not for you at a

i recently sat down with dr. dukan. this is not a quick-fix diet. >> no. it's quick to lose weight. consolidate, that's the key. the dukan die quiet works in four stages. the attack phase starts with encouraging weight loss, a lot of lean protein, and his special ingredient. oat bran, culled from the husks. it makes you feel full. >> full and satisfied. >> reporter: it helps flush out the calories. also helps the plumbing. >> yes. >> reporter: carbs. that's why dukan argues his diet is a balanced approach to life-long weight loss. the final phase claims you can eat anything you want, as long as you return to eating entrepreneur protein for one day a week. success stories, like with catherine middleton's mother, as a devotee. >> we go through pounds and pounds of chicken per week. >> reporter: marshall had been struggling to lose weight for ten years. and said nothing seemed to work until he tried dukan. dukan is suing for $20,000 in damages. not a lot by u.s. standards. but american laws are a lot higher burden of proof than french laws. >> he has a chance. french laws. >> he has a c