tonight on "nightline" -- sex ting bombshells. >> i've exchanged messages and photos of an explicit nature. >> now one talks exclusively to abc news. we've got the inside story of the sexting, the lies and the send button. weight loss wars. it's the diet that reportedly helped the mother of this bride get fit but now the question of whether it could be more harm than good has set off a legal battle. tonight, the scales of justice take on a whole that meaning. and mission afghanistan. the war is costing america about ten billion a month.

is it time for the united states to leave? diane sawyer is there with the men advising the president. good evening. i'm terry moran. i don't know what i was thinking. that's what democratic congressman weiner said today in a news conference in which he admitted tweeting an inappropriate photo to a woman over the internet. it was a breathtaking about-face from what weiner said last night here on "nightline." here's what he told abc's jonathan karl then. >> this is a twitter hoax. a prank that was done. i'm the victim of it. this poor girl is the victim. >> that didn't work so well. here is "20/20" anchor chris cuomo with the exclusive

interview that set this all in motion today. >> i had made terrible mistakes that have hurt the people i care about the most and i'm deeply sorry. i've exchanged messages and photos of an explicit nature with about six women over the last three years. >> it was the last thing anthony weiner was expected to say a week into allegations of a congressman's online sexual exploits. but there he was, taking full responsible for something he had, until now, claimed was the result of computer hacking. >> to be clear, picture was of me and i sent it. >> last week, a picture of a man in his boxers surfaced, along with claims it was sent by congressman weiner over twitter. at the time, weiner gave an angry press conference denying he gave the photo to a college student. >> as soon as you answer the question asked you, sir -- >> and then gave a confusing reto abc news. >> just tell me definitively, is that a photograph of you? >> i'm reluctant to say to you

definitively anything about this. >> claims and names and pressure mounted leading to today. a presser that was initially hijacked pie the man who launched the accusations, conservative blogger andrew breitbart. >> i want to hear the truth from congressman weiner. >> however, weiner eventually took the stage and took fault and apologized. going so far as to admit the name of one of the people involved. was megan broussard one of the women? >> megan broussard was one of the women. >> he said i'm an open book, maybe too open. >> 26-year-old megan broussard of texas is a single mother who says she first became acquainted with weiner on april 20th after seeing this video online. >> -- the fight and the argument every single day -- >> and commenting on his facebook page it was hot. now, what does that mean? >> i like to see someone who actually stood for something and was passionate about something. >> broussard said weiner

immediately responded to her facebook comment. a flirtatious internet relationship including she claims over 100 instant messages and eventually pictures. >> he was eager to hear about if i wanted him or thought he was attractive. >> how much of it was sex talk? >> i mean, he would attempt all the time. >> broussard said she asked for proof that the man online was really congressman weiner. >> i actually asked him to take a picture and write me on it so i would know. >> she says she then began exchanging a series of increasingly risque photos with a man she believed to be weiner. broussard sent these pictures which abc has licensed from her and says she received these photos and others in return. including the photo that set off this controversy last week. broussard says she also received this bare-chested shot of weiner with photos in the background

showing his wife and the clintons. and she says she received an explicit photo of a man's exposed private parts. when you got that photo, what did you think? >> i just thought it was risky, real risky. >> broussard says she decided to come forward with her story after watching weiner on tv last week trying to explain his predicament. >> it was a hoax. it was someone sending a picture of a wiener on weiners account. >> thought it was hysterical. >> why? >> i know he's lying. it would be one thing if he came out and said, so what? but now you're saying you got hacked. there's so many other women out there. >> she says she came forward because of concerns for her image as an aspiring nurse and that of her 3-year-old daughter. according to broussard, a friend got her in touch with andrew breitbart who shared her identity with abc news. >> i choose to investigate it. it took us about four hours with an editorial team to finally realize we had the story -- the story down solid. >> is this about politics for

you? >> no. >> do you dislike anthony weiner? >> no. >> do you think he's a bad guy? >> no, i don't think he's a bad guy. i just think he's got issues. >> today the congressman did something most people never have to do, he apologized in front of the national media, especially to the person he says he hurt the most, his wife, who's an aide to secretary hillary clinton. >> she made it very clear she thought it was very dumb and was not happy about it but she also -- and she's very disappointed. she also told me that she loved me and wanted us to, you know, pull through this. >> for "nightline," i'm chris cuomo in new york. >> the fallout from another political sex scandal, thanks to chris cuomo for that. just ahead, it's a world famous diet familiar from the best-seller list but could it carry previously unacknowledged health risks? depression is a serious medical condition.

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americans go on diets each year and each year there are tens of millions of arguments over whether this diet or that diet really works. most of those are friendly debates. but recently a disagreement about whether a diet works and even whether it is safe ended up in court when dr. pierre dukan, the inventor of the renowned dukan diet, sued a rival for libel. here's juju chang. >> reporter: in large part, because one reported devotee was a very slim mother of the bride and not just any bride, this one

had the most watched wedding in history. dr. pierre dukan's high-protein diet has hollywood star lets and commoners singing its praises. >> i tried weight watcher. i've tried them all. >> reporter: anna was a serial dieter. this was before, she says, a new diet changed her figure and her life. you look like a different person there. >> i am a different person. you don't know how much more confidence i've got. >> reporter: latest in the bulging pantheon of fad diets. each captures the hopes of people struggling to lose those annoying extra pounds. >> that is okay. that is okay. >> reporter: the diet was launched by this man, dr. pierre dukan, a french nutritionist who traveled across the pond to launch dukan here in america with its promises of quick results and easy rules. too much fat? >> too much fat. >> reporter: but back in france, dr. dukan's diet has come under attack by a richal nutritionist.

jean-michel cohen, a kind of french dr. oz who hosts a popular tv show and has a penchant for what he calls truth telling. >> if you pretend to have a solution for this problem, you're a liar. >> reporter: are you saying dr. dukan is a liar? >> perhaps the word liar is too much because i'm a gentleman but i think they're not understanding what they're doing because when you do this type of diet, you regain more weight than you lost. >> reporter: dr. cohen points to one online survey in france that found 75% of dieters who lost weight using dukan say they regained it within two years. dr. dukan dismisses it as a promotional quiz. even so, that's pretty much on par with most other diets. a university study estimates 8% of overweight dieters fail to maintain their weight loss for more than a year.

in a french magazine, dr. cohen argued the diet doesn't just fail, it's actually dangerous, warning that rapid weight loss due to eating exclusively high protein can lead to long-term health problems. dr. dukan was so offended by the criticism, he fought back and last week sued dr. cohen 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 teen. he suggests thursdays. you know the dukan diet involves vegetables and bread and other -- >> this is marketing, you know, because if -- >> reporter: no, it's not marketing, it's in the book. >> the problem is nobody does that -- >> reporter: you're saying because people don't listen to him exactly that he's dangerous. >> if you do the diet like it tells, then you don't lose weight. >> reporter: but the central question remains, is a high protein diet harmful? >> much of the initial weight loss on a high protein diet is

loss of body water and that dehydration can increase the risk of kidney stone formation, gal gal stone formation and can contribute to bone thinning. >> reporter: we asked dr. david katz, a nutritionist at yale university, to weigh in. >> the dukan diet essentially does exactly what the atkins diet did. very restrictive, use a lot of weight fast. then make it a diet you can live with and oopsy daisy maybe you'll start to gain back weight. not to worry, just go back to the beginning and start over. >> reporter: dr. katz argues this vicious cycle of yo-yo dieting is harmful and high-fat diets have been linked to heart disease, high cholesterol and even breast cancer it it's the atkins diet that allows you to eat high-fat proteins, a no no on the dukan diet. you don't let bacon -- >> no, of course. >> reporter: no fatty cheese. atkins, that's all -- >> at this time, big, big problem with the fat because you

can have cheese, ham, you can have -- >> reporter: which may be why just this week the american dietetic association's book review said the dukan diet is not dangerous. do you think that losing the case could ruin dr. dukan's reputation? >> i believe yes, and i'm very disappointed because this was not my objective. i didn't want to go to the court. he decided to sue me. i didn't want this. >> reporter: in a statement today, dr. dukan told us that dr. cohen actually didn't read my books or he's just looking to draw attention to him. i will let the diet speak for itself. >> no, not that -- >> reporter: regardless of what happens in the court of law, dr. dukan's reputation now rests in the court of public opinion. i'm juju chang for "nightline" in new york. do you think i'd let osteoporosis slow me down? so i asked my doctor about reclast because i heard

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73% of americans say the u.s. should withdraw. a substantial number of combat forces from afghanistan this summer. that's according to a new abc news/"washington post" poll out today. well, tonight diane sawyer is in afghanistan to talk with two men who have got a great deal of influence over what comes next in this war. part of our series, "afghanistan, can we win?" >> we are flying towards afghanistan on what's called the doomsday plane. his last trip as secretary of defense. robert gates says 4 1/2 years of ordering men and women into

battle is enough. do you know how muany have died since you became secretary of defense? >> as of yesterday, 1,255. and about 11,000 wounded. >> in afghanistan, he sat down for a giant intervijoint interv general david petraeus. are we winning this war? >> we're having a great deal of success of achieving the mission our forces have been given. >> we're making progress. we're loathe to use this very loaded term of winning or losing. overall, the momentum has changed. >> if we can't talk about winning, is it too much to ask of american men and women to put their lives on the line for the hope of a negotiated settlement with the people they're fighting? >> diane, we have not had a declared victory in a war with the possible exception of the

first gulf war, since world war ii. are the american people safer at the end because of the sacrifice these soldiers have made? have you succeeded in the mission? has become the substitute, if you will, for out and out victory. >> since secretary gates said he thought bin laden's death could be a game changer, we ask him, has it happened yet? >> not yet. it's really just too soon to tell. you know, it's only been a month. >> congress keeps saying in essence, hey, we've had a $10 billion a month is unsustainable. >> the cost is already coming down. we will be spending $40 billion a year less on these wars in 2012 than we did in 2011. congress is almost always impatient. i remember in the spring of '07, people saying this war is lost -- >> you said short-term thinking? >> yeah, absolutely. i can say that since i'm leaving in a few weeks.

>> as he prepared to leave his post, secretary gates shared a kind of airplane confession. from a kansas boy who still writes a personal note to the families of every fallen soldier. >> i go to the hospitals. i go to arlington. i see their families. so i feel the human cost and it's why i told somebody the other day maybe, in fact, it is time for me to leave, because these things have begun to weigh on me in a way that maybe i'm not as useful as i used to be. >> how do you experience it when you're home at night? >> well, that's when i do the condolence letters. >> it's just too much? >> yeah. it's time. >> it's time. secretary of defense robert gates. thanks to diane sawyer. thank you for watching abc news. hope you check in for "good morning america." they'll have more of that