this is "nightline" -- >> tonight who is bowe bergdahl. we unravel the mystery of the young sergeant. why did he allegedly abandon his post. why is the military stopping his parents from calling him and welcoming him back? and what did he write in his last note before disappearing? plus, big, fat surprise. is everything you know about dieting wrong? radical new research may show that fatty food is not only not bad, it's good for you. so, can you really eat this and still look like this?

but first, the "nightline" 5. >> with expedia you always get the lowest price. book any flight or hotel and if you find it for less we'll match it and give you $50 off your next trip. expedia, find yours. >> wait. prilosec is not made for immediate heartburn relief. zantac ♪ take the nestea plunge. nothing refreshes like nestea.

heat shields are compromised. weare those thrusters burning? that's a negative. what's that alarm? fuel cell two is down. i'm going to have to guide her in manually. this is very exciting. but i'm at my stop. come again? i'm watching this on the train. it's so hard to leave. good luck with everything. with the u-verse tv app, the u-verse revolves around you the u-verse revolves around you good evening. as we come on the air tonight, sergeant bowe bergdahl is recovering at a military hospital in germany. we learned today doctors are so worried about his mental health

after five years in taliban custody that they have told his family not to reach out even just to say welcome back. of all the questions swirling around this young man, perhaps the most intriguing is why did he leave his base in the first place? tonight, abc's martha raddatz probes the mystery -- who is bowe bergdahl? >> reporter: this is the bowe bergdahl the world knows. >> release me, please, i am begging you. >> reporter: pleading to the camera in proof of life videos released by the taliban. but this is the bowe bergdahl sherri horton knows doing something he loved. >> he was a wonderful partner. all the girls enjoyed dancing with him he was strong and steady. >> reporter: sherri was bergdahl's ballet teacher and later his roommate in idaho. >> he is a very interesting guy. very quiet. he was an observer you. couldn't put him in a cubby hole. he was kind of his own person. his own, he was very much an

individual. >> reporter: but tonight his family and friends can't even speak to him to say welcome back yet. the doctors at the military hospital in germany concerned that years of isolation and captivity have resulted in deterioration of the army sergeant's mental and physical health. and he has yet to be questioned by military authorities. how did bergdahl disappear? the commander-in-chief says that is beside the point. >> we still get an american soldier back if he is held in captivity. period. full stop. >> reporter: but that has not stopped the increasing questions about bergdahl's captivity. perhaps most importantly, did the young soldier dessert. sherri horton doesn't know the answer to that despite her unwavering support. >> the bowe i knew wasn't a quilter wasn't a deserter, this isn't what i would have seen him doing. i don't know for a fact that this is what he did.

nobody does. >> reporter: but those who served right next to him in afghanistan say he intentionally left his post. >> it was premeditated. it was thought out. >> reporter: cody full roomed with bergdahl before their deployment. >> he was not forcibly taken off the base. i don't think that somebody that deserted their platoon mates in a time of war should be haeblab dessert and get away with it. could he be punished? the army today said it is preparing a comprehensive review of bergdahl's disappearance and captivity, a review that will include speaking to the former prisoner. according to the chairman of the joint chiefs of staff, when bergdahl is ready to provide them we will learn the facts. like any american he is innocent until proven guilty. adding, our army's leaders will not look away from misconduct if it occurred. given the years in captivity, defense officials say, punishment is unlikely.

but an investigation could answer the question why would bergdahl leave the base? in the first hostage video, bergdahl was asked about soldiers who go awol or dessert. >> they are deserting, they know this is wrong, they just want to go home to their families. >> reporter: senior defense officials tell abc news, bergdahl wrote a note expressing disillusionment with the movement in afghanistan, and may have a better way to deal with it. that does not come as a surprise to nose who knew him. >> i guess in his mind he was dissatisfied with the united states army over there. >> there were times when it got to be a little much for him. he would go hike into the woods and just sit and meditate for an hour. >> reporter: and controversy continues to swirl about the deal. swapping bergdahl in exchange for five, high-level taliban

prisoners held at guantanamo, leaving many wondering did president obama make a bad deal? >> good afternoon, everybody. >> reporter: critics of the bergdahl deal say president obama gave up too much in return for sergeant bergdahl. >> these taliban operatives that we released from guantanamo were not foot soldiers. the deputy intelligence minister, two governors, a provincial chief, the top cadre of taliban leaders. i suspect they will return to terrorism. >> reporter: the men arrived in qatar to a hero's welcome. joined by the families. a qatari official saying the only restriction placed on them. a ban from traveling outside of qatar for a year. >> nearly a third of the detainees, left guantanamo, 200 of 600 are confirmed or suspected of returning to terrorism. today the president defended the decision. >> and we have confidence that we will be in a position to go

after them if in fact they are engaging in activities that threat in our defenses. >> reporter: in addition, gordon says that with the bergdahl deal president obama may have set a dangerous precedent. >> if the taliban could get five of top leaders out of guantanamo, imagine if they got a war hero. >> as a parent. i can't imagine the hard ship you guys have gone through. >> reporter: for bergdahl's parents standing beside the president in the rose garden over the weekend. no effort seemed outside consideration. how far would a father go? his father bob grew a beard and learned the language to connect with captors, even tweeting to a taliban spokesman just before his son's release. i am still working to free all guantanamo prisoners. abc news has learned that a video of bergdahl which emerged earlier this year, and seen only by the u.s. government, showed

an increasingly frail and disengaged bergdahl. for sherri horton, it has been painful to see the dancer she once knew fade away while being held by the taliban. >> he has always been a strong guy. and the last picture that were coming out. he was not. and so that's the difference i could see. at the time i actually stopped watching the videos. >> i want to go home. >> reporter: but the army will certainly be taking a hard look again at those videos. still so many questions remain. for "nightline," i'm martha raddatz in washington. >> intriguing and mysterious story. our thanks to martha for the report. next on "nightline," the woman behind a controversial new high fat lifestyle. she argues, you can have steak for dinner and lose weight. or chronic idiopathic constipation. linzess is thought to help calm pain-sensing nerves and accelerate bowel movements.

it helps you proactively manage your symptoms. do not give linzess to children under 6, and it should not be given to children 6 to 17. it may harm them. don't take linzess if you have a bowel blockage. get immediate help if you develop unusual or severe stomach pain especially with bloody or black stools the most common side effect is diarrhea, sometimes severe. if it's severe, stop taking linzess and call your doctor right away. other side effects include gas, stomach-area pain and swelling. bottom line, ask your doctor about linzess today. ♪ yeah ♪ don't stop now, come on mony ♪ come on, yeah ♪ i say yeah ♪ yeah

♪ yeah ♪ yeah ♪ yeah ♪ yeah ♪ yeah ♪ 'cause you make me feel ♪ like a pony ♪ so good ♪ like a pony ♪ so good ♪ like a pony [ male announcer ] the sentra with bose audio and nissanconnect technology. spread your joy. nissan. innovation that excites. ♪ mony mony innovation that excites. you've reached the age where you've learned a thing or two. this is the age of knowing what you're made of. so why let erectile dysfunction get in your way? talk to your doctor about viagra. 20 million men already have. ask your doctor if your heart is healthy enough for sex. do not take viagra if you take nitrates for chest pain... it may cause an unsafe drop in blood pressure. side effects include headache, flushing, upset stomach, and abnormal vision. to avoid long-term injury, seek immediate medical help for an erection lasting more than four hours. stop taking viagra and call your doctor right away if you experience a sudden decrease or loss in vision or hearing. this is the age of taking action.

viagra. talk to you doctor. bhe gets a ready for you alertfd the second his room is ready. books at laquinta.com, so he knows exactly when he can prep for his presentation. and when steve is perfectly prepped, ya know what he brings? and that's how you'll increase market share. any questions? can i get an "a", steve? yes! three a's! amazing sales! he brings his a-game! la quinta inns and suites is ready for you, so you'll be ready for business. the ready for you alert, only at laquinta.com! la quinta!

is it possible that everything we have long held as dietary gospel is backwards? tonight you will learn a mom/investigative reporter who says all of those high-fat foods we have been told to avoid are actually good for us, not only that, she says if you eat things like cheeseburgers, meatballs, and bacon, all my favorite foods, you may actually lose weight. here is my "nightline" co-anchor juju chang. ♪ >> reporter: it is an age-old debate, low carbs? or low fat? some dieters like gwenyth paltrow, and kim kardashian

advocate atkins to lose the pounds. some suggest kim kardashian could be on to something. you can still have the body without putting down the bacon. but the food revolution isn't just being waged by celebrities on the red carpet. >> we don't have any low fat foods in our fridge. >> reporter: but also by this unassuming new york city mom. >> we have heavy cream, half and half, whole milk, we don't have low fat milk. >> reporter: nina is author of "big fat surprise." >> the surprise is that saturated fat is really not bad for health. it does not cause heart disease. >> thank you. >> reporter: she is not a scientist, but she is on a mission to literally turn everything we know about nutritional guidelines on its head. >> mine family we basically eat the usda food pyramid turned upside down. they have meat at the top. we would put them closer at the bigger slab on the bottom. >> so we want to lunch with her

at a carnivore's delight, strip house steakhouse in new york. she spent the decade analyzing revered traditional and medical data to prove a radical idea. that fat may not be the devil we have been taught it is. >> when you take in carbohydrates your body does something come letly different than what it does when you take in protein or fat. it trigger the release of insulin, insulin is the king of all hormones for storing fat. >> you are saying my body reacts better to bacon than a bagel? >> exactly. >> reporter: how is it saturated fats became demonized. >> the story begins in the 1950s when the nation was terrified of the heart disease epidemic. >> the chief killer of americans is cardiovascular disease. >> reporter: the founding father of the low fat diet produced a landmark study to see what might cause heart disease. he examined 13,000 subjegts. but only looked at the diets of

a few hundred men. that study set the government on its path to recommending a low-fat diet in order to prevent heart disease. >> a lot of people are faithfully following the dietary guidelines. >> we have changed the obesity rates keep climbing. >> it tells you our working high pohigh -- hypothesis isworking. >> reporter: as a carnivore, she says she lost 10 pounds 2001 trying. what does make you fat? >> the best science that we have so far points to carbohydrates. if your diet ties high in carbohydrates, regardless of the kind that they are, it's harder for you to lose weight and you are at higher risk for heart disease and diabetes. >> many scientists are beginning to agree as does our waiter. >> what is your reaction to the fact that steaks may be good for you? >> it is great. we have known it for years here. >> reporter: not everyone thinks

we should throw out the low fat guidelines. >> i don't think it is enough to look at existing studies we have and say they're not perfect. that's not an indicator to go out and say sure let's eat more saturated fat. i think nutrition it is very difficult to get perfection. but i think where we are at, we have enough suggestive evidence to say -- you know what? eating a lot of saturated fat may not be the best idea. it's one thing to say, okay. saturated fat may be bad for you. is it responsible to say well let's stock up on high fat foods? >> the last decade of rigorous clinical trials and long term trials of two years really definitively showed that a high fat diet is better than a low fat diet. >> other experts say the trials are far from definitive. while the results show that high fat diets may not be harmful, they're not necessarily protective either. but that isn't stopping her, she is even changing the rules at home for her two young boys.

>> so a typical breakfast is bacon, or eggs, sausage, meatballs, sometimes. >> do you get sick of meatballs and sausage? >> no. >> reporter: do you wish you had cereal instead? you probably don't know what cereal is. >> reporter: she says she snacks all day. mostly on cheese. nuts and protein. >> i can eat red meat three times a day. i could have a cheeseburger for lunch. think nothing of having a cheeseburger for dinner. i don't think that's killing me. >> reporter: or is it? when we learned she didn't know her cholesterol levels, we asked if she would be willing to find out. >> hi, i'm here to see the doctor. >> reporter: she got tested off camera. >> the first door on the right. >> reporter: now her doctor is about to tell her the full results. >> what's the overall verdict? >> unless you have a very strong family history of premature heart disease, this looks fabulous. you are good. it is very, very low.

these are good numbers. >> yea! >> reporter: the self-appointed guinea pig is clearly relieved. >> so i should just keep doing what i am doing, right? whatever i am doing. however bizarre it sounds? >> in my judgment. whatever you are doing is working for you. >> reporter: like many physicians, the doctor remains cautious about going overboard on a high-fat diet. >> i couldn't say that eating in this fashion is appropriate for everybody. clearly if you do eat in this fashion, you probably should be checked by your physician at some point. >> that feels great. i am going to celebrate with a steak tonight. >> reporter: for "nightline," juju chang in new york. >> our thanks to juju chang. the "big fat surprise" is at bookstores now. >> the secrets of what america's first woman in space kept hidden during all those years in the spotlight. i'm trying my best.

seriously, i'm...i'm serious. request to come ashore. geico. saving people money on more than just car insurance.

♪ abe! get in! punch it! let quicken loans help you save your money. with a mortgage that's engineered to amaze!

"i wish i'd done this sooner." don't let that be you. you know your teeth are important. so don't put it off any longer. ♪

sally, we hardly knew you.

sally ride lived a life in the spotlight as america's first woman to go to space. now two years after her death comes a new book spilling some surprising secrets. >> liftoff of sts 7 and america's first woman astronaut. >> there was so much she kept hidden during all those years in the spotlight. >> it sure is fun. >> sally ride was the exuberant young physicist who in 1983 became the country any first female in space. [ indiscernible ] i'm going to drink chocolate milk running on the treadmill. >> and spent decades inspiring girls. during all that time there was a huge part of her life that was secret only to be revealed in a single line in her obituary two years ago. >> i think probably the thing most people were surprised to learn about sally was the fact that she had been living with another woman for 27 years. >> why was she so insistent on privacy on this score? >> there was no question in my mind, but that if sally ride had

been openly gay and if she had applied to nasa, number one she never would have been selected as an astronaut. number two never would have flown, and number three, never would have been first american woman in space. >> if she hadn't been private she wouldn't have made history. >> if she hadn't been private, she wouldn't have made history. >> lynn sherr first met sally ride covering the shuttle program. >> lynn sherr, abc news, florida. >> we hit it off immediately. >> reporter: she has written a new biography on ride which reveals many secrets including the fact amidst the hoopla of her first shuttle ride. >> the thing i will remember most about that flight, it was fun. and the most fun in my life. >> with her face on magazine covers, sally ride felt the need to see a shrink. >> i didn't realize the psychic price she paid for her fame. this was a woman who was genetically an introvert. she really was quite shy.

she sacrificed a lot to be the public figure that we wanted her to be. >> notwithstanding her shyness, behind the scenes, ride was capable of bold action. the new book reveals that on a trip to the eastern bloc at the height of the cold war, she defied strict orders from the state department and had a secret meeting with a russian female cosmonaut. >> sally had purposely worn that night under her blouse a t-shirt that she had worn on orbit during the sds 7 flight and said to vher i would look to give yo this. >> the investigation into the challenger explosion, she was handed a key document showing that nasa had knowledge of the faulty equipment believed to have led to the disaster. she wanted the troouth how to g out and wanted to protect her source as well. so she quitely leaked it. >> this was a smart, brave, at

times, sly, calculating woman? >> she was a great diplomat. she would have been an extraordinary, senator, congresswoman, president, you name it. >> sally ride died two years ago at age 61 from cancer. >> what do you conclude based on all these surprising things you learned about her in the course of writing the book? >> she was the person that i thought she was. but she was much more. she had so much more to her. there were so many deeper layers that she chose not to let too many of us know about. >> a remarkable woman. lynn sherr's new book called "sally ride, america's first

♪ [ barks ] whoo! mmm! ♪ ♪ oh, yeah [ whistling ] [ male announcer ] discover your new orleans. start exploring at followyournola.com. [ woman ] and i love new orleans!