We start with news from the world of weight loss. Scientists followed contestants from "The biggest loser." They found when the cameras went off, the weight often came back on. There are hard and... See More
We start with news from the world of weight loss. Scientists followed contestants from "The biggest loser." They found when the cameras went off, the weight often came back on. There are hard and surprising findings about our bodies. Here's Lindsey Davis. Reporter: It was a record-breaking moment. After shedding an astonishing 239 pounds, Danny cay hill became the contestant to lose the most weight ever on the biggest loser kwth. He went home with a quarter million dollars literally half the man he used to be. Your starting weight is 430 pounds. But it was what came after the afterphotos that proved the biggest shock to Danny and many of the other contestants. I didn't want anybody to know I was on the biggest loser. I didn't look like that anymore. Reporter: Little did they know their metabolism dropped. What would turn out to be a setback for the majority of contestants would prove to be a breakthrough to understanding why so many dieters struggle to keep off the weight they lose. In a new study that has everybody talking, scientists followed 14 of the 16 contestants from season 8 to the biggest loser kwth. Monitoring them for six years after the show. All but one gained some of the weight back. Four of them reported being heavier now than at the beginning of the show 123? When you attempt to fight your body weight, your biology will fight back. Reporter: This scientist says while they had a Normal metabolism at the start. Years after their metabolism slowed down. A mechanism he says the body uses to return to the original weight. Your body's metabolic rate will slow down. The number of calories you need is going to decrease. That makes it difficult for you to keep the weight off. Are you ready? Yeah. Reporter: Huge implications at a time when we're inundated with weight loss shows and celebrities going public with their struggles. I have eaten bread every single day. Reporter: Struggles that six years later Danny continues to face. I've had people look at me and tell me I ought to be ashamed of myself. I ought to walk around the block a few times. Reporter: As the winner he felt he also had the most to lose. 430 pounds before the show, 191 pounds right after. And since then he's gained back more than $100 pounds. Was this sustainable the weight loss that you had? It's not at all. When I went to the ranch I was there for 90 days. I quit my job. That's why I won the show and that's why I did what I did. I made it my job. But we're finding out that in order for us to keep our calorie burn at the same rate of a Normal person, that we're having to work out two hours a day. Reporter: He says in order to maintain his current 295 pounds, he has to eat 800 calories less than a typical man his size. I started eating like a Normal man would to be even. And I wasn't staying even, and that was what was dishartening, and I heard this from all the biggest loser contestants. It's difficult to keep weight off because your body doesn't want to do it and your desire to eat increases. It's the perfect storm for weight regain. Reporter: It might explain why there were plummeting levels of leptin among contestants. I kind of failed, here I went on national TV and became this person for people to look up to and now I've gained this weight. Reporter: She was also part of the study. She was 248 pounds before the show. Down to 173 at the end. And now back to 204 pounds. She says the results of study have helped her understand why it's taken so much effort to maintain her ideal weight. I have no idea what it's going to be like in 15 years as my metabolism continues to slow. I guess that's what I'm afraid of. Reporter: A fear that's led her to question her decision to join the show in the first place. Had I known what I know today, I probably would have not done the show. Reporter: But the creator of the biggest loser kwth defends his show. When people leave their show they're better off than being 400 pounds. I think the study could have done things a little differently. I would like them to take 14 people who kept the weight off. Reporter: He said the studies may have unintended negative results. Any time you tell somebody it's not them, it's something else, it gives them kind of a way to excuse the fact that they really want to eat that. Reporter: He says for him and many others it's helped relieve some of the crushing shame that came from the weight gain. I've heard from obese people all over Facebook and Twitter, everybody is messaging me going I knew this was going on. This makes me feel better about where I am. Not an excuse, but at least some of the shame is lifted and now we say where do we go from here. Reporter: One of Danny's closest friends on the show, 26-year-old Amanda. It's great the world knows that biggest loser contestants, we aren't lazy and didn't waste an opportunity and we're not just gaining weight back. Reporter: Over the course of the season she lost 87 pounds. Since the show wrapped seven years ago she's now gained back 12 of those pounds. Giving hope to many that weight loss can sometimes be maintained. And joining a list of celebrities who have gone public with weight loss that appears to be permanent, but she says it's a difficult daily struggle. To maintain my weight, I have to eat less than 1400 calories a day. It's almost nothing. I'm not full off that much. And I find myself in a constant, well, maybe I shouldn't eat right now, and it is almost like an eating disorder in way. It's like I have to restrict everything I do. Reporter: Like Danny, she breathed a sigh of relief when the most recent study was released. Finally an explanation for her struggle. She's concerned about the long term ramifications. No. Now I know I'm burning 600 calories less per day than I was six years ago. What does the future hold for me? It's like am I going to gain the weight back? Is my destiny going to be an over weight woman because my body is fighting me. Reporter: She says one of the most important motivational tools is of all things instagram. I've made sure that I go on social media a lot. I have over 83,000 followers on instagram. I want to be as real as I can with people. I found that going that keeps me accountable. Reporter: Tonight add to that one and a half million "Nightline" viewers. But no pressure. I'm Lindsey Davis in New York.
This transcript has been automatically generated and may not be 100% accurate.