Transcript for 'Fit to Fat to Fit' Trainers Gain Weight Before Helping Others Lose Weight

We've all heard of the pitfalls of extreme diets and wacky workouts. But what you're about to see is a master class in yo-yo dieting. Imagine dedicating your entire life to fitness and throwing it all away to help someone else. Here's ABC's nick watt. J.j., nice to meet you, I'm ray. Good to meet you, ray. Reporter: Fit to fat to fit is a weight loss TV show with a big fat twist. J.j. The trainer had to gain weight, a lot of weight, before helping ray try to lose it. I have something to tell you. I'm going to be gaining 60 pounds from where I am right now so that I can better understand and relate to where you're coming from. We both wear our emotions on our sleeves. Cry a lot. It's going to be an hour of two men sobbing. In one aspect it breaks my heart you're doing that to yourself. Part of me is just sad for you. Because I know what it's like. It sucks. I don't want anyone else to go there. I'm sick. You seriously might need to get a garbage can. You gained 65 pounds? 184 to 245. That is awesome. 61 pound in the space of? 120 days, 16 weeks. That cannot be good for you. No. No it's not recommended. What are you doing? Just trying not to die. It's the hardest thing I have ever done in my life. I was not prepared for what it did to me mentally, emotionally. It wrecked me in every way. Too much rice. You're kind of turning into a complainer. Reporter: J.j.'s wife Erika is a nutritionist. Was he a bit of a nightmare? Yeah, a big nightmare. Let's just say he was way worse than I am ever, pregnant. Reporter: J.j. Was closely monitored by doctors. Increase toll goes off the charts, triglycerides are up, you become pre-diabetic, medically obese. Reporter: This doesn't come without risk. There's very limited data on the deliberate weight gain and then weight loss of healthy individuals. Even if it's temporary over a long period of time, we don't know what the cumulative effects of that would be. Rise and shine. Nuh-uh. I'm going to sleep until dinner. It felt like he was a totally different person. I didn't feel like he was my husband anymore. When I watched it I was thinking, what this is guy doing this to himself? Because he's nuts. I get a small glimpse and a new perspective as to the struggles that come from that place that you are in before you start that weight loss process. You finally understand what it's like to be the fat guy? I was in a very, very dark place for most of that 120 days. How are you doing, man? I'm doing good. Four months ago J.J., he was frickin' ripped. J.j. Is in the pudgy boy's club. Reporter: Then they both beat the bulge together. Thanks, brother. Start off killing it. I didn't go through four months of hell to have you give up on me. Prove I didn't waste four months of my life. Find your breath, find your rhythm. You got it. Four months ago, this was so easy. Today I feel like I'm dying. Reporter: Now ray's motivation, apart from J.J. And the TV cameras, is a big one. My wife and I are hoping to adopt. I don't think someone's going to place a kid with us at 400 pounds. You know? I want to be that dad who goes out and goes hiking with his kids. I want my children to be in awe of the man I am. Like I am in awe of the man my dad is. And I'm not going to do that at 389 pounds. Reporter: With basically a food addiction. You think most of this was just what ray was putting into his body? Yeah. Yeah, 80%. All the cliches are true. It's hard work and exercise. And you cannot outwork a bad diet. Reporter: With this, a bit of this, a lot of this -- Start off killing it. Reporter: Not too much of this, ray lost a whopping 147 pounds in just five months. Make no mistake about it, the cardiovascular system can be impacted on dramatic weight gain or weight loss. Some of those impacts might be positive, some might be negative. Oh my god. 242, are you freaking kidding me? Yeah, yeah! Oh my gosh, yeah! You lost a decent-sized human being. That's right. You lost you. Yeah. Reporter: Again, that much, that fast, can be dangerous. Again, doctors monitored ray the whole way. Ray's wife Julie also lost 75 pounds doing the exercises J.J. Gave to ray and the food plans from his wife Erika. The plan was not for you to lose weight as well? We live together. It's our lifestyle. So it wasn't difficult to jump on board. Reporter: Their entire lifestyle now totally changed. Instead of just sitting around watching TV at the end of the night, we'll challenge each other to a max blank contest. Pushup position but on your elbows. See how long you can stay up. This is what you do for kick in the evenings? Sometimes, yeah. Reporter: J.j. Is now back to his fighting weight of a little over 190 pounds. And has his identity back. Your identity is the buff guy. That was my identity. Now I feel like I am so much more than just the buff guy. Right? I feel like I'm more well-rounded and I come from a place of love rather than a place of judgment. Haven't you done yourself lasting damage? Oh, man, I don't know. Besides the few stretch marks I have on my hips, that's the only thing that I'm still carrying from this little adventure. You wear them with pride? I wear them with pride, I love those stretch marks. Reporter: Ray and Julie are now all signed up hoping to adopt a kid any day now. You could gain it all back in five months. Don't think I don't know that. Reporter: Filming finished in October. Ray has not let himself go. He has, in fact, lost even more weight. I'm that like 13-year-old high schooler, flexing in the mirror after a shower. Reporter: On the day we visit for the first time in his life ray climbed a rope. That's what I'm talking about right there, dude, whoo! Reporter: I'm nick watt for "Nightline" in Salt Lake City. "Fit to fat to fit" premieres Tuesday and A&E, owned in part by our parent company Disney.

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