Reality Show 'Fit to Fat to Fit' Makes Trainers Gain Weight Before They Can Help Others Lose Weight

— -- It's a weight-loss show with a big, fat twist.

On the new A&E show "Fit to Fat to Fit," personal trainer JJ Peterson had to gain 60 pounds before he could help Ray Stewart try to lose weight.

"We both wear our emotions on our sleeve," Stewart told "Nightline" about his experience on the show.

"We cry a lot," Peterson told "Nightline."

"It's going to be an hour of two men sobbing," Stewart added.

Peterson went from 184 pounds to 245 pounds in 120 days, which he does not recommend doing.

"It was the hardest thing I've ever done in my life," Peterson said. "I was not prepared for what it did to me mentally and emotionally. It wrecked me in every way."

Once Peterson gained the weight, both he and Stewart then tried to beat the bulge together. Peterson was closely monitored by doctors during his weight gain.

"It went from a kind of funny...experiment to 'If you don't change what you're doing you will have a major cardiovascular event,'" Peterson recalled. "Your cholesterol goes off the charts and your triglycerides are up, and you become pre-diabetic and clinically obese."

Peterson's wife Erica Peterson, a nutritionist, joked that her husband was a "big nightmare" during the experience.

"I just felt like he was a totally different person. Like I didn't feel like he was my husband anymore," Erica Peterson told "Nightline." "I don't really care if he has a six-pack. I mean it's nice. It's more just like all the things he loves are health and fitness so when you take that away from him, he has nothing really to be excited about."

"I was in a very, very dark place for most of those 120 days," JJ Peterson said.

While working out together, Peterson and Stewart helped each other to see the light in their weight-loss journey. And now, Stewart has a new motivation, apart from Peterson and the TV cameras.

"My wife and I are hoping to adopt. I don't think someone's going to place a kid with us [if I weigh] 400 pounds," Stewart said. "I want to be that dad who goes out 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."

After just five months of exercising and dieting, Stewart lost a whopping 147 pounds. Losing that much weight that quickly can also be dangerous, so doctors monitored Stewart during his entire experience. But Experts caution that sudden weight gain or loss can be risky.

"There’s very limited data on the deliberate weight gain and then weight loss of healthy individuals," said Dr. Jennifer Ashton, ABC News’ Senior Medical Contributor. "So for someone to do that and subject themselves to that type of situation even if it’s temporary over a long period of time, we don’t know what the cumulative effects of that would be."

"Make no mistake about it, the cardiovascular system can be impacted on dramatic weight gain or weight loss," Ashton continued. "Some of those impacts might be positive, some might be negative."

Stewart's wife Julie also lost 75 pounds during the process.

"It was a nice benefit," Julie Stewart told "Nightline." "We live together. It's a lifestyle, so it wasn't difficult to jump on board."

Peterson is now back to his fighting weight of a little under 190 pounds.

"Now I feel like I am so much more than ‘The Buff Guy!' I feel like I'm more well-rounded and I come from a place of love, not judgment," Peterson said. "Besides the few stretch marks that I have on my hips, that's the only thing that I'm still carrying. I wear them with pride. [I] love those stretch marks."

Since filming ended this past October, Stewart hasn't let himself go and lost even more weight.

"I'm that 13-year-old high schooler flexing in the mirror after a shower now," Stewart said.

But Stewart is aware that it's possible he could one day gain it all back.

"Don't think I don't know that," Stewart said. "I've got 37 years of bad habits, and I've got six [to] eight months of good habits."