Trainers on A&E's Shocking New Fitness Show Packed on Pounds - Then Lost It With Clients

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Oh, how the tables have turned! ET has a sneak peek at the new A&E show that's putting trainers in their clients' shoes, Fit to Fat to Fit.

The show features an emotional and risky experiment -- 10 trainers gain an excessive amount of weight and try to lose it immediately in order to better understand and help their clients.

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Trainer J.J. Peterson packed on 61 pounds in 120 days, putting his health at risk. His method was simple: he stopped working out and ate as much as he could.

"It was usually six meals a day and 1,000 calories, roughly, at each of those meals," J.J. told ET. "The bloodwork got a little bit scary toward the end. I was pre-diabetic and clinically obese."

It took months for J.J. to get back into fighting form, and the journey proved to be more difficult than he imagined.

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"I thought in my head that I could still do these things -- I could still work out at high intensity or high level, and I fell on my face," J.J. said. "That was very eye opening for me. That was tough."

J.J.'s client, Ray, went from 389 pounds to 235 pounds. Watching J.J.'s struggle gave Ray more motivation to keep the weight off.

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"It was really cool knowing he was on the other side getting a taste for what it's like," Ray told ET.

Fit to Fat to Fit premieres Jan. 19 at 10 p.m. ET/PT on A&E.