Fallon Mercedes is a personal trainer and celebrity fitness correspondent based in Los Angeles, and she's never known what it's like to be overweight. But in order to empathize with her client JD, Mercedes is throwing her healthy eating habits and active lifestyle to the wind in order to pack on more than 40 pounds so that she can join JD on his fitness journey for A&E's groundbreaking reality fitness series "Fit to Fat to Fit."

In a series of sneak peeks for the Tuesday (March 8) night episode, Mercedes meets JD, a red carpet videographer who is close to 400 pounds. He wants to start a family and hopes that losing weight will help him find love, and Mercedes agrees to gain weight so she can help him realize his goals.

"I'm doing this project because I want to tell my clients that if I can do it, you can do it too," Mercedes says in the short clip.

A health professional tells Mercedes that by putting on 40 pounds in four months, she put her body into shock. Mercedes reveals how her new, high-calorie diet has affected her body.

"I had to have my tooth removed because I've been eating tons of sugar and I've had cramping, out of control bloating," Mercedes explains and she starts to cry. "It scares the crap out of me that I don't have control of my health."

"JD is the most unfit client that I've ever trained," Mercedes says in an extended clip. "We're gonna go through this weight loss process together so we have to be on the same team."

"My muscles are so sore," JD says. "She's pushing me a little bit harder than I'm quite ready for."

Mercedes coaches JD through a series of cardio workouts, from jumping rope to throwing punches at a punching bag to riding an exercise bike. But during a workout on the beach, JD collapses in the sand and it seems as though he's ready to give up on his weight loss journey and Mercedes gives him a pep talk.

"You have more to give," Mercedes tells JD.

"I don't," JD responds.

Watch the extended sneak peek below and be sure to catch tonight's episode of "Fit to Fat to Fit," airing at 10 p.m. on A&E.