On tonight's episode of A&E's groundbreaking new series "Fit to Fat to Fit," New York City personal trainer Katie Mack takes on the challenge of packing on 40 pounds in order to shed the weight alongside her client, chef Mateo. In a set of sneak peeks, Mack clashes with Mateo when she demands that he give up bacon while she's training him. But he starts to soften up when he realizes the emotional toll Mack's weight gain has taken on her.

"During the time that we're working together, what if I said you can't eat any more bacon?" Mack asks him during a consultation in the first sneak peek.

"What are you talking about?" asks Mateo. "That's not gonna happen. Don't tell me I can't eat what I want. I won't have it."

"I'm not God, but you can't eat what you want," she replies, her voice getting louder.

Mack opens up to the cameras about how her 40-pound weight gain has affected her emotions and her mood in the second, extended clip.

"I have no desire to work out or exercise," she tells cameras.

Mateo watches as Mack struggles to perform overhead kettlebell swings and push-ups and, together, they watch fitness videos that Mack recorded four months ago, before she packed on the pounds.

"In a very short time, I feel like you're gonna get back to there," says Mateo, but Mack is discouraged.

"She sacrificed her emotional well-being, and she can't put the pieces together and say that it was worth it right now," Mateo tells the cameras. "But she showed me the commitment and that's gonna make me wanna commit too."

Watch a clip below and be sure to catch tonight's episode of "Fit to Fat to Fit," airing at 10 p.m. ET on A&E.

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