"I was starting to cut out the carbs, so it worked out anyway," she tells PEOPLE in an exclusive video about her life in prison.
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Eating healthy wasn't always easy. In the morning, the options were oatmeal or bran cereal. For dinner, the choices ranged from hot dogs to tacos, and occasionally salad would make it onto the menu.
"In the beginning I tried everything, and then I started staying away from the meat," Real Housewives of New Jersey star says.
For much more from Teresa Giudice, plus an exclusive interview with her husband Joe as he prepares to serve his own prison sentence, pick up this week's issue of PEOPLE Magazine, on newsstands Friday
"In the wintertime, I would go to the gym," she says. "In the spring and summertime, I was constantly on the track, either walking or running."
In the gym, she would lift weights and use Jillian Michaels or Bob Harper workout videos.
By the time she was released in December, Giudice had dropped down to a size 2.
The prison offered exercise classes, and Giudice also participated in a competition against another fitness class.
"I did the longest plank, and it was 9 minutes and 19 seconds," she says. "So I was proud of myself for that."