"Changed my life. Changed the things that I've done."

Deciding to lose weight was a snap decision for Mark Gittleson. At 265 pounds, the Rochelle High School senior says he used to feel tired every day and wanted to make a change.

"I was just fed up. Fed up with just being old same self. Overweight. Unhappy," says Gittleson.

He started working out and eating healthier last January, and by April, in time for prom, he'd already lost 50 pounds.

"When we tried on, just tuxes, we were just like, oh my gosh. As much as he'd lost, and then he just kept losing it and losing it," says Mark's mom, Michele Rogers Gittleson.

Mark dropped about 90 pounds total, but his change hasn't only been physical.

"He's just more self-confident. That's just... that's right there, and he can just see it. And you can see the smile on his face," says Rogers Gittleson.

"Just how he walks down the hallways now. He has a little bounce in his step," says Rochelle High School guidance counselor Marianne Swanson.

"People have been noticing me a lot more lately. Before, you know, I never really got know like a girl that much, but now, it's like... changed a lot," says Gittleson.

Later this month, Mark will be leading a fitness class for teenagers, and Swanson says he may just be the ideal teacher.

"The kids will probably listen more to somebody from their own age group, as opposed to always coming from adults. And Mark's walked the walk and talked the talk," says Swanson.

Until then, Mark says he's happy to keep exercising.

"Just staying fit is my... that's my favorite thing now. It's addicting," says Gittleson.

Mark says starting the process of getting fit is difficult at first, but after a while, it can become almost second nature. That class he's teaching starts Jan. 12 and is geared toward students in junior high and high school.