02/11/2014 at 12:00 PM EST
When Rachel Frederickson revealed her new, size 0-2 figure on the Feb. 4 finale
of NBC's The Biggest Loser
, she thought she would wow friends, family and viewers – and maybe win the title of Biggest Loser.
She did win, but the 24-year-old voice actress also ignited a firestorm of controversy
about her dramatically slimmed-down figure, down to 105 lbs. from her starting weight of 260 lbs. (At 5'4", her height-weight ratio puts her BMI at 18, below the threshold doctors consider normal.)
Sitting down with PEOPLE in Los Angeles, just days after she made headlines for her striking transformation, Frederickson says she maintained a 1,600-calorie daily diet, working out six hours a day in the three months before the finale.
"Maybe I was a little too enthusiastic in my training to get to the finale," she tells PEOPLE exclusively in this week's cover story.
Asked point blank whether she has an eating disorder, she replies, "I am very, very healthy."
Producers say she was carefully monitored throughout her seven-and-a-half months with the show, including in the days before the finale. "Rachel passed all the required medical tests ensuring she was healthy," says the show's executive producer, Dave Broome.
Still, Frederickson's trainer, Dolvett Quince
, admits he was "shocked" by her appearance.
"The first thing that went through my mind was, 'That's just too much,' " he says.
For much more on this story, including personal photos of Rachel through the years and interviews with former
Biggest Loser contestants, pick up this week's issue of PEOPLE
Reporting by PATRICK GOMEZ and RAHA LEWIS