Katharine McPhee's Bulimia Battle

By Ericka Sóuter and Monica Rizzo

06/22/2006 at 08:00 AM EDT

American Idol's Simon Cowell called runner-up Katharine McPhee's voice "the best of the competition." What hardly anyone knew, however, was that she'd risked destroying that voice: She'd struggled with bulimia for five years, and the self-induced vomiting could have taken away her ability to sing.

At her worst point, she was throwing up as many as seven times a day, which is like "putting a sledgehammer to your vocal cords," she tells PEOPLE in its new issue.

Finally, after successfully auditioning for Idol last fall, McPhee, 22, decided to seek help. "When I made it onto American Idol, I knew that food – my eating disorder – was the one thing really holding me back," she says. "I was bingeing my whole life away for days at a time … So when I got on the show, I said, 'You know what? I can do well in this competition. Let me give myself a chance and just get ahold of this thing.' "

Her Aug. 18, 2005 Idol audition

Ray Mickshaw / WireImage

With the support of her parents, Peisha, 52, and Daniel, 57, and her actor boyfriend, Nick Cokas, 41, in October McPhee enrolled at Los Angeles's Eating Disorder Center of California, where she spent three months undergoing group and individual therapy, six days a week.

"I knew I had put off going to a treatment center long enough – I'd been struggling with bulimia since I was 17," she says. "Growing up in Los Angeles and spending all those years in dance class, I'd been conscious of body image at a young age, and I went through phases of exercising compulsively and starving myself. … Food was my crutch; it was how I dealt with emotions and uncomfortable situations."

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Finally, her program at the Eating Disorder Center helped her gain control of her relationship with food – but it wasn't easy. "I really had to surrender and give up having a free life to do the program, because I'd be there from 9 in the morning until 7 at night. … I remember that first night, my dad holding me, crying and saying, 'I don't know why you have to suffer through this, but it's going to be okay.' "

McPhee and boyfriend Nick Cokas in June

Marcel Thomas / FilmMagic

Now, McPhee, whose first single comes out June 27, has dropped 30 lbs. as a result of the "intuitive eating" approach she learned at the center. "I learned that there's no such thing as a bad food," she says. "If you look at a doughnut, people think it's a fattening food – why? Because if you eat it you'll get fat? No, you'll get fat if you eat 10 doughnuts."

Although McPhee admits she's still learning to deal with stress – "I still bite my nails," she says – she's much happier these days. Of getting treatment for her illness, she says, "That's why I say American Idol saved my life, because if I hadn't auditioned I don't think I would have gotten a handle on food."

Pick up the new issue of PEOPLE, on newsstands Friday, for the full story.

For more on Katharine McPhee, visit TeenPeople.com.