Katherine McPhee's weight was the subject of much water-cooler conversation this season on American Idol. In the new issue of People we find out that it's been a battle for a long time, and was much worse than anyone could have suspected.

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.' "

"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."

...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 Eating Disorder 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."

Good for Katharine. Hopefully her message will resonate with young girls suffering similar pressures...