"There are times when I go to the gym and really try, and there are times when I just don't," the actress, who turns 23 on Wednesday, tells W magazine in its May issue. "I gain a pound, I lose a pound. But I think I've developed a really good sense of when I'm doing something for myself as opposed to when I'm doing something because of other people's expectations of me.
"And honestly, even if I wanted to be anorexic, I just don't have what it takes. After four hours of being anorexic, I'd be like, 'It's been four whole hours! Feed me!"
Ferrera is on the rise. She won Golden Globe and Screen Actors Guild awards this year for her role as awkward assistant Betty Suarez on "Ugly Betty," and has appeared on a host of magazine covers.
Salma Hayek, an executive producer of the ABC comedy, charmed Ferrara into accepting the part.
"Honestly I never saw myself doing TV, but Salma was so convincing," says Ferrera, who's starred in the movies "Real Women Have Curves" and "Sisterhood of the Traveling Pants." "Salma is the kind of person who could sell you, like, a used stereo. She promised me that it would be done in the right way, and I just trusted her."