"I'm a big believer in feeding your body what it needs," Greene, who plays bloodsucker Alice Cullen, tells the July/August issue of Women's Health. "Deny yourself something and you're going to end up shoving your face full of it."
That's why the 23-year-old actress, who recently signed on as the new face of Avon's Mark cosmetics, doesn't obsess over her weight and refuses to slim down to scary-skinny proportions.
"A lot of people like to be supertiny," she explains, "but I don't want a child's body -- I want a woman's body that is extremely fit. It's so much sexier."
Greene shows what she means in "The Twilight Saga: Eclipse," which arrives in theaters June 30. The action-packed film required the 5-foot-5 star to get into fighting form with a regime that included cardio, squats, and planks.
She even challenged her cast mates to squat contests. "I'm a competitive little sucker," Greene laughs.
When it comes to fashion, however, Greene takes a more laid-back approach. "On the red carpet, it's complete glam. But at home I'm a jeans and T-shirt kind of girl," she tells the magazine, which hits newsstands June 29. "Simple can be beautiful."
In related news, read what Kristen Stewart told Elle about her red-carpet attitude.