Talk about a restrictive diet.
"It's intense. And sort of awful," says the actress, 24 (who plays a call girl in her next flick, Chloe, in theaters March 26). "Yesterday for lunch? Spinach. Just spinach. Spinach and some seeds."
Esquire notes that she "scrunches her nose" when lamenting about the seeds.
For the interview, the actress -- who appears in the mag photographed in skimpy lingerie -- brings her own tabbouleh salad in a plastic container to a "deserted bar" that doesn't serve lunch so she can avoid temptation.
She says she's under a lot of pressure to be pin thin.
"If I didn't run and work out, there's no way I would be this thin," she recently told Glamour magazine. "But I have to stay in shape because I'm an actress. It's f----d up and it's twisted, but I wouldn't get the roles otherwise. If I'd been a bit bigger, I don't think they would have cast me for Mamma Mia!"
The stress has taken its toll. While talking to Esquire, Seyfried pops half a Lexapro pill (which treats anxiety) -- and a birth control pill (she's currently dating Dominic Cooper).
"Yeah, yeah, I'm anxious," she says. "And yes, I use birth control."