The 29-year-old, who plays a struggling factory worker in the musical drama, said she went on the extreme diet to lose plenty of weight to play the character, reported Contactmusic.
"I'm doing some crazy weight stuff right now. I'm on day six of detox... This diet makes me break out, so I love that. Nothing like living on hummus and radishes and then be all, 'And I got a pimple, Yeah!," she said.
Hathaway previously said that she lived on kale and dust during shooting of "The Dark Knight Rises" in order to wear the skintight Catwoman suit.
"It was a psychological terrorist. The suit, thoughts of my suit, changing my life so I would fit into that suit... It dominated my year," she said.