"Have your cake and don't eat it, too" joke

Keira Knightley recently rejected suggestions that she was anorexic. She said her grandmother and great-grandmother both suffered from anorexia, so she was very aware of the problem, but Knightley assured the press, "I'm quite sure I don't have it..."
She continued, "...Because I eat and throw up, which is definitely bulimia and not anorexia." (She knows her eating disorder facts.)