The ending of the novel may seem dramatic and a bit weird, but it is actually quite simple: Marian is so extremely fed up with her situation at work, at home, in her relationship, with Peter, with her friends, and with life in general that her eating disorder did nothing but to draw the last straw. She basically snapped. How couldn't she? She was starving, and unhappy. So, her best bet was to do her final demonstration to Peter about her unhappiness: She faced her problem and baked a cake in the shape of a woman to represent how she felt: Consumed and edible. When she finally expressed herself, she basically told Peter that this was the way that he saw her, and to go ahead and eat the cake then. Since he was very shocked at her behavior and she was in the middle of what seems to be an anxiety attack, he leaves her be and she eats the cake herself. Now, this last eating part was basically a symbol of going back to the person that you were now that you have scared off what was eating you alive: Nothing is eating her from within now- She is back to eating IT!