The main idea, or main theme, of Margaret Atwood's novel The Edible Woman is that social expectations are seldom in tandem with our freedom of choice. As a result, the clash between what we want to be versus what social constructs nearly guide us to become will render us unable to separate what is real and what is a mere paradigm.
The novel, itself, was written during the strongest part of the Canadian Woman's Liberation Movement. Although there is no mention of such movement in the novel at all, Marian is an extract of the times, wondering exactly what is her role within a society that is changing as fast as she is.
Marian is a young, and very dissatisfied woman. She obviously finds no motivation nor "primal call" anywhere around her. Her job does not challenge her. Her girlfriends are all typical women who follow the tenets of female behavior as they are expected to be: One is a femme fatale, the other one, a submissive wife and mother. Marian's male companions are no different in terms of motivating her. The man whom she is going to marry bores her and the man with whom she has indiscreet sex simply serves as an alternate for a boring relationship.
Above all this, there are the expectations bestowed upon Marian: The fact that she has no other choice but to follow the roles of every woman that she has met in her life: She can either become a "
lose cannon" or a devoted wife. But Marian cannot find herself in any of those roles, therefore, she feels as if she is "eaten" alive.
We know that, in the midst of her crisis, she becomes fixated with food. She first begins to reject it naturally thinking that she may have just developed new aversions. However, the rejection that she experiences afterwards is purely psychological. Her refusal to eat, however, makes allusion to her refusal to allow society to dictate how her life is supposed to be.
Perhaps the most poignant moment in the novel is when Marian certainly comes to the realization of how mechanical and boring her life is and how little choice she would have if she follows all the "social rules" for women. Even when she begins to act almost insanely towards food she has no other choice but to say:
So I'm finally going mad," [...], "like everybody else. What a nuisance. Though I suppose it will be a change.
Therefore, the thematic relationship between Marian and food has to do with her changing nature and the clash between this nature and the imposition of society over her.