Joe Christmas is filled with self-hatred. He believes himself to be part black and cannot accept the fact that he is mixed. He craves order and acceptance, but can find neither in the segregated world in which he lives. He will not take the white McEachern's bread, worship his god, or take his name. But he cannot find a home with blacks either. Ingrained in him from the orphanage is the fact that he is unworthy, that he is the devil's child. His crazed, racist grandfather who worked as janitor at the ophanage implanted these notions into the young child's mind and permanately instilled a sense of inferiority and self loathing.
His one shot at love is with Bobbie, the waitress/prostitute, who turns on him when his conflict with McEachern becomes a threat to her business. Her cronies beat the young Christmas who has dreams of eloping with her to a bloody pulp, leaving him for dead. This incident together with that involving the scheming dietician and the submissive Mrs. McEachern prevent him from ever truly trusting women again. Thus his later relationship with Joanna Burden is doomed from the start.
Food is the way these conflicts are exhibited. Christmas refuses McEachern's food, just as he refuses to learn the catachism. He will not accept Mrs. McEachern's kindness because it is unpredictable and always displayed secretly as if it is wrong. So, he throws her offered food on the floor. We see this same pattern developing with Joanna Burden. She sets his food out for him in the kitchen while she eats in the dining room, as if he is inferior to her. Christmas reacts by throwing the food against the wall. He cannot allow her to control him.
If you look at one incident in particular, you get some idea of the various conflicts raging in Christmas' life that could certainly represent the conflicts of the human heart.
When he finds himself alone in the nurse's room at the orphanage, he finds the toothpaste and decides this time to eat a large quantity of it, giving in to the animal nature within that finds something sweet and consumes it. He knows he shouldn't, and he knows that when he sees the nurse come in with her lover that he mustn't be discovered.
He liked the nurse, thought she was pretty, but he knew she wouldn't bear him being there when she is doing something illicit. So when he vomits and gets caught, once again giving in to the animal nature of his body, it betrays the mental emotional side of him.
So too the heart often acts as the nexus of those two conflicts between the physical side of humans and the emotional side.