Good questions raised, we'd assume that something has happened to the internal systems of the zombie which mean they are no longer dependent on the normal foods which we eat, and now they eat flesh for sustenance. Flesh is high-fat and protein, this would drive their metabolic functions. The fact that their heart is no longer beating means that they would need to eat a large quantity of this for it to be distributed around the body by diffusion (travelling through the circulatory system without the blood flowing from where it's in high concentration to where it's in low concentration).
I would still consider human flesh "normal" foods as their is nothing special about it, but that is beside the point. I don't understand the idea of eating more will drive metabolic functions. Is this becuase more nutrients are available to be absorbed? if the heart isn't pumping i don't understand how anything would be able to get around the body. Im sure i missed something that you said that explaind this.