You would have to ask George Romero. Historically, I don't think Zombies ate people until Night of the Living Dead. Since then every zombie flick seems to copy the creatures from that flick. Sometimes they are slow and other times they are as fast as normal people. I like the book Breathers: A Zombie Lament. Apparently in that one the zombies eat flesh to reverse the effects of death on the body. Those zombies are able to talk (if through eating flesh their vocal capabilities are restored) eat regular food and probably do all the same bodily functions as regular humans.