Like they said in Zombieland, when someone contracts the virus, it makes them angry, hateful, adrenaline fueled, and gives them a really bad case of the munchies. Zombies aren't neccessarily the walking dead, they're just people that are infected with a virus that screws them up hard. They still need to ingest calories to prevent starvation. Now, the walking dead type zombie, I'm not so sure about, but I'm pretty sure brains is thier favorite food, though not for sustaining life, so to speak.