This is probably the hardest question to answer. An internet search reveals that no one knows what the hell they’re talking about. The most common answer is “Omg I don’t think zombies are even real.” Um, thanks.
The real controversy is whether zombies eat brains, or human flesh. I don’t know why these have to be mutually exclusive. I’m going to just go ahead and decide that zombies eat humans, period. They want to eat you, it doesn’t matter to them whether it’s your arm or your brain or your liver. (In fact, if we were purely using fictional zombies as examples, movie zombies ate flesh exclusively until the brain tangent was introduced in 1985′s “Return of the Living Dead” as a comedic concept.) I think the reason we associate them more strongly with brain consumption is that we’re much more terrified of losing our brains than any other body part. You eat my brain, I’m dead.
That being said, for the sake of a good argument, let’s go ahead and pretend that zombies only eat brains. The most convincing theories range from metaphorical (they crave the mind/consciousness/soul that they have lost) to scientific (it’s been recently discovered that glial cells in the brain can replicate cells and perhaps give them back some of the brain mass that they lose in decomposition) to metabolic (brains have fat or cholesterol that sustain them). Brooks, my most trustworthy zombie source, is pretty insistent about the fact that zombies don’t eat for nourishment, but rather that it’s some sort of instinct (I would compare it to a human’s need for sleeping; they won’t die immediately without it but it does serve some sort of basic survival purpose). My favorite theory is that they eat brains to prevent competition or to keep the zombie population from increasing to unsustainable levels, so that the only ones that survive are the people who are bitten and then escape.
The complete opposite theory, in the original “Dawn of the Dead”, is that zombies purposefully only consume about 40% of human bodies, so that enough is left to be fully mobile after reanimation. I disagree with this because I don’t think zombies are capable of a group mentality or organizing together. It seems more likely that they would prefer to be alone. It’s not like they care if their race survives.
Answer: they eat both, it’s an instinct, and brains have no nutritional value. Now everyone can stop wondering.