Even if you took the meaning of that idiom literally you'd still be wrong. If "you are what you eat" and you ate pork, taking it literally you'd be pork. There's no such noun as "dead". You can't "eat a dead".
However, even literally I suppose most of you is what you eat, since your body takes everything you eat and uses it to make new cells (but of course things like oxyhaemoglobin uses what you breathe, and you can also drink etc. etc.)
But then again, a lot of you is dead. Try rubbing your hands on the bone the fingers come out of together fast for a bit and look at all the dead skin you burn off.