From my post on Called to Communion:
Jesus points out that eating food is a physical process. It goes in physically, is physically digested, and then discarded. It doesn’t enter the heart and therefore does not defile. This isn’t dualism, just common sense. Now it follows that if something cannot cause spiritual harm by a purely physical process, it cannot cause spiritual benefit by a purely physical process.Read the whole post here: Real Presence - Does it Mean Cannibalism?