They say you are what you eat. Which makes sense. If you put crap in your body, then you’ll consist of crap. But I’ve also heard this put in terms of “mental food.” The things you look at become your focus and drive your thoughts. This is why things like pornography and bad language are so dangerous and pervasive.
But back to the original meaning…food. I wonder if the phrase meant just the general make-up of food; like fat, carbs, proteins, etc. Or if it also meant the consistency of food. I mean, if I eat more croissants, will I be more flaky? If I eat more pizza, will I be more pointed and sharp with people? If I eat more chocolate, will I be more sweet? If it were true, it could lead to an interesting change to the science of Psychiatry. Instead of drugs, you could give food. “You seem a bit bitter and angry about the way things having been going for you lately at work…have a piece of chocolate.”