The phrase "you are what you eat" is universally true.
It does not refer only to physical foods but also spiritual and mental ones.
Perhaps it would be better to say "you are what you consume".
And it was never as much true as in today's consumerist society of almost infinite choice. If all you consume is news of strife and violence you will become fearful and nervous (or "preemptively aggressive"). If you watch and read content predominantly from one side of the political spectrum, then that is what you will become. "Cognitive" or "filter" bubbles are a seemingly modern phenomenon but they are a clear manifestation of ancient "you are what you eat" principle.
And of course, if you eat crap food your physical body will become crap. If you consume crap information, your mental body will become crap as well.