in 1826, Jean Anthelme Brillat-Savarin wrote: ββTell me what you eat and I will tell you what you are.β Then, in 1864 Ludwig Andreas Feuerbach wrote: ββman is what he eats.β Really, "You Are What You Eat" means it's important to eat good food in order to be healthy and fit.
So, humorous meme-makers trying to get creative turn phrases around hoping to get a laugh. For example. there is this old saying, "An ounce of prevention is worth a pound of cure." Change a couple letters and it's: "An ounce of pretension is worth a pound of manure." Some may find it funny others may not. Good Luck memeing.