I believe it was “The Zombie Survival Guide” that postulated that while zombies eat, they don’t digest, and that the hunks of the unfortunates they had eaten were simply forced out of the anus by the pressure of fresh meat behind it.
Most of the solids in your shit is fiber, along with bacteria (a bunch of which which colonizes fiber in your colon, so the more fiber…) and stuff like dead red blood cells and bile byproducts.
I don’t eat much fiber usually so I don’t poop all that much (once daily, smallish amount). Fat, protein, and sugars are very completely digested.