system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If feces is the bits of food that our body couldn't digest or use, is it possible to eat a diet that our bodies can use 100% and never have to defecate? And, if so, could such a diet ever be healthy or would it always be deficient in some nutrients?
gpt
Ignoring water and focusing on the solid mass, more than half of your feces is composed of bacterial corpses and waste. The rest is about one quarter plant matter you couldn’t digest mixed with some complex carbs that weren’t fully broken down, and the remaining quarter is divided between protein and fat that wasn’t fully digested/absorbed; the exact fraction depends a lot on the person and their diet. 

Back to the top though, healthy stool is 50%-75% water, so really your feces consists primarily of water, bacterial biomass, and indigestible fiber. Even if the remaining bits could be *perfectly* digested and absorbed, you’d still have the same need to poop.