system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Could we only eat exactly what our bodies need, and never poop?

Poop is just waste, right?  So if we didn't eat the waste part of food, could we stop pooping, and eventually would humans evolve their butts off?  

I'm not kidding more than I'm kidding. I'm sort of kidding, just because it's butts and poop, but, I'd still really like to know the answer.
gpt
>Poop is just waste, right?

Wrong.  Or at least, poop is not just undigested food.  Feces contain various other biological waste products that we can't stop producing.  Things like bile, and broken-down, dead red blood cells.  Plus, roughly 40% of feces consist of dead bacteria:  You have **tons** of bacteria living in your intestines, helping you extract nutrients from food.  As those bacteria die, they get expelled.