#i feel its a pretty common headcanon in the UT community

[janeelyakiri]

Wait, then what happens to whatever they eat?

depends on the headcanon but my boys just absorb everything from the food.

CANONICALLY, monsters don't seem to need the bathroom. Their food doesn't go through them. And we learn this by talking to Big Mouth

This Audrey II lookin monster

In Neutral/Pacifist routes, they mention that human food 'goes right through you', that it's 'disgusting' and they'd love to try it sometime. So either monster food just doesn't work like our food, or monsters don't digest like we do.

From a quick search it appears if you call Papyrus in waterfall he says he hears water and asks if you're on the toilet. Immediately calling him back reveals he has no idea what a toilet is.