[markdown]

It's the end of the input. Think about it another way... say you chew on a slice of orange, swallow the juice, and spit out the pith. The pith isn't output of your body. It's just the end of the intake process.

Feaces isn't produced by your metabolic system. It's just the byproduct of intake. The stuff that your body didn't absorb.

[thaumasiotes]

Feces may not be produced by your metabolic system, but it is certainly not just a byproduct of intake. It includes red blood cells that have failed quality assurance and been filtered out of the bloodstream by the spleen; that is why it's brown.