[VanillaCafe]

The final bit that locked it in for me (and someone please correct me if this is wrong) is that going to the bathroom (which what many people assume is where weight leaves the body) is actually the last stage of "energy input" to the body and doesn't really play a role in "energy output".

[semi-extrinsic]

Well, your body does not utilize all the energy it could from the food and drink you ingest. Say if you get a condition where your digestive efficiency goes down - tapeworm, inflammation, whatever - all else equal, you're going to start losing weight fast. This is why gastric bypass surgery works.

[anm89]

I don't understand what you are saying here. The physical weight of the feces does leave your body. How is that an input?

[yesenadam]

Not long ago I googled why shit and piss are two things and not one. It just suddenly seemed odd to me one day. The answer seemed to be that the stuff we poo out never goes 'inside' our body. Imagining our bodies being like a tube or doughnut, what goes in our mouths just goes through the hole, staying in a sense on the outside. What comes out is just what we couldn't use as it passed along the conveyor belt. Piss, on the other hand, is the waste from (truly) inside us.

Hmm seems there are (at least) two levels of 'inside' here. It's quite a weird, topological subject hehe. It seems odd that we don't intuitively think of those as different degrees of insideness.

[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.