Agreed, Paul. So many times we want to approach things in an over-simplified manner.
I see no problem with the physics of it all, nor even with “calories in/calories out” (of course) - there’s no real ‘magic’ at work. We’re big chemical-soup containers and the minutiae of all that happens in us is practically endless, but it’s also that we really use energy for electro-chemical/chemical reactions, many of which make muscles contract and give off heat; not all that complicated.
Okay, but isn’t the human body plenty closed to make things work quite well, here? Outside of enormously rare or contrived circumstances, the energy we take in is overwhelmingly our food - that’s really about it. Then we either excrete it (not much of a thing except in wacky circumstances), store it as fat or use it (use it up) - things going toward more entropy and a happy and satisfied 2nd law of thermodynamics.
In this modern age I think many people are frustrated because we’re so ‘closed’ and efficient with energy; we don’t waste nearly enough of it.