[clejack]

So what you're saying is the laws of thermodynamics that I learned in school don't apply to biological systems?

I'm admittedly in no position to argue deeply about biology, or even physics for that matter, because I studied mechanical engineering. I dealt with physics in an applied manner.

With that said, this story comes to mind:

https://en.wikipedia.org/wiki/Angus_Barbieri

It seems to disagree with your idea that fat is not fuel. And the first law of Thermo seemed to be applying to this man. He had little to no energy input (via food) and a baseline energy expenditure from just existing, so his system burned stored energy via body fat.