Here's something that's always puzzled me. It seems almost inescapable that if two people of the same size have radically different metabolism, their body temperatures must be different. If anything, a skinny person should consume less energy because they have less skin.
Granted, this is me, a physicist, thinking simplistically. But it's what's always made me skeptical about metabolism.
There are too many aspects. Like gut flora and even enzymes present. Those could affect how and to what input is broken down. And then there are hormone responses and how body responses to those. In the end we are probably very far away of having complete understanding what is going on. And differences might be minute, but will add over years.
Granted, this is me, a physicist, thinking simplistically. But it's what's always made me skeptical about metabolism.