I have no idea.
Wait. I thought it was simple? Or maybe accurately modelling the energy balance of complex organisms is too complex to represent with three variables? Going back to something you said before:
People work hard to make it seem complicated and mysterious so that they have an excuse that they can tell themselves.
It think it is more the opposite. People (like you) try to make it seem much less complicated than it actually is so they can look down on and/or make fun of fat people.