You're making me think too much about Energy Balances and bringing in Mass Transfer and Heat Transfer now. It's harder to apply those rules to a human body, YOU KNOW? And now having to dig up medical terms too.

Bit busy the past few nights so haven't had time to chew on all of this. My only concept atm is that the body needs energy to move (exercise or normal movement) and that it's taking it from blood sugar. That blood sugar is in various forms though, depending on what the body is doing and what nutrients/fuel is available. Losing weight is the side-effect of this energy consumption. Building/losing muscles is a side-effect of this energy consumption. Building stamina is something I haven't quite figure out yet because haven't seen how it fits into the entire thing. Just need to min-max this energy consumption to get to what we want. Min-max : changing the body/environment to get maximum benefits/goal achievement with minimum losses/goals incomplete.

The bit that is interesting is how it all fits together. And there's something about blood sugar that plays a major role in all chronic diseases that is also interesting.