https://www.sciencedaily.com/releases/2012/09/120912125114.h...

Also

>Basal Metabolic Rate (BMR)

What increases or decreases BMR?

E.g. https://www.scientificamerican.com/article/6-years-after-the...

Why are there very slim people who struggle to gain weight regardless of how much they try to eat more?

What causes hunger pangs for people when they have half a million calories strapped to their waist?

>I'm guessing the reason nobody runs these studies is that it's solved science?

Exactly this type of thinking has stymied research into very interesting questions.

One of the article's main points is that the obsession with the calorie-only model (with the assumption that it's solved) has stymied even asking these fat-metabolism questions in the first place (outside of veterinary science, where they actively trying to make animals that get fat very quickly).

But yes, your body is free to store/burn mass and energy in muscles, tissue, glycogen and whatnot if it chooses to store the excess at all.

Of course if you force an environment of extreme depletion (like bootcamp as mentioned elsewhere), you body will get to its adipose tissue, but that's not going to solve the obesity epidemic. Most veterans get just as fat as the rest of the population after they leave the service.

I personally can feel when my glycogen has been depleted when working out. Once it's gone I look a bit anemic and /pushing/ beyond certain limits becomes impossible. Normally it's a precursor to me entering ketosis, which pretty much guarantees I'll be burning fat or muscle only from then out.