This still boils down to calories in/out though. Metabolism goes under the 'out' column. It fluctuates, sure, but it's just as much an 'out' variable as exercise is.
The body has a choice: burn the stuff now or lower metabolism and store the stuff for later. You can lower your intake, and the system can decide to lower metabolism to store some more. Obviously this can't go on forever, but maybe it can go on enough to impact other factors, like availability of glucose to run willpower.
Or not. Figuring out how and if those adjustments are made and how to control them is the next step after you say "calories in = calories out", which is only a first and smallest step in figuring it out.
