From brain perspective, "reserve power" would be when it ends up relying on ketone bodies, which start to be produced in higher numbers when you have been in high glucagon, low insulin condition for a longer period of time. Long enough that the liver burned through its glycogen stores and the liver cells redirect oxaloacetate to gluconeogenesis (producing glucose from stuff in the blood) to the point where the cells become unable to finish its own metabolism of free fatty acids. It then turns the intermediate products it can't use itself into ketone bodies.
That part can be regulated with nutrition, glucagon and insulin, but having plenty of glucose won't replace sensations from the digestive system itself.
I actually do feel it, I still feel hungry if I eat lots of something that has little caloric value. So eating lots of vegetable and water doesn't fool my body into satisfaction.