When we need energy, can’t the body just burn the stored fat? Isn’t that the whole point of stored fat? Why will we feel hungry, weak, lightheaded, etc. at all? I understand if the body doesn’t have enough fat (if you’re super skinny), it would make sense to feel hungry, but I don’t understand why would that be the case if there’s enough fat to go around.
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If you are feeling weak and lightheaded from hunger, and something frightening and truly urgent happens that you have to deal with, you will stop feeling those things almost immediately as your liver starts dumping stored energy as glucose into the bloodstream.
There’s less stored carbohydrate on hand than stored fat (because it’s bulky), but it’s also quick enough to pull out, that if the situation warrants it, it can be spiking your blood sugar within seconds. If you’re fairly fit, there’s probably enough there to fuel an hour of heavy exercise.
But, when faced with the shortage, your brain can ask you to go eat (make you uncomfortable and hungry) or can conserve energy by asking you to keep still (making you cold/weak/tired) – it might do both, but the homeostasis process is just trying to guard your energy stores. If the food is convincingly unavailable, burning fat to make you function will eventually happen (and the sensations will ease up) but it’s not the first choice. Same, incidentally with cold – first try is to make you feel uncomfortable and ask you (that way) to put on a sweater – but if the sweater or warm refuge is unavailable, giving up and burning calories to make heat will eventually happen (and you might even feel warm and comfortable again at that point).
The precise dynamics have an element of context/acclimatization to them, as well as inputs like hormonal gender – testosterone is a nudge towards guarding your fat stores less. Some people have a body composition set point above which mobilizing fat stores is far easier.
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