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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Your body keeps that fat as a reserve, and it wouldn’t be much of a reserve if you burned through it just going about your normal day.
In practice when you get hungry for long enough your body will swap over to burn fat mode. You may then feel a bit less hungry and you won’t feel so weak.
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It has to do with insulin. Insulin is a fat storage hormone. When your insulin level increases from eating carbs, it signals your body to store fat and you cannot burn it effectively until it lowers again.
Carbs (sugar) cannot be stored very well in your body and you must constantly be eating them to provide energy. So when this energy is depleted you feel hungry until you eat more or until your insulin levels lower enough to start allowing your body to burn the stored fat.
So either constantly need to eat, or stop eating long enough to get into fat burning stage (ketosis).
Much like a car there is a bit of lag between when you hit the pedal and when the engine revs and you start moving. The body will burn fat but it can’t do so immediately on demand, there are processes that need to start up and that take some time.
For the first time in our human existence, we aren’t living hand-to-mouth, in constant fear of scarcity or starvation. Those inherited and annoying urges to stuff yourself when possible, saved your ancestors.
It can burn fat, but an overweight person is usually that way because they eat too much and are constantly replenishing their fat stores. When they’re losing weight they’re also not eating as much which is what your body also uses for energy. If you’re going through your fat stores faster than you can eat and digest food you’ll start to feel tired and hungry.
First the body goes to find energy in the food you have eaten, when that is empty, it takes a little while before the body goes into ketosis, which basically means you are now burning fat. To keep your body in ketosis, you need to keep you carb intake close to zero. It takes at least a day or two before your body enters this emergency fat burning mode.
There is a lot of energy in that fat, so you can live surprisingly long on 20kg extra fat, but its a slow process that itself takes energy. So you wont have ANY energy to spare. Imagine being on a life raft for 3 months, thats about the level of energy you are able to exert.
A) it takes time to break down fat into usable energy
B) the brain is wired to control eating behavior so it’s consistent. Even if you do survive without eating for a few more hours these mechanisms don’t care about it. Evolutionary wise that’s adaptive since it forces you to think about gathering food, because in the wild food is scarce.
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.
Answer: your body really doesn’t want to burn fat. There are 3 energy stores in your body: carbs, fat, muscle. You burn carbs first because they are the easiest things your body knows what to do with (look up simple vs complex carbs). Once you run out of carbs to burn, your body switches to fat. This is called ketosis and is what people on no carb diets are trying to achieve. Ketosis is absolutely a survival response related to starvation and temperature regulation and can come with some nasty side effects (look up keto flu).
Those sensations: hunger, weakness, headaches, etc. are all your body’s signals telling you that things are out of whack. Through the processes I described above, it will try to maintain homeostasis but that can only go on so long so you still gotta eat.
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