system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does fat and energy work? How is it that you burn fat by exercising, but you can have fat and still be out of energy. If I just stop eating, why isn't it that I can be full of energy until my body is out of fat?
gpt
This will be a little complicated.

Fat is energy, but it's not immediately usable as energy; since cells are the smallest living unit, fat has to be broken down into parts that the cells can use before it is actually usable. Of course, breaking down molecules takes time, as well.

Breaking down the fat releases the usable parts (glucose) into the bloodstream. Your body automatically tries to keep the blood glucose level steady. Why? Because too much blood glucose is inefficient; too little blood glucose is dangerous.

So when you exercise, you are using up blood glucose. Once the level gets too low, your body will start breaking things down to bring the levels back up. It starts with carbohydrates, because those are easily broken down into glucose. Once those are almost gone, then it starts breaking down the fats. If the fats are running out, then it will turn to proteins and muscle. Light exercise will burn up the carbohydrates. Strenuous or extended exercise will start burning up fats. Flight-or-fight, life-or-death situations will most likely cut into your muscles and proteins.

Because of this order of energy consumption, your body doesn't like to use too much energy. It doesn't like to start burning up fat, because that is the "reserve" energy that the body can use before it goes into full panic mode (burning muscle).

So when you exercise a lot, your body will grudgingly start breaking up fat. That's why you have to do a lot of strenuous exercise to lose fat; you have to force the body to use its reserve energy.

The other side of this coin is energy storage. After a meal, when your blood is full of glucose, your body will start storing the energy away for a rainy day. Your body will start turning the extra blood glucose into fats, and storing those fats away for later use. That's why you don't have a huge burst of energy after a meal; all that extra glucose that you are digesting is being turned into fats.