system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we get hungry when we still have energy stored in our bodies?

Like fat reserves. I have an amount of fat on my body but why do I still get hungry every day when my body can just use that fat as energy instead? Doesn't the body use that energy anyways when you don't eat?
gpt
Our bodies have a kind of order they like to use energy in.  Obviously your body will use the energy in your food before the energy it has stored as fat, but it goes a little deeper than that. 

Your body likes to keep around a teaspoon of glucose (sugar) in your blood at any time.  Too much sugar can cause serious problems, and so can too little, so your body manages the amount of sugar in the bloodstream constantly.  So what happens when you eat a meal that has sugar in it?

When you eat a meal, your stomach quickly absorbs the sugar. Your body now has more sugar in its blood than it really wants, so it releases a hormone called insulin.  Insulin is like a key that opens your cells’ gas tank and lets sugar come in.  The problem is that your cells can only take on so much fuel, so all the extra basically gets stored in your fat cells, both to keep you safe and to burn later.

So what stops your body from using that fat? Well, as long as you have any extra sugar in your blood stream your body will *always* use that first.  It’s like a safety mechanism. Unfortunately, people snack a lot, and when they snack, they will use the sugars in their snacks for fuel instead of stores fat.  If you are constantly eating, you will always be using those fuels. 

Two things to consider:

The first is you need to understand that all carbohydrates are essentially sugar.  A tablespoon of sugar, starch, and ‘complex’ carbohydrates are all broken down into glucose. Enzymes in your mouth will start to break starches down immediately, and your stomach will quickly break down more complex carbohydrates. You can think of complex carbohydrates as time-release sugar.  Fiber slows down the absorption of sugar somewhat. 

The second is that when you eat a meal with a lot of carbohydrates, a lot of the calories end up in storage unless you burn them pretty quickly.  So you may have eaten a 700 calorie meal, but you maybe only got 400 calories worth of energy because you couldn’t process it enough. So that meal should have been enough to  keep you going for hours, but since a lot of it got stored, it doesn’t keep you going for as long, and you get hungry sooner than you might like. If you ignore the hunger for a while, you will eventually start burning fat, but most people opt for a snack because being hungry doesn’t feel good. 

This is why you see people have so much success losing weight with things like intermittent fasting; they give their bodies enough time to actually burn some of the stored fat instead of just eating more food.