Where does fat go when you burn it?

Fat is a macro nutrient. Like protein and carbohydrates, it doesn't just sit around. It has an impact on a physiological level. So much so that in the absence of a calorie controlled diet, I've even been using some sort of macro-modulator to help with fat loss. But let's dive in a bit further to see where this macro goes once burned.

Where does fat go when you burn it?
Where does fat go when you burn it?

As you probably know, it's pretty well established that muscle, especially the metabolically active type, stores significant amounts of glucose (aka glycogen) when the body is in an anaerobic (sprinting) state. 

So it makes sense that a large amount of carbohydrate will be converted to glucose before the muscles stores are depleted. And it's also pretty well established that fat is burned very quickly and efficiently for energy. 

So it's a given that fat metabolism must be active when the body is in a fasted metabolic state such as a fasted overnight fast.

But here's where the real story of fat metabolism comes in; glycogenolysis and ketogenesis are not merely anaerobic, they are most definitely anaerobic. So let's take a look at the basics.

First, here's a little background regarding glycogenolysis. Glycogen, you may recall, is a form of glycogen, a type of storage carbohydrate in the muscle tissue. 

The type of glycogen in muscle tissue takes the form of long, linear chains of glucose molecules. Glycogen is created from glucose when the body breaks down glycogen storage. Glycogen degradation is what happens when you start a diet that results in a loss of muscle mass. Glycogen breaks down in response to being burned for energy.

In other words, muscle does not use glucose, it uses glycogen. So when you start a diet and your muscle stores run low, glycogen breaks down and is converted to glucose and that's what the body uses to fuel energy.

During this process, some of the glycogen is converted to a ketone body called acetoacetic acid (in most people). 

This is actually the acetoacetic acid portion of the TCA cycle, with the pyruvic acid coming from glycolysis. The pyruvate from glycolysis is also converted to acetoacetic acid during the TCA cycle, and the body then uses acetoacetic acid as the starting fuel for Krebs cycle metabolism.

So once we understand this simple fact of metabolism, it becomes pretty easy to accept that once again, the fasted body uses insulin in a manner similar to the fed state when looking to burn fat. 

This is the ketosis that many people talk about, and it is a fact that in a fasted state the insulin levels are high and the glucagon levels are low.

In fact, the best way to understand what insulin does is that it allows for the transport of glucose into the cell, whereas glucagon allows for the transport of glycogen to the cell. These are two very different states for insulin, with glucose being the opposite of glycogen.

Insulin also moves fatty acids into fat cells, while glucagon allows for fatty acids to move out of the cell and use for energy.

That's why insulin is so critical in the fasting state, as it allows for glucose to fuel cells that are converting pyruvate into acetoacetic acid, rather than into ATP. Insulin increases mitochondrial activity, and by doing this it provides for higher levels of adenosine triphosphate (ATP) for the cell. 

This allows for the body to burn fat. Insulin also allows the body to burn off and use stored glycogen as fuel.

And in fact, insulin is responsible for moving glucose into fat cells so that the cell can burn it. The other body fat cells also have insulin receptors, and the body will use insulin to increase their fat burning activity.

But in the fed state, the insulin levels are low. Glucagon is not low, so the body will continue to burn stored glycogen for energy. Fat cells do not have insulin receptors, and so they will not be able to be activated to use fat as fuel. 

That's why we need to have a low carb or ketogenic diet to stimulate insulin so that the body can use that fat stored in our fat cells to fuel all of the processes in the body.

The best way to understand how insulin works is to see how low levels of insulin will stimulate fat cells to actually burn fat.

 And then on a ketogenic diet or ketosis diet the low insulin levels will allow more glycogen to be transported into the cell so that it is available for immediate use. The ketones in the blood cause the body to burn stored glycogen in the liver and muscle as fuel.

Insulin is a very important hormone for fat storage and burning. It is also important for how much sugar the body uses and stores and how it processes glycogen. Insulin is very useful for fat burning when it is low, but also is very harmful to weight loss when insulin is high.

Insulin and Hunger/Energy Levels

We've seen the importance of insulin in fat burning and in the storage of glycogen as energy. One of the things that is very important to know is what happens with insulin when you are hungry.

When insulin is low, the body is not hungry and does not really care about storing food in the muscle and liver. But when insulin is high, it's as if you are super hungry.

The body will use protein first if insulin is low, and the pancreas will secrete insulin to help transport glucose into the muscle and liver for immediate use.

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