What Happens When We Fast? (Part 2)

In the last article, we made an entrance to the fasting physiology. We talked about hormones that are part of the fasting process.

In this article, we’ll talk about gluconeogenesis and ketosis. Part 3 is going to be all about autophagy. Alright, let’s begin. 


I assume you are not familiar with that word. Let me explain quickly. 

It is simply our body’s process of producing glucose. 

The human body has the ability to produce glucose from multiple molecules. I don’t want to confuse you with some weird names. Let’s get into the point. 

When we fast, we are deprived of ingested food energy. Simply because we don’t eat, right? So, what ends up happening is our body starts to use its own stored energy. 

Where is this stored energy come from? Glycogen stores and adipose tissue(fat).

Our body uses glycogen stores first because it is far easier to use than mobilizing fat. 

After about 24 hours our glycogen stores run out. So, we need to produce glucose. Thankfully, our body is able to produce glucose without glucose.

So, we need glucose, don’t get me wrong. But we don’t need to consume glucose to produce glucose.

After glycogen stores run out, our body starts to break down proteins to produce glucose.

Ups! “But you say all the time, we don’t break down our muscles during the fast”.

Don’t be afraid, it is temporary and not a big deal for your muscles.

Besides, people usually think if we breakdown proteins, it must be our muscles. 

As a matter of fact, your body has proteins that are not muscle. There are tons of excess proteins within your body. So, It doesn’t have to be your muscles.

So, this gluconeogenesis process continues during the fast. Our body never stops producing glucose. But it shifts the source of this glucose.

I mentioned earlier that glycogen stores run out after 24-48 hours. 

You might wonder that are you going to burn fat if you fast less than 24 hours? 

Don’t worry, you will. Thanks to catecholamines, our body doesn’t need to run out its glycogen stores to burn fat. 

If you are stranger to catecholamines and hormones, please check my previous article. I talked about hormones that play a part in fasting.

Let’s get into the topic. After glycogen stores are empty, we start to burn fat. Our body starts to break down adipose tissue(fat). 

Let me inform you about fats quickly. 

Fats are stored as triglyceride form within our bodies. Triglyceride molecule contains three fatty acid molecules and one glycerol molecule. 

Our body breaks down these triglyceride molecules and releases free fatty acid into the bloodstream along with the glycerol.

But there is a problem. Not every tissue of our body is able to use fatty acids as a source of fuel, like brain for example. 

Because of that, our body needs to turn these fatty acids into something else. So that we can use them as fuel.

And that’s where the KETOSIS comes in.


These fatty acids go to the liver after broken down. Our liver starts to process called Beta-Oxydation. 

After this process, ketone bodies show up. And they go to the bloodstream to be used as fuel by different tissues of our body. 

We are talking about 2 different ketone bodies. Beta-hydroxybutyrate and acetoacetate. But mainly we use beta-hydroxybutyrate. 

Ketosis is a very important mechanism for our bodies. It is sort of an adaptation to survive in hunger times. 

People think if they don’t eat for two days, they are going to die due to starvation.

But our body is a genius. It knows how to survive and its survival way is ketosis.

I want to talk about these ketone bodies more because it is so important. 

Ketones are miraculous things. Many benefits of fasting come through ketosis. 

I’ll tell you the top 2 benefits of ketosis.

1-Preservation of muscles

Besides growth hormone, ketone bodies play a part in the preservation of muscles during the fast.

There is a branch chained amino acid called Leucine. You probably heard it before. It is a major component of BCAA supplements along with Valin and Isoleucine.

Our muscles have plenty of leucine. It is important for us to preserve our muscle. Luckily, ketones block the leucine oxidation within the body. So, our body doesn’t have to break down muscles thanks to that.

2-Increased brain functions

We’ve been told that our brain uses only glucose as a source of fuel and we need glucose to think properly.


Our brain is able to use ketones as a source of fuel. But here is the thing. 

These ketone bodies increase the production of BDNF(Brain-Derived Neurotrophic Factor). This molecule promotes nerve-cell growth basically. Which means it makes us SMARTER.

And there is one more thing about ketones, and it is miraculous.

Our body uses ketones more efficiently than glucose. It spends less oxygen to oxidize and creates more energy. 

Which means ketones are better energy source than glucose and here is the proof.

This might not sound so miraculous but it is extremely good for your overall health and well being.

Anyway, thanks for reading this so far, you are awesome. We will talk about autophagy and its benefits in the next article.

Until then, take care…


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