What Happens When We Fast? (Part 1)

Fasting is so popular nowadays. But people usually afraid of fasting. The idea of fasting scares them. In this article, I will explain the physiology of fasting and how our body responds to it.

First of all, we need to know what insulin is and how it works.

Insulin is a major anabolic hormone within our bodies. It is produced in pancreas Beta-cells. It opens our cell’s gates so that nutrients can come in from the bloodstream.

Our body produces insulin whenever we eat something. So, when we produce it, we’re on storage mode, therefore, we store fat, build muscle, and all kinds of anabolic stuff.

Insulin shows it’s effect by activating anabolic pathways. The most important thing that insulin does, is activating mTOR(Mammalian Target of Rapamycin). 

MTOR is basically an anabolic switch that turns on whenever we eat. It allows us to build muscle, store fat, etc. Basically it plays a role in every anabolic thing within our body including tumors, cancers, etc. 

We’ll discuss mTOR later in the autophagy case. 

During fasting our insulin level is low, because we don’t eat simply. And more importantly, we are under physiological stress. In this case, we force our body to use its own stores for energy rather than food energy. And our body produces counter-regulatory hormones as a response to low insulin. 

What are these hormones?


First of all, I want to talk about glucagon. Because glucagon is antagonist of insulin. This means when we are depleted insulin, glucagon is produced immediately by alfa-cells of the pancreas. It sends signals to the liver and fat cells. In response, the liver breaks down its glycogen stores and produces glucose, and fat cells break down fats and produce fatty acids. And they send them into the bloodstream so that our body uses them to create energy. 

Glucagon is only a part of the insulin-related side of fasting. There is a stress-related part also.

As you know, our body is under stress during fasting. And our body produces some other counter-regulatory hormones to deal with stress. Catecholamines and Cortisol

Cathecolamins stand as Adrenaline(Epinephrine) and Noradrenaline(Norepinephrine). 

We all know about Cortisol. For those of you who don’t know, Cortisol is a stress-related hormone that is produced by our body whenever we are under any kind of stress, whether it’s psychological or physiological. 

And there are Noradrenaline and Adrenaline. In fact, these hormones are a part of the central nervous system. And Noradrenaline is not an actual hormone, it is a neurotransmitter but to better understanding, you count both of them as a hormone. 

Adrenaline and noradrenaline are basically “fight or flight” hormones like Cortisol. Our body increases their production when they are needed such as stress conditions. In this case, we are talking about fasting stress.

What all of these counter-regulatory hormones do, they activate a specific enzyme called “Hormone Sensitive Lipase” (HSL). This enzyme is everywhere within our bodies where there is fat. And once this enzyme is activated by these hormones, we start to burn fat. 

By the way, Cortisol makes us burn fat as well as the others, during a fasting period of course. If it is chronically elevated, then we start to gain weight.

So, What do we’ve got here?

Glucagon, Cortisol, Noradrenaline, and Adrenaline, all four of them is elevated during fasting. And they play a huge role in the fat-burning process. 

So, the difference in fasting appears more clearly now. 

We don’t get any of these hormonal benefits while we are in a caloric deficit, eating 5 times a day and snacking all day long. Because our insulin level is high all the time and we can’t burn fat. As a result of that our basal metabolic rate goes down and we end up regaining weight that we lost. 

By doing fasting, we produce Adrenaline and Noradrenaline as I mentioned earlier, and thanks to these two our metabolism doesn’t slow down. It goes up even.

There is another hormone that increases like crazy during fasting. 

Human Growth Hormone (HGH)

HGH is very important in muscle preservation case especially. You probably hear “Fasting breaks down muscles” all the time. 

As a matter of fact, it is quite opposite. Think of this from an evolutionary standpoint. In ancient times, It’s not easy to find food like nowadays. So, we needed to hunt in order to feed our bodies. In this perspective, fasting was part of our lives and we were hunting fasted of course. So, if we didn’t preserve our muscle, we couldn’t hunt because hunting requires muscle power. 

We had to adapt that hunger and preserve our muscle. So, our body found the solution in producing much more HGH than usual which allows us to maintain our lean body mass. We are talking about huge increases.

For example, our growth hormone level increases 5 times in response to 2 day fast. 

I tried to explain to the hormonal changes mostly within our bodies during fasting. I hope this article would be useful for you. We’ll talk about ketosis, autophagy, gluconeogenesis, and the other stuff in the next part. 

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Take care…



  1. Hi Caesar, very good introduction to fasting and why insulin is such an important piece of it! I like that you explain these medical terms in simple phrases. I know the major benefits of fasting but I’ve never learned about insulin, so this was very helpful to me. Looking forward to your next blog post! Cheers, James

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