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Energy and blood sugar. Everything you need to know!

Updated: Jun 12, 2022

This post is about energy!

More specifically it’s about how our body generates energy from glucose (aka blood sugar), how energy is stored and used when needed, and which factors regulate this whole process.

I must confess that it took me some time to write this post. The human body is so complex and so fascinating at the same time that I found myself deep down the rabbit hole, and let me tell you, it was hard to get out of it. :)

The best way to start this story is to introduce carbohydrates, the macronutrient category glucose is a part of.

Carbohydrates (carbs), together with proteins and fats, are macronutrients present in certain foods and drinks. They are needed by our body to generate energy so that it can function properly and stay healthy (to keep it simple).

They are called carbohydrates because, at the molecular level, they are made of carbon, hydrogen, and oxygen atoms.

Carbs can be found in two forms, simple or complex.

Simple carbohydrates can be divided into:

  • Monosaccharides, consisting of one molecule of sugar. Some examples of monosaccharides are glucose, fructose or galactose.

  • Disaccharides, consisting of two monosaccharides combined together. The three major disaccharides are sucrose (glucose combined with fructose), lactose (glucose combined with galactose), and maltose (glucose combined with glucose).

Complex carbohydrates consist of three or more monosaccharides combined together, this group includes starch and fibers.

Carbohydrate digestion turns both simple and complex carbohydrates into glucose.

Given their different structure, simple carbohydrates are digested faster than complex ones, which is why monosaccharides are considered a fast way to get energy.

Now that we know what glucose is, let’s focus on how it is turned into energy.

Energy production process

Glucose, generated by carbohydrates' digestion, enters the bloodstream through the gastrointestinal wall.

As the blood glucose (blood sugar) level rises, the pancreas releases insulin, a hormone that “opens up” the cellular gates and allows the glucose to pass from the bloodstream and into the cells. And so the process of energy production can begin.

Within the cell (cytoplasm), glucose is transformed into pyruvate and ATP. ATP stands for adenosine triphosphate and it is the energy needed for our body to function.

This transformation happens through a complicated 10 steps energy production process called glycolysis which leads to the creation of 2 molecules of energy (ATP) from each molecule of glucose.

When oxygen is not present (anaerobic conditions), energy comes with the additional production of lactate. As we’ll see later on, lactate can be “recycled” to generate even more energy.

In the presence of oxygen, more ATP is generated in the mitochondria (the energy power plant of the cells).

The energy storing process

Not all available glucose is transformed into ATP, part of it is stored in case of future needs.

The energy storing process is called Glycogenesis and is triggered by insulin also.

The energy storing process is simple but clever. Cells are always craving energy and would suck up all available glucose if they could find it, so the logical trick is to “hide” it from them!

Glycogenesis is the process that chains the glucose molecules together to form glycogen. This way glucose is still available but in a form that is not directly usable by cells.

Energy storage in the shape of glycogen exists in the liver and muscle cells.

The energy that fuels cells can come from two main sources, glucose and fat. Some cells (like red blood cells) can only rely on glucose as an energy source, making it extremely important to keep the glucose level in the bloodstream within a certain range.

Red blood cells can not use fat to generate energy because they have no mitochondria which is the part of the cell where fat (fatty acids) is converted into ATP.

How and when do we use the stored energy?

During fasting, as in between meals or overnight, the blood glucose levels are maintained within the normal range by making use of the stored energy. I.e. the glucose stored as glycogen.

The stored energy is made available through a process called Glycogenolysis.

Glycogenolysis is triggered by a hormone called glucagon and it is pretty straightforward. The chains of glucose molecules (glycogen), built during the energy storing process, are now broken down again, thus making the glucose available.

The glucose freed up in the muscle cells, stays there to provide energy for muscle movements. The glucose freed up in the liver cells enters the bloodstream so that it can be used by other cells and converted to energy (ATP)

How long does stored energy last?

After about 18 hours of fasting or after about 90 minutes of intense exercise, the energy storages (glycogen) are depleted and another source of energy is needed.

The Alternative Energy Sources available are lactate, glycerol, and amino acids. The process which converts them into glucose is called Gluconeogenesis and occurs in liver and kidney cells. The newly generated glucose is then released into the bloodstream and reaches the cells where the energy production process starts again.

The Alternative Energy Sources are made available in different ways:

  • Lactate, as mentioned above, is a result of the anaerobic energy production process taking place in the cells.

  • Glycerol comes from adipose tissue where triglycerides are broken down into their two principal components, fatty acids, and glycerol.

  • Amino acids are the result of muscle proteins breaking down (muscle proteolysis). Not all amino acids can be converted into glucose (only the so-called glucogenic ones).

According to some estimations, gluconeogenesis produces around 50% of the needed glucose after 14 hours of fasting (the other 50% comes from energy storage). The production reaches 64% after 22 hours and up to 84% after 42 hours.

A tough balance act

Keeping the glucose levels in the bloodstream within a certain range requires hard work. It is a continuous increase and decrease of its concentration.

  1. Gluconeogenesis and glycogenolysis are both triggered by glucagon and they both increase blood sugar because they “produce” glucose.

  2. Glycolysis and glycogenesis are both triggered by insulin and they both reduce the blood sugar because they“consume” glucose. Insulin also has a role as an inhibitor of the production of glucagon and in that way slows down the production of glucose.

We finally reached the end of the story but…not really :)

As mentioned before, glucose is not the only source of energy. Fats play a very important role here as well, but this is definitely a story worth telling in another post!


Each of us is lucky enough to be exposed to many learning opportunities every day. Sometimes we catch them, sometimes not.

I started this blog to slow down, focus, reflect on these learnings and make them available to myself and whoever is interested.

My previous posts:

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