Have you ever wondered what our body has to go through after we eat and during the time we do not? I will explain in light terms, how our insides work in these two instances.

Why do we eat?

Eating is an essential process without which we would not be alive. The simple reason for eating is energy. The more complicated explanation is that we need different types of molecules so our body, our cells can transform it into energy in the form of ATP (adenosine triphosphate) (1). This molecule is the unit of energy which all cells use - no matter of what species they are. Metabolism is a name for all the chemical processes taking place in an organism (2). 

There are 2 main types of metabolism: 

Anabolism utilises energy to make macromolecules (large molecules). Catabolism releases energy when these are broken down into simpler molecules (3).

The biggest difference in metabolism between fed and fasted state is the direction in which molecules flow. Are they being broken down or are there more being made? 

In simple terms, when we eat our body takes the molecules from food we ate and does two things. It uses these for direct and fast energy, and it transforms the left-over ones into larger molecules. These so-called macromolecules are then stored until there are no “easy” small molecules that could be used for energy. You might have guessed that this happens after a long time of not eating. During fasting the macromolecules are broken down so that energy can be released and made from the simpler molecules (4).

Fasting

Let us investigate it with more detail. It is the night or early morning before you have breakfast. You are fasting but your body still has all the energy needed for you to move in your sleep, dream, think, scroll on Instagram, or get up and make coffee. How? Your body uses your energy storage. Energy storage are the macromolecules made from simple molecules before your fasting. To make these, the body incorporates ATP inside.   That is why when these macromolecules are broken down, the energy (ATP) is released. The most efficient way to store energy is fat or in scientific terms lipids. Fat (adipose) tissue releases this fat into the bloodstream and other tissues and organs, such as liver, muscle, heart take this fat and by a process called oxidation release energy from it. Another player in terms of energy release is the liver. Liver upon receiving a signal by glucagon (the main hormone controlling the molecular breakdown) breaks down so called glycogen into glucose and generates new glucose from other molecules (5). 

Fed

Now you have had breakfast. Let us imagine something high in sugars (carbohydrates) like a toast with jam or cereal. This type of meal contains mostly the simple molecule glucose. Glucose in your bloodstream causes pancreas (an organ behind the stomach) to release a hormone called insulin. Insulin tells the cells all over your body to take the glucose from your blood and convert it to energy. At the same time the fat tissue instead of releasing fat will take up the left-over glucose and turn it into fat to store for later. Of course, liver will have a role too. Its role is to create glycogen from glucose. All the energy we get from a meal is either directly used or incorporated into macromolecules (4). 

All of these processes are strictly regulated by hormones and enzymes creating pathways that all have their own names. If you are interested, feel free to check the sources and google more about these topics. There are also great YouTube videos for you who are visual learners. 

Radka 

Sources:

(1) https://www.ncbi.nlm.nih.gov/books/NBK553175/

(2) https://www.ncbi.nlm.nih.gov/books/NBK546690/

(3) https://pmc.ncbi.nlm.nih.gov/articles/PMC7545035/

(4) https://med.libretexts.org/Bookshelves/Basic_Science/Cell_Biology_Genetics_and_Biochemistry_for_Pre-Clinical_Students/03%3A_Fed_and_fasted_state/3.01%3A_Fed_and_Fasted_States

(5) https://www.ncbi.nlm.nih.gov/books/NBK534877/