For example, bears can hibernate or travel very long distances, like migratory birds, and all this without food.
Humans are relatively well equipped to cope with food scarcity. This allowed our ancestors to survive through very difficult times – times when food was not readily available. 

In several species, different fasting strategies, which we will see in detail, have been experimented with and sometimes give impressive results regarding aging and the development of age-related problems. This is the case, for example, of rhesus monkeys[1].

• Glucose, a type of sugar, comes from the carbohydrates we eat. These carbohydrates are degraded into glucose by endogenous enzymes but also by the action of the intestinal microbiota. They are sometimes modified further to be used by our organs as an energy source.
  Most excess glucose is stored as glycogen in the liver (which produces it) and in skeletal muscle. This glycogen is a glucose polymer: a large molecule composed of small glucose molecules. Glycogen serves as an “energy reserve” and can be released into the blood as glucose (blood glucose).

• Lipids, or fat, for their part, come from the reserves present in adipose tissue. Fatty acids can be directly assimilated by some organs.
  Not all organs are able to assimilate directly the fatty acids present in adipocytes, which contain on average 87% of lipids (mainly triglycerides). These fatty acids are transformed, if necessary, into ketones that can be used by certain organs such as the brain[2,3].

Glucose is used first and foremost as long as its level is sufficient. In the event of a lack of glucose, the ketones take over but are less directly consumable. These mechanisms are regulated by the action of hormones, which we will see in detail later.

The storage of energy in our bodies can be compared to the difference between a restaurant meal and a frozen meal. The restaurant meal corresponds to glucose, easily accessible and directly edible, while frozen meal refer to fat. They need preparation before consumption.

The liver is the vital organ involved in this energy management. Responsible for the production of bile, it also performs synthesis and degradation functions for many molecules.

Synthesis functions are, for example, the transformation of glucose into glycogen. It’s glycogenesis. The liver can, on the other hand, release glucose into the bloodstream through the reverse process.

Fasting is therefore a thousand-year-old practice, whether forced by lack of food or encouraged by religious precepts. This does not make it an argument for judging its effectiveness.

We will try to clarify this in the next parts, now that we have some basics on the main mechanisms of energy storage and use.

[CAUTION]Before starting to fast, seek advice from health professionals, such as your doctor. Depending on practices and individuals, fasting can be dangerous to your health if it is not carried out properly and could lead to deficiencies. In the case of certain diseases, it must be even more closely controlled.

Uncontrolled changes in diet can be dangerous. In the case of an undetected deficiency, this can lead to more serious complications.
For example, a 38-year-old woman decides to adopt a fasting technique without knowing that she has an iron deficiency. Unfortunately, she does not include iron-rich food in her new diet and finds herself a few years later with edema of the lower legs or her period might stop.
Before drastically modifying your lifestyle and trying a fasting technique, consult your doctor.