In the first article (Fasting, article 1: the biological basics of reflection) we mentioned that “it is normal to have periods without eating and it would be very beneficial for health, to be hungry and to resist it” I will explain why, here. Let us also recall the statement of benefits in which I highlighted two important points:

“There is mounting evidence that eating over 6 hours and fasting for 18 hours can cause a metabolic change from switching from glucose to using ketones as an energy source, with improved resistance mechanisms, better longevity and reduced incidence of diseases including cancer and obesity ”(de Cabo and Mattson, 2019).

Why does starving yourself have beneficial metabolic effects? Why could this increase our health longevity and prevent important illnesses?

First, let’s start with a few definitions. Fasting which could be understood as caloric restriction is not scientifically understood. We define calorie restriction as a reduction in calorie intake, from 20 to 40% of normal intake, keeping the frequency of eating. Fasting is a lack of food, a reduction in frequency, but not necessarily a reduction in daily caloric intake. So eating 3 meals at normal hours, but lighter meals (-20 to 40%) is calorie restriction. Eating normally, but over a very short period of the day (e.g. over a period of 6 hours, with 18 hours without eating), is fasting. I don’t think it’s necessary to have very long fasts to get the full benefit. We should consider periods of 18 to 72 hours. We will come back to this in a future article.

Secondly, we will differentiate glucose metabolism from ketone metabolism mentioned in the statement from Cabo and Mattson (2019). Glucose is the easiest source of energy for our cells. Thus, our body has developed, during evolution, a taste of sugar allowing it to easily identify the best sources of energy. Glucose is used by cells following the production of insulin, which is the messenger to let it in. The liver transforms part of it into a glycogen reserve for short-term energy needs. The body’s energy intake therefore depends on circulating glucose, glycogen reserves and fat reserves (long-term reserves). The body therefore does not use fat stores, unless it is obliged to. During a fast, depending on the level of activity, after 12 to 24 hours, the blood glucose will have dropped by 20% or more, the glycogen in the liver will have also dropped and the fat breakdown mechanisms will be started (lipolysis). The breakdown of fat (long-term energy stores) will produce what are called ketones which are used as an energy source when glucose runs out. So, running out of glucose forces the body to use a fat-breaking machinery.

Let’s compare our cells to factories. If factories still receive glucose, they will not be equipped to use fat as an energy source. If factories frequently lack glucose, they will keep all the equipment they need to use fat as an energy source. People who eat less glucose are therefore less likely to store fat, because they are more likely to use it as an energy source. It’s an epigenetic factor. Epigenetics is sort of the ability to easily use the information written in our genes when cells need it. The genes commonly used by our cells are more easily accessible. We talked about it in an article on genetics (Don’t be a slave to your genetics).

Thirdly, it’s not just the energy metabolism that changes; there is an improvement in resistance mechanisms. Here, without going into details, know that when the body detects that it is running out of energy, it stimulates several maintenance and repair mechanisms to be able to get through a difficult period. These mechanisms, such as cell recycling (autophagy), are very beneficial for health and are linked to reducing the risks of many diseases, including cancer.

In conclusion, the reasons why fasting is beneficial for health is the fact that it improves energy metabolism, it promotes less adiposity (less body fat) and it stimulates maintenance and repair mechanisms. The amount of body fat (up to a certain limit and by age) is directly correlated with the increased risk of many diseases. Thus, fasting acts on two complementary facets:

  • it reduces the risk of disease by facilitating better body composition and better metabolism,
  • it stimulates our natural maintenance and repair mechanisms.

It goes without saying that all this is true, as long as outside of fasting periods, you have a healthy balanced diet. Fasting cannot be used to compensate for poor diet (e.g. eating junk food once a day). Fasting should also help reduce total daily calorie intake, which is important for reducing the body’s urge to age and improving several health parameters. Fasting should therefore be considered a food mode that can be part of a healthy diet, according to a certain frequency.

In the next few articles, we will cover the topics of desired intensity and implementation in a structured manner.



  • de Cabo et Mattson, 2019. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medecin. 381 (26), 2541-2551. 2019 Dec 26.
  • Longo et Mattson, 2015. Fasting: Molecular Mechanisms and Clinical Applications. Cell Metab. Author manuscript; available in PMC 2015 Feb 4. Cell Metab. 2014 Feb 4; 19(2): 181–192.
  • Longo VD, Panda S. Fasting, Circadian Rhythms, and Time-Restricted Feeding in Healthy Lifespan. Cell Metab. 2016;23(6):1048–1059.
  • Mattson, Longo, et Michelle, 2017. Harvie Impact of Intermittent Fasting on Health and Disease Processes. Ageing Res Rev. 2017 October ; 39: 46–58.