In the first two articles, I told you about the biological basis of fasting (article 1) and the reasons why fasting can be beneficial for health (article 2). We will talk here about the different types of fasting in order to better differentiate what it is.

From a scientific point of view, we mainly define two types of fasting: intermittent and periodic.

Intermittent and periodic fasting

Intermittent fasting is shorter, but generally provides for a closer frequency. It can be for example a fast of alternation of a day (fast every other day) or every other day, or even, of 18 hours in a day, every other day or during a week. Periodic fasting lasts for several days (2 to 7 days or more), for example every two weeks or more.

The big difference between these two approaches will be the intensity of the metabolic changes induced. The body gradually adapts to the lack of energy from glucose and the intensity of the changes will be more pronounced the longer the fasting period. Thus, intermittent fasting will cause more frequent changes, but less intense, while periodic fasting allows deeper, but less frequent changes. It also depends of course on the level of activity during the fast. You are probably wondering: Can we do physical activity on an empty stomach?

Ah yes! A recent study shows a greater benefit from physical activity carried out before lunch in obese or overweight men. Improving the use of fat as an energy source is the basis of the benefits identified. Likewise, the use of fat during a fast will depend on the duration of the fast and the level of activity of the person.

 Not for everyone

There are still some health risks, and note, fasting is not an approach that works for everyone. Fasting is not applicable among others, for young people, for people with low weight, those undernourished, with a very advanced age, or with a weak immune system. It is always important to discuss this with your doctor. We will return to the contraindications for fasting in a future article.

We can then ask ourselves: is it better to have an impact that is lighter and more frequent or one that is stronger, but less frequent? Fasting studies follow a large number of biological markers, not only according to the type of fasting, but also according to the objective pursued. Fasting to fight obesity does not have the same goals and considerations as fasting for longevity in a healthy person. The person’s condition, lifestyle, physiological parameters and goals are very important.

First two simple recommendations:

  1. If you wish to try to fast for periods longer than 24 hours, it is best to be followed by your doctor or a specialized clinic.
  2. If fasting is used in the context of medical treatment (e.g. cancer, obesity, metabolic problems or rheumatoid arthritis), it is very important to have a periodic follow-up from your doctor.

As long as the diet is healthy and balanced

So if it is to help control body weight or to reduce the risk of disease (cancer, type II diabetes, inflammatory diseases, hypertension, nerve degeneration, metabolic syndrome), intermittent fasting of 18 hours per day for a few days a week or 24 hours a day, every other day, can give good results. I remind you again that this is true to the extent as it is about healthy eating. Fasting cannot compensate for poor diet. In addition, a diet too high in calories or too rich in protein could cancel the effects of fasting on longevity.

12, 15 or 18 hours without eating

Professor Valter Longo, who directs the Institute of Longevity at the University of Southern California in Los Angeles, recommends eating for periods of 12 hours or less in his recent book “The Longevity Diet”. He teaches in gerontology and biology, with a specialty for understanding the processes of primary aging and the effects of food on it.

Thus, the 12-hour period without eating allows the body to enter its cycle of using its reserves, which is very beneficial for health. Remember that for an equal caloric intake, it is beneficial that it is consumed over a shorter period of time. This time period could also be 6, 8 or 10 hours a day. You can now understand that without calling it fasting, grouping meals over the shortest period of time is very beneficial. An intermittent fast of 18 hours represents food intake over a period of 6 hours.

Here Professor Longo’s recommendation is a maximum of 12 hours. This is easily achievable. Even if you have supper relatively late, it is also possible to have lunch later: finish supper at 9 p.m. and breakfast at 9 a.m. the next morning. It should be noted that this recommendation is widely accepted by researchers in the field of longevity, but it is not applicable to some people who could end up with too low blood sugar levels or drops in blood pressure. In some cases, improving other health conditions would be a prerequisite for implementing this recommendation. For example, physical activity, with weight loss if applicable, could improve glucose management enough to help reduce the length of eating periods during the day.

We will discuss more specifically the fasting protocols in the next article.



  • 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, Valter, 2016 (en français 2018). Le Régime de Longévité. Editions Actes Sud Format : 336 pages.
  • 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.
  • Robert M Edinburgh, Helen E Bradley, Nurul-Fadhilah Abdullah, Scott L Robinson, Oliver J Chrzanowski-Smith, Jean-Philippe Walhin, Sophie Joanisse, Konstantinos N Manolopoulos, Andrew Philp, Aaron Hengist, 2020. Lipid Metabolism Links Nutrient-Exercise Timing to Insulin Sensitivity in Men Classified as Overweight or Obese. The Journal of Clinical Endocrinology & Metabolism, Volume 105, Issue 3, March 2020.