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Fasting seems to be the new trendy thing to do. Whether that’s fasting for a few hours, a few days, or intermittently, a host of benefits are associated with it. But are the benefits real?
During a fast, the body experiences physiological adaptations. In the absence of food, the body needs to use the resources it has to be able to continue functioning. During the first few hours (or days, depending on your activity, age, etc.) of fasting, the body uses its glycogen reserves (complex carbohydrates stocked in the muscles and liver) as energy. When these are used up, the body turns towards its fat reserves and ketones to obtain the energy it needs.
With this change in energy source, the body changes its functions so that its energy needs are reduced: systems involved in growth and development are put on hold and the emphasis is placed on repairing and maintaining the body. This change of use of available energy source forces the suppression of growth hormones, which could have a role in preventing aging. That’s where the hypothesis that fasting can increase your life by 20% comes from.
The thought of prolonging life is certainly interesting… but not eating for several days isn’t very realistic in everyday life. To respond to this problem, different types of fasting have appeared, including intermittent fasting.
Those following this type of fast can proceed in two different ways: they can decide not to eat (or eat very little) during a day or two a week, alternating with days when they eat normally. Or they can block periods during one or more days a week when they don’t eat (for example, not eating between 15h and 7h the next morning). In theory, you could even say the “fast” of not eating between dinner and breakfast (literally breaking the overnight fast … providing there is no a snack in front of the tv in the evening!) could be beneficial.
That said, studies don’t enable us to establish what type of fast is best, or which is more beneficial, if there are benefits for humans. Most studies conducted on humans come from the Longevity Institute at the University of South California (USC), under the direction of Valter Longo. Despite promising results, there is too little data coming from human studies to justify a recommendation to fast to increase longevity in humans.