Fasting potentials are the most interesting topics in the Nutritional Era. Fasting consists of the catabolism of lipids, proteins, and carbohydrates to maintain blood glucose levels in a normal range. The action mechanisms of fasting were firstly understood in minor organisms and later in humans. Nutritional interventions of caloric restriction could attenuate age-associated epigenetic alterations and could have a protective effect against cellular alterations, promoting longevity and health span. While most fasting studies point out the weight and fat mass decreases, it is important to define specific guidelines for fasting and non-fasting days to enhance adherence, minimize the dropout rates of the interventions, and maximize body composition improvement. Although the panorama of evidence on fasting and caloric restriction is wide, there is a lack of a safe fasting protocol to guide physicians in its prescription. The main goal is to identify a how to use guide, a major posology of fasting, inserted within a huge dietetic personalized strategy leading to an optimal and healthy nutritional status.