Fasting is a thousand-year-old ancestral practice that consists of voluntarily abstaining from food and/or drink for a set period of time. This deprivation has an impact on the body's various physiological systems. The biological mechanisms associated with fasting are complex and remain largely mysterious to the general public. These mechanisms include autophagy (a powerful cell regeneration process), hormone regulation, mitochondrial dynamics and metabolic reprogramming. Why and how can fasting act on neurodegenerative pathologies, metabolic disorders, cardiovascular disease, gastrointestinal disorders, as well as on reproduction, autoimmunity and cancer? Is fasting dangerous? What are its limits? For the first time, scientists tackle these fundamental questions in a comprehensive book, describing the advantages and disadvantages of fasting on the various biological systems of the human body. Get ready to discover the unique therapeutic potential of fasting, as well as its limitations, based on proven scientific data.