In all developed countries, and ours is not an exception, nutrition is very topical as contemporary society shows a growing interest in the problems posed by human nutrition and is becoming aware of its close relationship with health, putting highlighted by the impressive progress made in scientific knowledge on the nature of nutritional processes in recent years. However, these truly spectacular advances are sometimes difficult to convey as a message and can sometimes even be contradictory.On the other hand, and unfortunately, this plausible interest, undoubtedly motivated by the legitimate desire to enjoy a long and healthy life, is accompanied by the proliferation of all sorts of recommendations based on myths and irrational beliefs with complete neglect of the principles established by the scientific study of Nutrition and, in many cases, in flagrant contradiction with the solidly documented knowledge that we currently have.Thus, a series of fallacies, myths, "magic diets", etc. have arisen, which constitute one of the greatest problems of nutrition in industrialized societies. Although there is no doubt that many people of good faith can disclose inappropriate information, it is also true that supported by these doctrines there are very lucrative businesses that can lead to irreversible pathological situations. The problem is aggravated if we take into account that the tendency to consume this type of food and diet has increased notably in recent years. Information to the population that does not arrive through the appropriate channels can lead to a serious health problem.In this Book we propose to make a critical review of the different "magic diets", including in this term all those that, without any scientific basis, claim to possess unprovable qualities that are not in accordance with the current knowledge of the Science of Nutrition or they exaggerate properties and effects that are not evident or proven. It is indisputable that magic is opposed to the concept of Science.Within the "magic diets", those with the greatest popularity are the weight-loss diets that can constitute in most cases not only a fraud but also a danger to health. These diets are a consequence of the current obsession with losing weight and achieving the so-called "ideal weight" that the media advocate so much, putting physical appearance before health. It should be remembered that although obesity is a problem of the first order, thinness below certain limits, judged by the most widely used index currently - the body mass index [weight (kg) / height2 (m)], less than 20 - is associated with a higher prevalence of certain diseases and an increase in the mortality rate. Many people concerned about losing weight are, paradoxically, within the appropriate weight ranges and even inadequate for low.In this book, when we speak of "magic diets", we are not referring exclusively to weight loss diets, although we grant them a great extension justified by the reality of their problem, but also to all those that seek to increase longevity and healing of various pathologies -whose treatments are often incompatible with said diets- and which we analyze in the chapter on "foods and cure-all diets". The virtues attributed to foods or "natural" products that are not duly verified are also studied. It is obvious to remember that it would be difficult to imagine that after an exhaustive study of the natural products of the earth or of the waters, as they are known today, qualities as important as those attributed to these foods could emerge that had not been taken into account by those who investigated them.