The attempt to search for the word "diet" on a popular online bookstore opens up a world of revolutionary programs. Limited to the first results from tens of thousands of titles present, we have: the diet of super metabolism, the diet of longevity, the paleo diet, the protein diet, the diet of hormones, the vegan diet, the smart-food diet, the fasting diet, the smart diet...There is something weird: in the history of mankind, there has never been a maniacal attention to what we eat that is equal to that of our time. And, if you think about it, being able to easily get diverse foods has never been easier. Your great-grandfather didn't have the chance to taste goji berries. Likely, he had problems getting meat every day.On the other hand, however, the diseases associated with poor nutrition are like a biblical catastrophe almost unknown in other eras: a recent FAO report shows that within roughly ten years, one in three people in the world will suffer from food-related issues: obesity, diabetes, cardiovascular disease, and cancer. And as we get closer to our daily lives, many of us struggle to keep our weight under control and get easily tired, anxious and without energy. Furthermore, many even age early despite trying to eat (more or less) healthily... almost as if their bodies were not forgiving the slightest cheat.What happened to our relationship with food? Can we say that it has become ill, amidst super-chef reality shows, food fashions that last a few seasons, organic markets with absurd prices and waves of colored junk foods? Why do the diets listed above contradict each other, yet each of them sells itself as the absolute truth?Maybe we need to draw a line and start over. Our bodies live by an impressive series of chemical reactions; everything we are, feel, and become comes from them. The food we ingest is not burned in a furnace like wood, but is transformed into thousands of molecules that flood our blood and react with our body.Fattening, slimming, getting old, getting sick, feeling tired... they are simply consequences of these reactions. For this reason, this book aims to understand how our diet should really be structured to keep us healthy, thin, and healthy, but starting from how our body works, and how, from a scientific point of view, different types of food and diet can have different effects on us."Scientific" is a key word: we believe that what distinguishes hoaxes from truth is precisely scientific research; in fact, every statement made in the book is accompanied by a reference to a text, study, or scientific article. The material listed can, among other things, be an excellent starting point for those who want to deepen the concepts expressed.The ambition of this book, therefore, is not to be yet another work on the miraculous diet for a spectacular body or for living a hundred years eating only this and that food, but a way to understand, in a scientific but enjoyable way, how our body and our metabolism work and, only in relation to this, which foods should be consumed and in what quantities. We will go so far as to give readers the tools to build their own balanced diet based on their own needs; the knowledge that we will use for this purpose will be built step by step in the different chapters.