We are all increasingly bewildered by the simple question of what to eat. Despite advice from experts, governments and dieticians about the dangers of too much fat, sugar, protein and lack of exercise, our nutrition, and the global obesity crisis, is getting worse. Why can one person eat a certain meal and gain weight and another eat exactly the same food and lose pounds? Genes provide part of the answer, but we have been overlooking one vital aspect of diet that lies within us. Scientists have begun to understand the importance of the permanent residents in our guts: the thousands of previously unknown but essential microbes whose job it is to digest our food and keep us alive. The hidden world of the microbiome and the common misconceptions about fat, calories, vitamins and nutrients. Only by understanding how our own microbes interact with our bodies can we overcome confusion about diet and nutrition to regain the correct balance of our ancestors. Mixing cutting-edge discoveries, illuminating science and his own case studies, Spector shows why we should stop listening to the myths of diet fads and instead embrace diversity for a healthy gut and a healthy body.