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. Most diets work only in the short term, and the scientific differences between individual responses to food remain unexplained. Why can one person eat a certain meal and gain weight and another eat exactly the same food and lose pounds? Genes provide only part of the answer; there is an enormous piece of the puzzle missing. We have been overlooking one vital aspect of diet that lies within us. Thanks to recent breakthroughs scientists have begun to examine 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. Drawing on the latest science and his team's own pioneering research, Professor Tim Spector explores the hidden world of the microbiome, and demystifies the common misconceptions about fat, calories, vitamins and nutrients. Only by understanding what makes our own personal microbes tick and interact with our bodies can we overcome the confusion of modern diets 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 abandon fads and instead embrace diversity for a healthy diet, a healthy gut and a healthy body.