Summary: The intestinal microbiota, also known as the invisible organ of metabolism, accompanies humans from birth and is essential for normal development. In this paper, we present the current state of knowledge on the intestinal microbiota, presenting its functions, the factors that determine its composition and its influence on the development of diabetes and obesity. We also reveal how mice helped to solve the mystery of different body weights in twins and why some people find it so difficult to 'lose' weight. We also summarise the results of recent studies evaluating the possible links between the intestinal microbiota and the prevention and treatment of diabetes. And all this is described in a friendly (if not humorous) manner for a better understanding of this very difficult topic.