The aim of this thesis was to provide more insights into the role of the gut microbiota in human metabolism and particular in the modulation of insulin sensitivity. For this purpose, we completed a literature review and consequently carried out three original clinical human studies with three different approaches, all in subjects suffering from metabolic syndrome. First, in chapter 3, we focused on identifying novel microbes and microbiota-derived metabolites influencing insulin sensitivity. Second, in chapter 4, we investigated the synergetic effect a specific diet-regimen (Mediterranean diet) in combination with lean donor fecal microbiota transplantation (FMT). We confirmed the previously described beneficial effects of the Mediterranean diet on metabolic markers. However, although the combination of Mediterranean diet with lean donor FMT resulted in increased levels of donor-specific bacterial species, it failed to induce synergistic beneficial metabolic effects. Finally, in chapter 5, we unravelled the impact of the duodenal infusion of a single commensal bacteria strain on metabolic health and intestinal homeostasis. The introduction of A. soehngenii L2-7 resulted in an elevated post-prandial GLP-1 response, diminished peripheral glucose variability and increased postprandial levels of secondary bile acids. It also had remarkable robust effects on the duodenal mucosa transcriptome with hundreds of differentially expressed genes. Overall, this thesis contributes a small piece of the exciting and extensive puzzle that comprises the role of gut microbiota in human metabolism. Hopefully future research will help us to further improve our understanding and enable us to move toward innovative patient-tailored therapeutic and preventive strategies.