The pathophysiology of obesity and obesity-related diseases such as type 2 diabetes mellitus (T2DM) is complex and driven by many factors. One of the most recently identified factors in development of these metabolic pathologies is the gut microbiota. The introduction of affordable, high-throughput sequencing technologies has substantially expanded our understanding of the role of the gut microbiome in modulation of host metabolism and (cardio)metabolic disease development. Nevertheless, evidence for a role of the gut microbiome as a causal, driving factor in disease development mainly originates from studies in mouse models: data showing causality in humans are scarce. In this review, we will discuss the quality of evidence supporting a causal role for the gut microbiome in the development of obesity and diabetes, in particular T2DM, in humans. Considering overlap in potential mechanisms, the role of the gut microbiome in type 1 diabetes mellitus will also be addressed. We will elaborate on factors that drive microbiome composition in humans and discuss how alterations in microbial composition or microbial metabolite production contribute to disease development. Challenging aspects in determining causality in humans will be postulated together with strategies that might hold potential to overcome these challenges. Furthermore, we will discuss means to modify gut microbiome composition in humans to help establish causality and discuss systems biology approaches that might hold the key to unravelling the role of the gut microbiome in obesity and T2DM.

No abstract

The intestinal microbiota has been linked to the development and prevalence of steatohepatitis in humans. Interestingly, steatohepatitis is significantly lower in individuals taking a plant‐based, low‐animal‐protein diet, which is thought to be mediated by gut microbiota. However, data on causality between these observations in humans is scarce. In this regard, fecal microbiota transplantation (FMT) using healthy donors is safe and is capable of changing microbial composition in human disease. We therefore performed a double‐blind randomized controlled proof‐of‐principle study in which individuals with hepatic steatosis on ultrasound were randomized to two study arms: lean vegan donor (allogenic n = 10) or own (autologous n = 11) FMT. Both were performed three times at 8‐week intervals. A liver biopsy was performed at baseline and after 24 weeks in every subject to determine histopathology (Nonalcoholic Steatohepatitis Clinical Research Network) classification and changes in hepatic gene expression based on RNA sequencing. Secondary outcome parameters were changes in intestinal microbiota composition and fasting plasma metabolomics. We observed a trend toward improved necro‐inflammatory histology, and found significant changes in expression of hepatic genes involved in inflammation and lipid metabolism following allogenic FMT. Intestinal microbial community structure changed following allogenic FMT, which was associated with changes in plasma metabolites as well as markers of . Conclusion: Allogenic FMT using lean vegan donors in individuals with hepatic steatosis shows an effect on intestinal microbiota composition, which is associated with beneficial changes in plasma metabolites and markers of steatohepatitis.

In a population with only overweight and obese adult subjects, 46.8% were diagnosed with prediabetes and 11.9% were newly diagnosed with diabetes. Exactly, 5.6 and 20.7% of total population met the diagnostic criteria of the OGTT for diabetes and prediabetes, respectively, but did not meet the diagnostic criteria of the HbA1c. These data suggest that not performing an OGTT results in significant underdiagnose of T2DM in an overweight and obese adult population.