A database and a biomaterial bank will be established to assess, if the role of the gut for the development of obesity and obesity-related diseases such as fatty liver are of relevance in humans, e.g. for classification of afflicted individuals regarding risk or outcome after intervention. During 3 years, the investigators will include 480 obese individuals (4 cohorts of 120 individuals) who undergo a treatment program (either surgical or non-surgical), and follow them up for at least 3 years. Every 6 months, a visit is planned at the obesity center where the initial intervention has been performed. Within the visits, anthropometry (Body Mass Index (BMI), excess body weight (EBW), bioelectrical impedance analysis (BIA)), quality of life (SF36 score, Impact Of Weight On Quality Of Life (IWQOL) questionnaire), and sample collection for laboratory analyses (inflammatory markers, hormones, flora composition, micronutrients, metabolomics etc.) will be performed. Patients will be recruited from 3 centers in Germany. The long-term expectation from this model project is to find gastrointestinal parameters allowing to predict outcome and sustainability of different intervention strategies. In addition, intervention programs will be compared regarding long-term outcome and quality of life changes.