Obese individuals that undergo major intervention such as a low-calorie formula diet program or bariatric surgery with the result of substantial weight loss (> 10%) are included and followed-up for at least 3 years. Weight changes and excess weight loss as well as measures of quality of life are monitored. In addition, biomaterials will be collected from these individuals every 6 months for measurement of parameters related to obesity-associated gastrointestinal (GI) impairments such as change in GI hormone levels, change in GI microbiota, or enhanced bacterial translocation. Moreover, micronutrient and metabolomics analysis will be performed. This project allows comparison of non-surgical and surgical intervention and enables to asses the anticipated relationship between obesity and the GI tract in humans in the future.

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.

Inclusion Criteria:

• Adults (18-65 years at time of inclusion)
• Obesity defined as BMI > 30 kg/m2
• Obesity treatment (multidisciplinary life style intervention with LCD or bariatric surgery) resulting in an estimated body weight loss of >10% of the initial body weight within 6 month

Exclusion Criteria:

• Body weight loss after intervention < 10%
• Follow-up period < 1 year or number of consultations within 3 years < 3