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.