Obesity and type 2 diabetes mellitus (T2DM) are reaching epidemic proportions in the developed world. In morbidly obese patients only surgical treatment (bariatric operations) leads to a sustained weight loss and relief of co-morbidities in the majority of patients. One of the most frequently performed operations is the laparoscopic proximal Roux-en-Y gastric bypass (LRYGB). There is still lack of knowledge why some patients respond much better than others to an identically performed procedure. Therefore, a number of variations of this operation have been introduced over the past 50 years. Increasing the length of small bowel being bypassed has the potential to improve the effect of the operation but buries the risk of nutrient deficiencies. The metabolic effect of LRYGB occurs, in part, independently of weight loss. The mechanisms underlying metabolic improvement through metabolic surgery are not yet fully understood.

Inclusion Criteria:

• informed consent as documented by signature
• patients with BMI of 35 kg/m2 or higher who comply with the regulatory rules for bariatric surgery in Switzerland

Exclusion Criteria:

• general contraindications to kind of surgery
• known or suspected non-compliance, drug or alcohol abuse
• inability to follow the procedures of the study, e.g. due to language problems, psychological disorders etc. of the participant
• participation in another study
• age < 18 years or > 65 years
• BMI > 60 kg/m2
• height < 145 cm
• CL length of < 180 cm as measured intraoperatively
• ASA physical status classification > III
• inflammatory bowel disease