This proposed trial is designed to answer the following questions: First, is biliary reflux to the distal esophagus present before OLGB and does it increase after the procedure? Second, does performing an OLGB with or without anti-reflux sutures make a difference in (biliary) reflux exposures of the distal esophagus? To answer these questions the investigators plan to perform a randomized, controlled trial involving two groups. Group A will undergo an OLGB without anti-reflux sutures and consists of 50 patients. Group B will receive an OLGB with anti-reflux sutures and also consists of 50 patients.

Gastroscopic evaluation for inflammation and reflux will be performed before and one year after the operation utilizing multilevel intraluminal impedance pH-monitoring (MII-pH) and intragastric Bilitec 2000™. Furthermore, the study will be blinded to the patient. Long-term weight loss, the resolution of comorbidities and the incidence of surgical complications will serve as secondary endpoints. Follow-ups will be performed at 3, 6, and 12 months postoperatively to assess all primary and secondary goals.

Obesity, and especially its comorbidities, has unarguably become the number one threat to human health in the modern world. Western lifestyle leads to an increased prevalence and thus to a higher mortality (i.e. due to cardiovascular diseases).

The positive effects of gastric bypass surgery on excess weight loss and comorbidity resolution are well-known. In contrast to the standard laparoscopic Roux-en-Y Gastric Bypass (RYGB), a newer method, the laparoscopic Omega Loop Gastric Bypass (OLGB), has emerged over the past years. It is believed to be the simpler method involving only one anastomosis (instead of two) and therefore potentially reducing morbidity and mortality whilst maintaining comparable excess weight loss. However, since this new type of gastric bypass is similar to the former Billroth II resection (BII), the carcinoma risk is a concern. The OLGB is different from the BII resection in many ways. For instance, it involves creating an approximately 10 cm long narrow gastric pouch which could prevent the suspected underlying pathogenetic mechanism: biliary reflux to the gastric tube and subsequently to the esophagus. Biliary reflux is suspected to stimulate squamous esophageal cells and Barrett's epithelial cells to produce inflammatory mediators and therefore cause oxidative stress, DNA damage and apoptosis which might lead to adenocarcinoma.

Worldwide, there are currently two different ways to perform an OLGB: with or without anti-reflux sutures, which involve sewing the biliopancreatic limb to the staple line of the pouch using V-Loc (non-absorbable) moving upwards as far as easily possible without creating any tension.

This proposed trial is designed to answer the following questions: First, is biliary reflux to the distal esophagus present before OLGB and does it increase after the procedure? Second, does performing an OLGB with or without anti-reflux sutures make a difference in (biliary) reflux exposures of the distal esophagus?

Inclusion Criteria:

• Age between 20 and 70 years
• Body Mass Index ≥ 35.0 kg/m2
• Exclusion of psychological, endocrinological and anesthesiological contraindications
• Willingness to be assigned to either of the two groups
• Follow-up anticipation
• Willingness to attend all follow-up visits
• Written informed consent

Exclusion Criteria:

• recent or ongoing cardiovascular event within the last 6 months (myocardial infarction, acute coronary syndrome, angioplasty or coronary bypass operation, insult),
• pulmonary embolism or thrombophlebitis within the last 6 months, other bariatric procedures, e.g. gastric banding, Sleeve Gastrectomy, etc., cancer of various origins (exceptions: basal cell carcinoma or carcinoma in situ, disease-free over the last 5 years),
• significant anemia or coagulopathy,
• serum creatinin ≥ 1.5 mg/dl.,
• serum bilirubin or phosphatase elevated or ALT above 3 times the upper limit, stomach, biliopancreatic operations, splenectomy or colon resections, gastric or duodenal ulcer within the last 6 months,
• intraabdominal sepsis (except uncomplicated appendicitis or diverticulitis over 6 months prior to the study),
• organ transplantations,
• anamnestic HIV infection, active TBC, active malaria, chronic Hepatitis B or C, liver cirrhosis,
• alcohol or drug addiction (except caffeine, nicotine) in history,
• acute psychiatric disease interfering with the proposed trial,
• other chronic disease which might interfere with the participation.

Also, patients who are pregnant or plan a pregnancy within the following two years as well as patients who have been included in another study, will be excluded.

Note: Exclusion can be performed by a study coordinator at any time.

Hiatal hernia is not considered an exclusion criterion in this study.

Analysis will be performed on anonymized data only. A list cross-linking the patient number to the name will be kept secure and is only available to the principal researcher. Written informed consent will be kept for 15 years.