Background: Severe obesity is an increasing global epidemic. To date, metabolic bariatric surgery (MBS) is the only treatment for severe obesity with good and sustainable long-term weight loss and remission of obesity related comorbidities. The gold standard procedure is laparoscopic Roux-en-Y gastric bypass (LRYGB), but since 2014, laparoscopic sleeve gastrectomy (LSG) has been the most common MBS procedure in the world. This transition took place before any available longterm data for LSG. As obesity is a chronic disease, the long-term outcomes are of vital clinical importance in outcome assessment and procedure selection.

Aims: The main purpose of this thesis was to assess long-term outcomes of MBS. The first and second study assessed the outcomes of the Sleeve vs. Bypass (SLEEVEPASS) randomized clinical equivalence trial at 7 and 10 years. Firstly, the aim was to compare both weight loss (percentage excess weight loss, %EWL) and quality of life (QOL) and their possible association after LSG vs. LRYGB at 7 years. The second predefined 10-year analysis assessed weight loss and remission of comorbidities, QOL, and overall morbidity and mortality. In addition, a special focus was added to evaluate both symptoms and endoscopic findings of gastroesophageal reflux and more specifically the prevalence of Barrett’s esophagus (BE). The third aim of this thesis was to assess the feasibility of Individualized Metabolic Surgery (IMS) score in facilitating procedure selection in patients with severe obesity and type 2 diabetes (T2DM) using a large merged individual patient 5-year data of the two largest RCTs comparing LSG and LRYGB (SLEEVEPASS and SM-BOSS).

Results: The mean %EWL was 47% after LSG vs. 55% after LRYGB at 7 years, and 44% vs. 51% at 10 years, respectively. The two procedures were not equivalent, but the difference was not clinically relevant based on the predefined equivalence margins. There was no difference in long-term QOL or in the remission of T2DM, dyslipidemia, obstructive sleep apnea, or complication rate or BE prevalence between LSG and LRYGB at 10 years. Hypertension remission was superior after LRYGB. Esophagitis was more prevalent after LSG. In all IMS score severity stages, there was no difference between the procedures in T2DM remission rates.

Conclusions: %EWL was greater after LRYGB, and the procedures were not equivalent for weight loss, but both resulted in good and sustainable weight loss at long-term. Long-term QOL and remission of comorbidities were similar between the procedures except for hypertension and esophagitis, where LRYGB had superior outcomes. IMS score did not facilitate procedure selection.