Intensive Weight Loss Intervention Versus Bariatric Surgery for Adults With Severe and Complex Obesity: the LightBAR Randomised Trial

Please note that participants need to be invited in order to take part in the trial. Inclusion Criteria: * Aged 18 to 60 years (inclusive) at time of screening. * Eligible for and willing to undergo bariatric surgery, i.e., fulfilment of criteria for bariatric surgery from the respective national health authorities: * DK: BMI ≥ 35 kg/m2 with one or more of the following: T2D, severe hypertension, sleep apnoea requiring treatment, symptomatic arthrosis in lower extremities, female infertility related to overweight, or BMI\>40 kg/m2 with other strong medical reasons for weight loss (28). Prior to surgery, an 8% weight loss is required as well as smoking cessation. * UK: BMI of 35 kg/m2 to 40 kg/m2 and other significant disease (e.g., type 2 diabetes or high blood pressure), or BMI ≥40 kg/m2. Has been or is willing to receive intensive management in a specialist tier 3 obesity service (29). * Fit for anaesthesia and surgery. * Informed consent. Exclusion Criteria: * Prior bariatric or hiatal surgery, not including intragastric balloons or duodenal-jejunal bypass sleeve (Endobarrier™ or similar) if the device has been removed \>1 year before screening. * Use of any WLM (including liraglutide and semaglutide for diabetes) within last 3 months. * Conditions that contraindicate or complicate total diet replacement (including type 1 diabetes or other diabetes requiring basal bolus insulin therapy or insulin pump therapy (for Denmark) and any diabetes requiring insulin therapy (for UK), phenylketonuria, or other conditions requiring strict adherence to special diets). * Conditions that contraindicate or complicate treatment with GLP-1 receptor analogues (including history of pancreatitis or known allergies). * Conditions that contraindicate or complicate bariatric surgery (GI motility disorders, large abdominal wall hernia, large hiatus hernia (\>5cm), Crohn's disease, liver cirrhosis, or other conditions preventing laparoscopic bariatric surgery e.g. multiple previous abdominal surgery). * Conditions that contraindicate or complicate study adherence and bariatric surgery (mental disorder, unstable psychiatric disease, recent history of alcohol/medication abuse, cancer treatment within 5 years). * Pregnant or planning pregnancy in the next two years or currently breast feeding. * Not achieving a 5% weight loss within 12 weeks prior to randomisation. * People taking part in other research involving multidisciplinary obesity treatment that would compromise their participation in this trial. * Another member of the household enrolled in the trial. * Diagnosis of or treatment for severe eating disorder within the last 6 months.