With this trial, the aim is to assess the benefits and harms of a non-surgical intensive weight loss intervention that includes total dietary replacements, behavioural support and weight-loss medication compared with bariatric surgery for people with severe and complex obesity. The interpretation of the results will help inform future care pathways for people with obesity in whom bariatric surgery is currently the only available effective treatment option.

In the LightBAR trial, an intensive weight loss (IWL) intervention will be compared with bariatric surgery. The IWL consists of three phases:

'Induction' phase (week 0-12 after randomisation): total dietary replacement (TDR) programme and behavioural support with weight loss medication (WLM) if rate of weight loss is insufficient.

'Weight loss continuation' phase (week 13-32 after randomisation): progression of dietary programme including reduction in use of TDR products, reintroduction of healthy foods, with behavioural support, introduction of physical activity, WLM (as required).

'Maintenance' phase (week 33-104 after randomisation): Continued healthy diet and physical activity with WLM (if required), with return to induction phase if weight regain occurs induction, weight loss continuation, maintenance. The IWL lasts two years, and includes total dietary replacements, behavioural support, and weight loss medication. Bariatric surgery will be standard Roux-en-Y gastric bypass or sleeve gastrectomy.

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.

After the results have been published, we aim to make a depersonalised dataset publicly available on e.g. ClinicalTrials.gov and/or the European Union (EU) Zenodo database (https://zenodo.org/). The final choice will reflect which platform(s) that are compliant with current legislation at that time.

When the results have been published

Researchers with a protocol for their planned study

Due to the nature of the interventions, it will not be possible to blind the participants or the healthcare providers administering the interventions.

Blinded outcome assessment will be employed for the primary outcome and anywhere else possible. When assessing clinical outcomes, the participants will be asked not to disclose their assigned intervention group to the outcome assessor. Further, they will be provided with a plain white t-shirt, or similar, to wear during clinical assessments to cover the presence of surgical scars.

Statisticians and investigators drawing conclusions will be fully blinded. We will conduct the statistical analyses with the intervention groups coded as e.g., 'A' and 'B'. We will write two abstracts while the blinding is intact; one assuming the experimental intervention group is 'A' and the control intervention group is 'B', and one assuming the opposite. After these two abstracts have both obtained consensus, the code will be broken by the data manager.