In this trial, the aim is to assess the clinical benefits and harms, as well as cost-effectiveness of an intensive weight loss (IWL) intervention that includes total dietary replacements, behavioural support and weight-loss medication compared with existing weight management programmes within primary care for people with severe and complex obesity.

In the LightWAY trial, an intensive weight loss (IWL) intervention will be compared with with usual care. The IWL lasts two years, and includes total dietary replacements, behavioural support, and weight loss medication in 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.

Usual care will differ between the two countries (Denmark and the United Kingdom).

In Denmark, participants will receive a pamphlet on current obesity management guidelines from the Danish National Board of Health, and will be advised to contact their GP for potential referral to local municipality-based obesity management programmes. Furthermore, a notification will be sent to the participant's GP, informing that the participant has been enrolled in the trial and is randomised to usual care. The availability and structure of current obesity programmes varies between municipalities.

In the United Kingdom, participants are eligible for referral into an NHS Tier 3 weight management service. These services are commissioned by integrated care boards, and can therefore differ slightly across the 42 regions within the UK. These are specialist weight management clinics that provide non-surgical, intensive medical management with a multidisciplinary approach to care. These clinics often consist of specialist doctors, nurses, dieticians and physiotherapists/exercise therapists, and include psychological support.

Please note that participants need to be invited in order to take part in the trial

Inclusion Criteria:

1. Age ≥18 years and ≤60 years old at screening.
2. Has severe and complex obesity i.e. BMI>35 or >32.5 in people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds and with one or more of these specific adiposity-related chronic diseases of cardiovascular disease, type 2 diabetes, hypertension, non-alcoholic steatosis, or sleep apnoea.
3. Provides informed consent.

Exclusion Criteria:

1. Intending to become pregnant in the next two years or pregnant or breastfeeding.
2. Use of WLM or GLP-1 agonist treatment within the last three months.
3. Currently being treated for cancer other than oestrogen antagonist therapy or non-melanoma skin cancer.
4. Prior bariatric surgery, not including laparoscopic gastric banding, intragastric balloons or duodenal-jejunal bypass sleeve (Endobarrier™ or similar) if the device has been removed >1 year before screening.
5. Diagnosis of or treatment for severe eating disorder within the last 6 months.
6. Any other disease that markedly compromises the participant's ability to adhere to the treatment programme or follow-up or is likely to mean that weight loss will not improve the person's length or quality of life, such as conditions limiting life expectancy.
7. Conditions that contraindicate or complicate TDR (including type 1 diabetes or other diabetes requiring insulin therapy, phenylketonuria, or other conditions requiring special diets).
8. Conditions that contraindicate or complicate GLP-1 treatment (including history of pancreatitis)
9. Taking part in other research involving multidisciplinary obesity treatment would compromise participation in this trial.
10. Another member of the household enrolled in the trial.

After the results have been published, the aim is 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