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 obesity class I or uncomplicated obesity class II or higher.

In the LightCARE trial, an intensive weight loss (IWL) intervention will compared 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 will be offered to discuss weight management programmes available in their area with their GP practice; the programmes available referral routes vary slightly from place to place. Tier 2 weight management services are mostly commissioned by local authority, and therefore differ slightly across the 333 local authorities in England. These local community-based weight management services provide diet, nutrition, behavioural advice.

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. BMI ≥30 kg/m2 or ≥27.5 kg/m2 in people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds (as reported by the participants).
3. Informed consent.

Exclusion Criteria:

1. Has severe and complex obesity, i.e., obesity class II (BMI≥35 if white or 32.5 if non-white) with one or more of these specific adiposity-related comorbidities: cardiovascular disease, type 2 diabetes, hypertension, non-alcoholic steatosis, or sleep apnoea (Appendix 1).
2. Intending to become pregnant in the next two years, or pregnant, or breastfeeding.
3. Use of WLM or GLP-1 agonist treatment within the last 3 months.
4. Currently being treated for cancer other than oestrogen antagonist therapy or non-melanoma skin cancer.
5. 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.
6. Diagnosis or treatment for eating disorder within the last 6 months.
7. 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.
8. Conditions that contraindicate or complicate total diet replacement (including type 1 diabetes or other diabetes requiring any insulin therapy, phenylketonuria, or other conditions requiring special diets).
9. Taking part in other research involving multidisciplinary obesity treatment that would compromise participation in this trial.
10. Conditions that contraindicate or complicate GLP-1 or GIP agonist treatment (including history of pancreatitis)
11. 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

Due to the nature of the interventions, it will not be possible to blind the participants or the healthcare providers administering the interventions. In all other aspects of the trial, blinding will be employed. Outcome assessors will be blinded to group allocation. Participants will also be requested not to disclose their allocation when outcomes are assessed.

Statisticians and investigators drawing conclusions will be fully blinded. The statistical analyses will be conducted with the intervention groups coded as e.g., 'A' and 'B'. The Steering Committee ill 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.