The goal of this study is to quantify the body composition and organ fat distribution changes with obesity interventions in adolescents with severe obesity. The main question the investigators aim to answer is if there is improvement in organ health with weight loss intervention. The investigators will be comparing the degree of these changes between children being treated with GLP1 receptor agonists in addition to lifestyle changes and those managed with lifestyle changes alone. Participants will undergo MRI body composition at baseline and after 9 months.

Inclusion Criteria:

• Age 12 to <18 years at the time of enrolment
• BMI >99.6th centile or >2.67 SDS
• Informed consent from parent(s) (or person with parental responsibility) and assent from the child or young person for participants aged less than 16 years (or)
• Informed consent from the participant if aged ≥ 16 years.

Exclusion Criteria:

• Body weight above upper limit of the scanner (>250 kilograms)
• Needs general anaesthesia for the MRI scanning
• Any contraindications for MRI- Metal implants/claustrophobia
• Previous use of GLP-1 receptor agonist
• Refusal to consent or refusal to assent from a competent older adolescent.

Eligible adolescents under the care of the tertiary weight management service (Complications from Excess Weight service) at Alder Hey Children's hospital will be offered to enrol in the study once a decision has been taken whether or not they are being started on GLP1 receptor agonist treatment.