JOULE - Metabolic Adaptation to Weight Loss in Response to a Behavioural Lifestyle Program With or Without Semaglutide in Adolescents With Obesity

Inclusion Criteria: * Youth aged 12-17 years * Diagnosed with obesity (BMI ≥2 standard deviations above WHO Reference median) * Enrolled in Growing Health Weight Management or Next Step pediatric clinics (ie. have at least one year of the program remaining). Exclusion Criteria: * Any contraindications for MRI (i.e. claustrophobia, implanted metal, metallic injuries recent tattoo or weight \>300lb. Ineligible for imaging visit only. * Use of atypical anti-psychotics. * Use of the following medication classes: (i.e. Beta-blocker medications, Steatogenic medications, Anti-hyperglycemic medications, HIV drugs, Antidepressants, anxiolytic drugs, anti-psychotic drugs, Thyroid drugs, Antiemetic or amphetamine, dextromethorphan and metoclopramide.) * Elevated alanine aminotransferase (ALT) \> 5 x upper range of normal at screening. * Use of glucose lowering or any anti-obesity medication in the previous 3 months. * Known monogenic, syndromic or hypothalamic causes of obesity. * Diagnosis of type 1 or 2 diabetes mellitus. * Prior bariatric surgery or liver transplantation. * Alcohol intake exceeding 3 drinks per week or reported cannabis use. * Recent history of cigarette smoking (previous 3 months) - ineligible for imaging only. * History or family history of multiple endocrine neoplasia 2 or medullary thyroid cancer. * History of pancreatitis. * Presence of untreated endocrine disorder. * History of an eating disorder and / or history of suicidal ideation * History of a cardiac condition that precludes exercise testing or unable to have exercise testing done in the GHWM pediatric clinic, inability to use a cycle ergometer. Participant who does not have a peak power value obtained in the GHWM pediatric clinic Female who is pregnant, breast-feeding or intends to become pregnant. Female who refuses to use a means of contraception if sexually active. Participation in any interventional clinical study within 90 days before screening.