Cold Acclimation As a Modulator of Brown Adipose Tissue Function in Adults with Obesity

Inclusion Criteria: * BMI\>30kg/m2 (for participants with obesity) * BMI\<25kg/m2 (for lean participants) * Waist circumference \>94 cm for men (for participants with obesity) * Waist circumference \> 80 for women (for participants with obesity) * Glucose ≥7.8 and \<11 mmol/L after OGTT (for participants with obesity) Exclusion Criteria: * Being regularly exposed to cold environments for the last 3 months (e.g. having a job as a ski monitor, fishmonger, woodcutter, etc.; or doing sports/leisure activities outdoors for prolonged periods of time, etc.) * Having gone for winter/ice swimming and bathing more than once every 2 weeks during the last 3 months. * Inability to undergo scanning (e.g., claustrophobia) * Pregnancy * Smoking (or use of nicotine based products) * Hypo/hyperthyroidism * Malignancies * Immune disorders * Viral or bacterial infections * Abnormal cardiovascular status, arrhythmia and/or long QTc in ECG, abnormal cardiac murmur, previous history of cardiovascular disease * Allergy to lidocaine or epinephrine, or other local anaesthetics * Weight change (intentional or not) over the last 6-months \> than 5% of body weight * Any other cardiovascular, pulmonary, orthopaedic, neurologic, psychiatric or other conditions that in the opinion of the local clinician/researcher, would preclude participation and successful completion of the protocol, or that would negatively impact or mitigate participation in and completion of the protocol.