Extended Data Fig. 3. In vivo study protocols.
(a) Fifteen participants (nine female, six male) aged 18-35 years with a normal BMI (18.5-25 kg/m2) were recruited to a randomised, double-blind, placebo-controlled crossover study to determine the effect of the SSRI sertraline (50 mg for 7 days) on BAT activity. At each study visit, BAT activity was quantified using 18F-fluorodeoxyglucose (18F-FDG) PET/MRI and thermal imaging while whole body energy expenditure was measured using indirect calorimetry (IC). Participants spent 1 hour in a warm room, then 3 hours in a cold room to activate BAT. After 2 hours of cold exposure, 75 MBq of 18F-FDG was injected intravenously and a PET/MRI scan performed 1 hour later. (b) To determine the effect of obesity on circulating and adipose serotonin concentrations during cold exposure, male and female participants aged 18-40 years who were either normal weight (BMI 18.5-25 kg/m2) or obese (BMI 30-55 kg/m2) were recruited to a case-control study (n = 10/ group). Participants lay supine in a warm room for 2 hours prior to transfer to a cold room for 2 hours to activate BAT. BAT activity was measured by thermal imaging and whole body energy expenditure was measured by IC. Subcutaneous abdominal white adipose tissue (scWAT) biopsies were obtained following 105 minutes in each room.