Effect of Caffeine on Cold-stimulated Brown Adipose Tissue Activity

Inclusion Criteria: * BMI 18.5 to 25 kg/m2 * Able to give informed consent as documented by signature * Moderate caffeine consumption (1 to 3 cups of coffee per day) * Increase of EE in response to mild cold of ≥ 5% of REE Exclusion Criteria: * Known hypersensitivity or allergy to caffeine * Concomitant medication other than prescription free analgesics (paracetamol and NSAID) and oral contraceptives * Other clinically significant concomitant disease states (e.g., renal failure, hepatic dysfunction, cardiovascular disease, cardiac arrhythmia, hypertension, diabetes mellitus, hyper- or hypothyroidism) * History of depressive disorder or anxiety disorder * Smoker / habitual tobacco use * Habitual excessive alcohol use * Regular consumption of caffeine containing energy drinks * Weight change of \>5% within 3 months prior to inclusion * Systolic blood pressure \>140 mmHg and/or diastolic blood pressure \> 95 mmHg. * Resting heart rate \>90 bpm * Hypersensitivity to cold (e.g. Raynaud syndrome) * Known or suspected non-compliance, drug or alcohol abuse * Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant. * Enrolment of the investigator, his/her family members, employees, and other dependent persons * Enrolment into another study using ionizing radiation within the previous 12 months. * Pregnancy or lactation