Diagnosed with morbid obesity (defined as BMI > 40 kg/m2 or BMI > 35 kg/m2 with at least one serious comorbidity (2013 NIH compendium guidelines)) Scheduled for medically supervised Very Low Calorie Diet Age 30 years and older Ability to lay in supine or elevated position for 5 hours Willingness and ability to comply with the protocol Exclusion Criteria Any condition that may interfere with the definition 'healthy subject' according to the investigator's judgment (for healthy control group only) Participation in Weight Loss Management Program within the last 6 months prior the first study day Presence of fever within the last 3 days Untreated metabolic diseases including hepatic or renal disorder unrelated to the primary disease Presence of acute illness or metabolically unstable chronic illness unrelated to the primary disease