Using intravenous and oral administration of stable isotope tracers, we will investigate the importance of altered fat absorption and altered whole body fat metabolism for the reduced postprandial systemic TAG concentrations in non-diabetic patients in weight stable phase after RYGB.

Inclusion criteria, RYGB

• Age > 18 years
• RYGB > 12 months prior to inclusion
• Weight stable (± 3 kg during the last month)
• HbA1c < 48 mmol/mol before surgery, and no history of diabetes
• HbA1c < 48 mmol/mol and fasting plasma glucose < 6.1 mmol/l at inclusion

Inclusion criteria, CON

• Age > 18 years
• No former bariatric surgery
• Weight stable (± 3 kg during the last month)
• HbA1c < 48 mmol/mol, fasting plasma glucose < 6.1, and no history of diabetes

Exclusion criteria

• Thyrotoxicosis or inadequately treated hypothyreosis
• Haemoglobin < 6.5 mmol/l at inclusion
• Pregnancy (pregnancy test in fertile woman before enrolment) or breast feeding
• Medication affecting the planned examinations which cannot be paused during the study period.
• Adherence to extreme diets with excessively high or low contributions from certain macronutrients (e.g. ketogenic diet)
• Complications restricting eating behavior (e.g. postprandial hypoglycemia, vomiting or strictures)
• MR contraindications