Adipose Tissue Storage in the Rapid Remission of Hepatic and Cardiac Metabolic Dysfunction After Bariatric Surgery

This clinical assay will include 5 visits: the screening visit and four 9-hour postprandial metabolic sessions (A0, A1, B0 and C0) before and after surgery: * initial visit: screening * before surgery: 2 metabolic sessions A0 and A1 (without/with niacin) will be performed, in random order, at least one week interval. * 12 days post surgery: 1 metabolic session B0 (without niacin) * 1 year post surgery: 1 metabolic session C0 (without niacin) Each metabolic visit will last 9 hours with: * perfusion of stable tracers, * ingestion of a liquid meal * Positron-Emitting-Tomography (PET) acquisitions using radiopharmaceuticals such as \[18F\]-fluoro-6-thia-heptadecanoic acid (\[18F\]-FTHA) and \[11C\]-palmitate, * MRI acquisitions.\[18F\]fluoro-6-thia-heptadecanoic acid (FTHA). The niacin will be given during metabolic visits A1 as a regulator of lipids metabolism. During these visits, the subjects will ingest 150mg every half hour for 6 hours. Niacin will be used as a pharmacological suppressor of dietary fatty acid (DFA) spillover in order to determine the role played by this mechanism in the reduction of postprandial endogen glucose production (EGP) in T2D after bariatric surgery.