Amino acid profile and obesity: impact of fat and weight loss

Background: In subjects with obesity insulin resistance (IR) is present at the level of all tissues, especially muscle, liver and adipose tissue. In addition, severe obesity is often associated with reduced muscle mass and sarcopenia due to high protein catabolism with amino acid release (AA) and lipoxicity. Bariatric surgery leads to significant weight loss and improvement in glucose and lipid metabolism, glycaemic control, IR, beta cell function, and in many cases, to the remission of type 2 diabetes (T2D). However, it is still not known if the metabolic improvements after bariatric surgery are already present a few days after surgery or if they are mediated by weight loss achieved after several weeks. Moreover, there is a need for biomarkers of response to BS for early evaluation and prediction of metabolic changes. Thus, the aim of this thesis project was to evaluate the short- and long-term effects of bariatric surgery on metabolic fluxes (glucose and lipid fluxes), indexes of insulin resistance, plasma aminoacid (AA) concentrations and lipid composition. Method: two cohorts were studied: first cohort (n=20) was studied three times (baseline, 1 week after diet and 1 week after surgery); second cohort (n=18) was also studied three times (baseline, 1 year FU_1 and ~ 9 years FU_2 after surgery). Each time subjects underwent infusion of stable isotope tracers of [6,6-2H]-glucose and [2H5]-glycerol infusion for the measurement of endogenous glucose production (EGP), hepatic_IR (EGPxIns)), lipolysis, adipose tissue IR (as Lipo- IR=(RaGlycerolxIns) or Adipo-IR=(FFAxIns)), peripheral insulin sensitivity as fasting glucose clearance (EGP/glucose). AA composition was measured by GC-MS and lipidomic profile by LC- MS QTOF with the quantification of mono- (MAG), di (DAG and tri-acylglycerols (TAG), phosphocholines (PC and LysoPC), ceramides (Cer), phosphoethanolamines (PE and Lyso PE) and sphingomyelins (SM) concentrations. Results: We observed already 1 week after surgery a significant improvement of Hep-IR and adipose tissue-IR. That results were confirmed in the second cohort studied 1 year (FU_1) and up to 9 years of follow-up (FU_2). GNG_AA concentrations were decreased already 1 week after surgery and were correlated to EGP. Hepatic IR improved as one week after surgery and was further decrease at FU_2.Some AA (alanine, tyrosine and proline) which we found decreased at FU_1 and FU_2 were already decreased 1 week after surgery. The concentrations of the aromatic AA, Phe and Tyr that are mainly metabolized in the liver, were decreased slightly after 1 week, and more significantly after surgery FU_1 and FU_2. Tyrosine was decreased already one week and was correlated to EGP. The lipid profile changes were observed mainly at FU_1 and FU_2, while only TAGs concentrations were decreased at 1 week. The degree of TAG saturation (ie TAG(0-1)/TAG(2-7)) correlated with Hep-IR, fatty liver index (FLI; but not Lipo-IR after surgery FU_1 and FU_2). At FU_1 and FU_2 we observed a decreased in in ceramides and SM Conclusion: Bariatric surgery induces metabolic improvement not only in long term but also independent or weight loss particularly at the level of hepatic glucose metabolism by changing mainly GNG_AA. Improvement in lipid metabolism was limited to TAG profile in short term FU, while in long term FU all lipids were decreased, lipid composition was changed toward a decrease in the saturate/unsaturated fatty acids abundance in the different lipid compounds, a decrease in ceramides and SM.