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• Roux-en-Y gastric bypass (RYGB), but not sleeve gastrectomy, reduces plasma PCSK9 levels. • Plasma PCSK9 changes after RYGB are not associated with changes in LDL-C. • Changes in plasma PCSK9 are correlated with changes in glucose parameters after bariatric surgery. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a master regulator of low-density lipoprotein cholesterol (LDL-C) metabolism, acting as an endogenous inhibitor of the LDL receptor. While it has been shown that bariatric surgery differentially affects plasma LDL-C levels, little is known of its effects on plasma PCSK9 concentrations. Therefore, the present study aimed to: (i) investigate the effect of sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB) on plasma PCSK9 concentrations; and (ii) correlate baseline or postoperative plasma PCSK9 concentration variations with anthropometric and metabolic parameters. Fasting plasma PCSK9 levels were measured by ELISA in morbidly obese patients before and 6 months after bariatric surgery. Patients were recruited from three prospective cohorts (in Nantes and Colombes in France, and Antwerp in Belgium). A total of 156 patients (34 SG, 122 RYGB) were included. Plasma PCSK9 , LDL-C and non-high-density lipoprotein cholesterol (non-HDL-C) levels were significantly reduced after RYGB (−19.6%, −16.6% and −19.5%, respectively; P < 0.0001), but not after SG. In all patients, postoperative PCSK9 change was positively correlated with fasting plasma glucose (FPG; r = 0.22, P = 0.007), HOMA-IR (r = 0.24, P = 0.005), total cholesterol (r = 0.17, P = 0.037) and non-HDL-C (r = 0.17, P = 0.038) variations, but not LDL-C. In contrast to what was observed for glucose parameters (FPG, HOMA-IR), correlation between PCSK9 and non-HDL-C changes after RYGB was independent of total weight loss. RYGB, but not SG, promotes a significant reduction in plasma PCSK9 levels, and such changes in circulating PCSK9 levels after RYGB appear to be more associated with glucose improvement than with lipid homoeostasis parameters. [ABSTRACT FROM AUTHOR]