BACKGROUND: Obesity is a critical risk factor for chronic kidney disease and cardiovascular disease. The study aimed to explore the relationship between endothelial function, assessed by flow-mediated dilation (FMD), and kidney function, estimated using cystatin C-based eGFR (eGFRcys), in individuals with severe obesity undergoing bariatric surgery. METHODS: Sixty-five individuals with a BMI >
35 kg/m RESULTS: Bariatric surgery significantly improved BMI, FMD (p <
.001) and eGFRcys (p = .042). Before surgery, eGFRcys was positively correlated with FMD (r = .30, p = .011) and inversely correlated with cf-PWV (r = -.26, p = .020), while eGFRcr showed weaker or non-significant associations with vascular variables. eGFRcys increased post-surgery, correlating positively with improvements in FMD (traditional: r = .26, p = .038
allometric: CI [.19, .82], p = .003). Multivariable mixed models confirmed the robust association between eGFRcys and FMD calculated by allometric scaling, independent of BMI or blood pressure changes. In contrast, eGFRcr showed no significant association with FMD changes. CONCLUSIONS: Bariatric surgery enhances both renal and vascular health in individuals with severe obesity. Cystatin C-based eGFR correlates more strongly with endothelial function improvements than creatinine-based eGFR. These findings highlight the utility of cystatin C as an integrative marker for assessing renal and vascular risk in populations affected by obesity undergoing metabolic surgery.