Effect of income level on stroke incidence and the mediated effect of simultaneous diagnosis of metabolic syndrome diseases; a nationwide cohort study in South Korea

Background: This study aimed to determine whether a simultaneous diagnosis of main components of metabolic syndrome (MetS) (hypertension, diabetes mellitus, and dyslipidemia) plays a mediator between income level and stroke. Methods: We used the National Health Insurance Service National Sample Cohort database from 2006 to 2015. The mediator variables were the number of main MetS components diagnosed simultaneously (two or more/three or more). We used a weighting approach method of causal mediation analysis to apply counterfactual frameworks to the Cox proportional hazards regression model. Results: A total of 213,526 people were included with 1,690,665.3 person-years of followed up. Compared with the high-income group, the risk of being diagnosed with two or more components of MetS significantly increased in all other income groups [middle-income OR 1.05 (95% CI 1.02–1.08); low-income OR 1.09 (95% CI 1.05–1.12); Medical Aid beneficiaries OR 1.39 (95% CI 1.32–1.47)]. A lower level of income was significantly associated with a higher risk of stroke compared with the high-income group [middle-income HR 1.15 (95% CI 1.07–1.25); low-income HR 1.19 (95% CI 1.10–1.29); Medical Aid beneficiaries HR 1.63 (95% CI 1.48–1.80)]. In the Medical Aid beneficiaries, simultaneous diagnosis of the main metabolic components acted as a significant mediator between income levels and stroke incidence, with 26.6% mediated when diagnosed with two or more diseases and 21.1% when diagnosed with all three. Conclusions: Co-diagnosis of MetS components played a significant mediator role between income level and stroke incidence.