Longitudinal associations of sustained low or high income and income variability with incident cardiovascular disease in individuals with type 2 diabetes: a retrospective population-based cohort study

Background and Aims Longitudinal change in income is crucial in explaining cardiovascular health inequalities, but there is limited evidence for cardiovascular disease (CVD) risk associated with income dynamics over time among individuals with type 2 diabetes (T2D). Methods Using a nationally representative sample from the Korean Health Insurance Service database, 1,528,108 adults with T2D aged 30-64 years and no history of CVD were enrolled between 2009-2012 (mean follow-up of 7.3 years). Using monthly health insurance premiums information, income levels were assessed annually for 4 years before the baseline year. Income variability was defined as the intraindividual standard deviation of the percent change in income across 5 years. The primary outcome was a composite event of incident fatal and nonfatal CVD (myocardial infarction, stroke, and heart failure) using insurance claims. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated after adjusting for potential confounders. Results Sustained low income (i.e., lowest income quartile) over 5 years was associated with increased CVD risk (HRn=5years vs. n=0years 1.38, 95% CI 1.35-1.41; Ptrend<0.0001), whereas sustained high income (i.e., highest income quartile) was associated with decreased CVD risk (HR 0.71n=5years vs. n=0years 95% CI 0.70-0.72; Ptrend<0.0001). High-income variability was associated with increased CVD risk (HRhighest vs. lowest quartile 1.25, 95% CI 1.22-1.27; Ptrend<0.001). Individuals who experienced an income decline across 5 years leading up to baseline had increased CVD risk, particularly in a decrease to the lowest income level (i.e., Medical Aids beneficiaries), regardless of initial income status. Sensitivity analyses, including potential mediators, such as lifestyle-related factors and obesity, supported the results. Conclusions Among non-elderly Korean adults with T2D, sustained low income, higher income variability, and income declines were associated with increased CVD risk. Our findings highlight the need to understand better the mechanisms by which income dynamics impact CVD risk among individuals with T2D. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported by a grant (K.H., 2022F-6) from the Korean Diabetes Association. The funder had no role in considering the study design; the collection, analysis, or interpretation of data; the writing of the report; or the decision to submit the article for publication. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee/IRB of Soongsil University, Seoul, South Korea gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present work are contained in the manuscript.