Income dynamics and risk of early-onset dementia: a nationwide cohort study

BACKGROUND: The relationship between income dynamics-including sustained income level, income changes, and income variability-and early-onset dementia (EOD) has not been well established. This study investigated the association between income dynamics and the risk of EOD. METHODS: This cohort study included 2,247,461 adults aged 40-60 years who underwent health examinations in 2012 through the Korean National Health Insurance Service, with a median follow-up duration of 5.4 years till 2018. Income status was assessed for 2012 and the 4 preceding years using monthly health insurance premiums, and income-related parameters were derived from this pre-baseline 5-year window. Income variability was defined as the intra-individual standard deviation of four consecutive annual percentage changes. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated to evaluate the relationship between income dynamics and EOD risk, adjusting for potential confounders. RESULTS: Sustained low-income status over five years was associated with an increased risk of EOD (HR for 5 years vs. 0 years 1.63, 95% CI 1.49-1.78; P-trend < 0.001), whereas sustained high-income status was associated with a reduced EOD risk (HR for 5 years vs. 0 years 0.55, 95% CI 0.51-0.59; P-trend < 0.001). Notably, higher income variability was linked to a greater risk of EOD (HR for highest vs. lowest quartile 1.37, 95% CI 1.28-1.48; P-trend < 0.001). The decline in income-particularly to the lowest level (Medical Aid beneficiaries)-was associated with an elevated risk of EOD, irrespective of the initial income status. CONCLUSIONS: Sustained low income, income decline, and greater income variability were associated with higher EOD risk. These findings may help identify socioeconomically vulnerable middle-aged adults for targeted dementia risk assessment and prevention.