Synergistic effect of smoking and education on incident dementia among older adults: The Shanghai aging study

Background: The association between smoking and dementia incidence remains controversial, while education is considered a cognitive safeguard. This study aims to explore the joint effect of smoking and education on incident dementia. Methods: We enrolled 1647 community-dwelling residents, and divided them into three groups: non-smoker (Smoking Index [SI, cigarette-years] = 0), mild-smoker (0 G SI G 265), and severe-smoker (SI >= 265). Their education was categorized to high or low level (formal education > 9 or <= 9 years). Dementia diagnosis was established according to the Diagnostic and Statistical Manual of Mental Disorders IV. Cox proportional hazards models were used for the analysis, and three indicators of additive interaction between low education and severe smoking were calculated. Results: We identified 167 cases (10.1 %) of incident dementia during the 8544.2 person-years of follow-up. After adjusting for age, gender, and APOE epsilon 4, a multiplicative interaction between severe smoking and low education was observed (Hazard Ratio (HR) = 4.01, 95 % CI: 1.29-12.48, P = 0.017), as well as an additive interaction between the two risk factors, with Relative Excess Risk due to Interaction was 2.63 (P = 0.017), Attributable Proportion due to Interaction was 0.60 (P G 0.001), and Synergy Index was 4.46 (P = 0.022), and compared to highly educated non-smokers, less educated severe smokers had a 3.38-fold increased risk of dementia (HR = 4.38, 95 %CI: 2.50-7.67, P G 0.001). Conclusion: Severe smoking and low education may have synergistic effect on incident dementia. Targeted tobacco control interventions are crucial for dementia prevention.