The Mediating Effect of Smoking on the Association between Income and Dementia among Japanese Older People.

Introduction: Health inequalities in dementia have been reported. Smoking is a risk factor for dementia and is disproportionately distributed in marginalized populations. This study examined the mediating effect of smoking on the association between income and dementia among older Japanese people. Methods: This longitudinal study was based on the Japan Gerontological Evaluation Study from 2010 (response rate = 64.3%) to 2019 (follow-up rate = 98.5%). A total of 44,083 participants independent in daily life were analyzed (mean age: 73.7; women 53.2%). The outcome variable was the incidence of dementia between 2010 and 2019, and the explanatory variable was equivalent income measured in 2010. Causal mediation analyses with a Cox proportional hazard model were performed to evaluate the mediating effect of smoking status in 2010. Multiple imputation was performed for the missing data. Results: Among the participants, the incidence rates of dementia were 16.2% for men and 18.2% for women. Low income was associated with the incidence of dementia (total effect in excess relative risk, ERR [95% confidence interval (CI)]: 0.095 [0.032-0.157] in overall participants, 0.102 [0.011-0.192] for men, 0.082 [-0.003 to 0.168] for women). Causal mediation analyses showed that smoking mediated the association between income and dementia (natural indirect effect in ERR [95% CI]: 0.007 [0.004-0.011] for overall participants, 0.007 [0.002-0.013] for men, and 0.005 [0.001-0.009] for women). The proportions of the mediating effect were 7.7% for all participants, 7.3% for men, and 6.4% for women. Conclusions: Our results showed that smoking partially explained the association between income and dementia. There is a possibility that smoking cessation may contribute to reducing health inequalities in dementia.