Poor oral health and dementia risk under time‐varying confounding: A cohort study based on marginal structural models

Abstract Background Bidirectional association between oral health, including tooth loss and oral hypofunction, and cognitive impairment can induce time‐varying confounding in association with dementia. This study aimed to investigate the association between oral health and the development of dementia among older adults, considering cognitive impairment as a time‐varying confounder. Methods This nine‐year follow‐up cohort study targeted independent older adults aged ≥65 years who participated in the Japan Gerontological Evaluation Study. The exposure variables were self‐reported poor oral health (≤19 remaining teeth, edentulousness, chewing difficulty, swallowing problems, and xerostomia), which were assessed in 2010 and 2013. The outcome variable was the incidence of dementia between 2013 and 2019, which was obtained from the municipalities' administrative database. Furthermore, in 2010 and 2013, we considered cognitive impairment as a time‐varying confounder and treated it based on a marginal structural model (MSM), including possible baseline confounders. Oral health and cognitive impairment were assessed using the items of Kihon checklist. We employed a Cox proportional hazards model with a stabilized inverse probability weight and estimated hazard ratios (HRs) and 95% confidence intervals (CIs). Results Among 37,556 participants (mean age: 72.8 years [1 SD = 5.5], males: 46.8%), the dementia incidence rate was 2.2/100 person‐year. The proportions of those with ≤19 remaining teeth, edentulousness, chewing difficulty, swallowing problems, and xerostomia were 61.3%, 11.0%, 24.4%, 14.7%, and 19.2%, respectively. From the regression analysis based on MSM, ≤19 remaining teeth (HR = 1.12, 95% CI = 1.03–1.23), edentulousness (HR = 1.20, 95% CI = 1.09–1.32), chewing difficulty (HR = 1.11, 95% CI = 1.02–1.21), and xerostomia (HR = 1.10, 95% CI = 1.01–1.20) were significantly associated with an increased risk of dementia; however, swallowing problems were not significantly associated with dementia onset ( p > 0.05). Conclusions Even after considering time‐varying confounding by cognitive function at baseline and follow‐up, we observed significant associations between poor oral health and increased risk of dementia among older adults.