Is <scp>BMI</scp> a Mediator in the Association Between Subjective Masticatory Function and Functional Disability? Findings From the Shizuoka Kokuho Database Study

BACKGROUND: Poor subjective masticatory function (SMF) and abnormal body mass index (BMI) have been shown to be associated with functional decline in later life. Oral dysfunction can precede weight loss and frailty, and BMI often shows a U-shaped association with disability. However, whether BMI is on the pathway from SMF to disability remains unclear. OBJECTIVES: To examine whether BMI mediates the association between poor SMF and incident functional disability. METHODS: We conducted a retrospective cohort study using the Shizuoka Kokuho Database of health checkups, medical claims and long-term care insurance data in Shizuoka Prefecture, Japan. Residents aged ≥ 65 years who attended health checkups, excluding those with prior long-term care certification, were eligible. SMF was assessed at the baseline checkup, BMI at the subsequent checkup and the outcome was new long-term care certification. Time-to-event mediation models estimated total, natural direct and natural indirect effects. Sensitivity analyses used BMI category transition as the mediator. RESULTS: Among 169,319 participants, 22.1% reported poor SMF. Poor SMF was associated with a higher risk of disability (hazard ratio [HR] 1.07; 95% confidence interval [CI] 1.03, 1.11). Low and high BMI were each associated with disability; only the low-BMI pathway contributed to mediation. The proportion mediated was 11.1% (95% CI 4.6, 46.4). Findings were consistent in sensitivity analyses using BMI transition. CONCLUSIONS: Poor SMF was associated with a higher risk of functional disability, partly accounted for by low BMI. Preventing low BMI among older adults with impaired mastication-together with oral-function care-may help to preserve independence.