Sex Differences in the Association Between Body Mass Index and Dementia Risk in Community-Dwelling Japanese People Aged 40-74 Years.

BACKGROUND The association between body mass index (BMI) and dementia risk is heterogeneous across age groups and might be influenced by sex. OBJECTIVE This study aimed to clarify sex differences in the association between BMI and dementia risk in community-dwelling people. METHODS This cohort study with an 8-year follow-up targeted 13,802 participants aged 40-74 years at baseline in 2011-2013. A self-administered questionnaire requested information on body size, including height, weight, and waist circumference (the values of which were validated by direct measurement), socio-demographics, lifestyle, and disease history. BMI was calculated and categorized as < 18.5 (underweight), 18.5-20.6 (low-normal), 20.7-22.7 (mid-normal), 22.8-24.9 (high-normal), 25.0-24.9 (overweight), and≥30.0 kg/m2 (obese). Incident cases of dementia were obtained from the long-term care insurance database. A Cox proportional hazards model was used to calculate multivariable-adjusted hazard ratios (HRs). RESULTS The mean age of participants was 59.0 years. In men, higher BMI was associated with lower dementia risk (fully-adjusted p for trend = 0.0086). In women, the association between BMI and dementia risk was U-shaped; the "underweight," "low-normal," and "overweight" groups had a significantly higher risk (fully-adjusted HR = 2.12, 2.08, and 1.78, respectively) than the reference ("high-normal" group). These findings did not change after excluding dementia cases which occurred within the first four years of the follow-up period. CONCLUSION Overweight/obese women, but not men, had an increased risk of dementia, suggesting that sex differences in adiposity might be involved in the development of dementia.