Body mass index, weight change in midlife, and dementia incidence: the Japan Public Health Center‐based Prospective Study

INTRODUCTION: Insufficient evidence exists on the sex‐specific associations of body mass index (BMI) and weight change through midlife with dementia incidence, especially in Asian populations. METHODS: For 37,414 Japanese residents aged 40 to 59 years, BMIs at baseline (year 1990 or 1993) and 10‐year follow‐ups were obtained. Weight changes between baseline and 10‐year follow‐ups were determined. Disabling dementia incidence from 2006 to 2016 was ascertained using long‐term care insurance (LTCI) certifications. Hazard ratios (HRs) were computed. RESULTS: Increased dementia risk was observed with obesity at baseline and with underweight at 10‐year follow‐ups. Weight loss after baseline was at greater risk than weight gain. No sex difference was observed. DISCUSSION: In both sexes, obesity in midlife increased the risk of developing dementia with increasing impacts of weight loss after midlife. A healthy body weight throughout adulthood is beneficial for dementia prevention. HIGHLIGHTS: Obesity in midlife is a risk factor for incident dementia. Weight loss is a bigger risk factor than weight gain in later midlife. Association of BMI and weight change in midlife with dementia does not vary by sex.