Background The age-specific association between obesity and risk of dementia in Chinese middle- and older- aged adults is unclear. We aimed to disentangle this association via examining dementia risk associated with overweight and obesity at ages 45, 55, and 65 years, and by comparing age-based body mass index (BMI) and waist circumference (WC) trajectories of participants with dementia with their matched counterparts who are free of dementia.

Methods Our analysis was based on the China Health and Retirement Longitudinal Study (CHARLS) wave 1 (2011) to wave 4 (2018), with 18927 participants and 597 cases of dementia. Dementia was defined using confirmatory factor analysis. A Cox regression model was used to estimate hazard ratios (HRs) of BMI (underweight (< 18.5 kg/m²), normal weight (18.5 - 23.99 kg/m², reference), overweight (24 - 27.99 kg/m²), and obese (≥ 28 kg/m²)) or WC (abdominal obesity (≥ 90/85 cm in men/women) or not (< 90/85 cm in men/women, reference)) associated with incidence of dementia over 7 years. A mixed effect model was applied to analyse BMI-age trajectories between the dementia group and matched controls. All analyses were adjusted for gender and educational level. Birth cohort effects were further tested in the mixed effect model additionally.

Results Overweight (HR = 1.43, p = 0.016), obesity (HR = 1.83, p = 0.002), and abdominal obesity (HR = 1.57, p = 0.001) at age 55 years were associated with risk of dementia incidence, but not in other age groups. The dementia group’s age-based BMI trajectory tended to be higher (0.33, p = 0.045) at midlife but declined more rapidly with age than the control group, resulting in a lower BMI than that of the control group after age 65. The younger cohort with a baseline age <55 years showed a slightly increased trend in BMI during follow-up, whereas the opposite trend was observed in the older cohort.

Conclusion BMI over 24 kg/m² and abdominal obesity in midlife are risk factors of dementia but not in older age. This association may be influenced by a birth cohort effect. Managing BMI may help reduce the burden of dementia in the future.