Body mass index levels and changes before and after dementia diagnosis and risk of all-cause mortality: a nationwide cohort study

Dementia is a progressive disease associated with various health risks, including increased mortality. Although weight changes have been linked to adverse outcomes in older adults, limited research has examined the impact of body weight status after dementia diagnosis and its changes before and after diagnosis on mortality. This study aimed to address these gaps. Using nationwide data from the Korean National Health Insurance Service, we included 37,717 individuals newly diagnosed with dementia between 2010 and 2016, comprising 29,982 with Alzheimer’s disease (AD) and 3,220 with vascular dementia (VaD). Participants were followed up until the end of 2019. Multivariable Cox proportional hazards regression models were used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause mortality based on body mass index (BMI) status after diagnosis and BMI changes before and after diagnosis. During a mean follow-up of 4.1 years, 21.7% of individuals with all-cause dementia died. Compared to those with normal BMI (18.5–22.9 kg/m²), underweight individuals (< 18.5 kg/m²) after diagnosis had a higher risk of all-cause mortality (HR 1.57, 95% CI 1.46–1.69). Excessive weight loss or gain was associated with the highest mortality risk—for example, weight loss from obesity to underweight yielded the highest risk (HR 2.09, 95% CI 1.26–3.46). Conversely, maintaining obesity (≥ 25 kg/m²) (HR 0.69, 95% CI 0.65–0.74) or modest weight gain from normal or overweight to obesity (HR 0.84, 95% CI 0.76–0.93) was associated with lower mortality risk. These associations remained consistent across dementia subtypes. Subgroup analyses revealed stronger associations in middle-aged individuals and in women with all-cause dementia. This large-scale cohort study suggests that excessive weight loss is significantly associated with the risk of all-cause mortality in individuals with dementia, while excessive weight gain may also be a marker of increased risk. Maintaining stable weight or modest weight gain is associated with lower mortality risk. These findings highlight the importance of monitoring body weight in dementia patients, especially during the critical period following diagnosis, to reduce mortality risk.