Quantifying the Dementia Burden Attributable to Excess Weight in the United States.

INTRODUCTION: Studies assessing the risk of dementia associated with excess weight have found mixed results due to methodological challenges. This study sought to quantify this relationship while avoiding the confounding that is attributable to reverse causality.

METHODS: This cohort study used data from the Health and Retirement Study (1992-2018) analyzed in 2024-2025. The exposure was maximum BMI over 10 years of weight history before the start of follow-up. Participants were followed for 16 years for incident dementia, death, or censoring. Population attributable fractions for dementia were calculated using hazard ratios (HRs) from multivariable Cox proportional hazards models and prevalence of overweight and obesity.

RESULTS: The final sample comprised 3,734 individuals (57.1% female, mean age 64.7 [SD 4.4]) and 395 incident dementia cases. Compared to those with a maximum BMI in the normal weight range (18.5-24.9 kg/m), the class II/III obesity group (35+ kg/m) had almost double the hazard of dementia over follow-up (adjusted [a]HR 1.89, CI 1.31, 2.73), the class I obesity group (30-34.9 kg/m) had 42% increased hazard (CI 1.01, 2.00) and the overweight group (25-29.9 kg/m) had 22% increased hazard (CI 0.89, 1.67). If all individuals with a maximum BMI in the overweight or obesity range had been in the normal weight group, 22.1% (CI 1.8, 38.2) of dementia cases could have potentially been averted.

CONCLUSIONS: The contribution of obesity to the burden of dementia is larger than current evidence suggests. Investigating this association without accounting for the complexity of the temporal relationship between obesity and dementia can result in erroneous conclusions.