High Body Mass Index as a Causal Risk Factor for Vascular-Related Dementia: A Mendelian Randomization Study

Context:
Obesity is associated with a high risk of vascular-related dementia with metabolic risk factors as potential mediators, but questions of causality remain unanswered.

Objective:
We aimed to determine whether high body mass index (BMI) is a causal risk factor for vascular-related dementia, and whether any effect is mediated by hypertension, hyperlipidemia, hyperglycemia, and low-grade inflammation.

Methods:
Prospective cohort studies of the general populations from the Copenhagen area and from across the United Kingdom and consortia data were included in the study. Interventions included one-sample mendelian randomization (MR), two-sample MR, and MR in mediation analyses. Both individual-level and summary-level data was used. Main outcome measures included risk of vascular-related dementia, Alzheimer's disease, and ischemic heart disease.

Results:
In a meta-analysis of 2 one-sample MR studies, the odds ratio (OR) for 1-SD higher BMI in predicting vascular-related dementia was 1.63 (95% CI, 1.13-2.35). In a two-sample MR study, the OR for vascular-related dementia per 1-SD higher BMI was 1.54 (1.10-2.16) using the inverse-variance weighted, 1.87 (1.22-2.85) using the weighted median, and 1.98 (1.21-3.22) using the weighted mode methods. Results from MR analyses including extended numbers of genetic variants were directionally consistent. Finally, systolic blood pressure mediated 18% (95% CI, 10%-61%) and diastolic blood pressure mediated 25% (13%-75%) of the genetic effect of BMI on vascular-related dementia.

Conclusion:
Observationally (U-shaped) and genetically (linearly), high BMI is associated with a higher risk of vascular-related dementia, an association partly mediated through high blood pressure. This suggests that high BMI and high blood pressure are important modifiable risk factors for dementia prevention.