Body Shape and Alzheimer's Disease: A Mendelian Randomization Analysis

Area of Science:

• Neuroscience
• Genetics
• Public Health

Background:

• Obesity is linked to cognitive decline and Alzheimer's disease (AD).
• Observational studies show conflicting associations between body shape and AD, hindering causal inference.
• Residual confounds and reverse causation complicate understanding the obesity-AD relationship.

Purpose of the Study:

• To investigate the causal association between body shape and Alzheimer's disease (AD) risk.
• To utilize Mendelian randomization (MR) to overcome limitations of observational studies.
• To assess the impact of various anthropometric traits on AD causality.

Main Methods:

• Two-sample Mendelian randomization (MR) analysis using genome-wide association study summary data.
• Instrumental variables included single nucleotide polymorphisms (SNPs) associated with body mass index (BMI), waist-to-hip ratio (WHR), body mass index-adjusted WHR (WHRadjBMI), and waist circumference (WC).
• Data from the GIANT consortium and International Genomics of Alzheimer's Project (IGAP) were analyzed using inverse-variance weighted methods.

Main Results:

• No statistically significant causal effect was observed between body shape metrics (BMI, WHR, WHRadjBMI, WC) and Alzheimer's disease risk.
• Odds ratios for AD per 1-SD difference were: BMI 1.04 (0.88, 1.23), WHR 1.01 (0.77, 1.33), WHRadjBMI 1.12 (0.89, 1.41), and WC 1.02 (0.82, 1.27).
• Simulation analyses indicated no bias from survivor effects on the BMI-AD relationship.

Conclusions:

• Mendelian randomization analyses do not support a causal relationship between body shape and Alzheimer's disease risk.
• These findings contrast with previous observational studies, highlighting the need for further research into underlying biological mechanisms.
• The study underscores the complexity of the obesity-cognition-AD pathway.