Assessing causality in the association between attention-deficit/hyperactivity disorder and obesity: a Mendelian randomization study

Background/Objectives Attention-deficit hyperactivity disorder (ADHD), one of the most common neurodevelopmental disorders in childhood and adolescence, is associated with obesity in observational studies. However, it is unclear whether ADHD contributes to, results from or is merely correlated with obesity. This study evaluates the presence and direction of a causal effect between ADHD and obesity. Subjects/Methods We performed a bidirectional two-sample Mendelian randomization using summary data from consortia of genome-wide association studies to investigate if ADHD ( N = 55,374) has a causal effect on body mass index (BMI) in childhood ( N = 35,668) and adulthood ( N = 322,154–500,000), and vice-versa. The main analysis was performed using the inverse variance weighted (IVW) method. As sensitivity analyses, we used other Mendelian randomization methods that are more robust to horizontal pleiotropy (i.e., MR-Egger, weighted mode, and penalized weighted median estimators), as well as stratified the analysis by the putative mechanisms of genetic instruments (i.e., pathways involved or not in neurological processes). Results The IVW method indicated a positive causal effect of BMI on ADHD: β = 0.324 (95% CI 0.198 to 0.449, p