Exploring the complex link between obesity and intelligence: Evidence from systematic review, updated meta-analysis, and Mendelian randomization.

Obesity is a major public health concern associated with a higher risk of various comorbidities. Some studies have explored the impact of obesity on cognitive function and, conversely, how lower intelligence might increase the risk of later obesity. The aim of this study is to analyze a complex relationship between body mass index (BMI) and intelligence quotient (IQ), employing a comprehensive approach, including a systematic review, meta-analysis, and Mendelian randomization (MR). We extracted the data from Medline and Embase to identify relevant studies published since June 22, 2009. MR analysis relied on genetic databases such as the Genome-Wide Association Study (GWAS) and the Genetic Investigation of Anthropometric Traits (GIANT) to explore potential causal relationships. The systematic review and meta-analysis encompassed 34 and 17 studies, respectively. They revealed a substantial correlation between obesity and reduced IQ, particularly notable among school-age children (mean difference -5.26; 95% CI: -7.44 to -3.09). Notably, within the IQ subgroup, verbal IQ also exhibited a significant association with a mean difference of -7.73 (95% CI: -14.70 to -0.77) in school-age children. In contrast, the MR did not unveil a significant causal relationship between BMI and IQ, both in childhood and adulthood. This comprehensive analysis underscores a significant correlation between BMI and IQ, particularly in school-age children. However, the MR analysis implies a potentially weaker causal relationship. Future large-scale cohort studies should address potential confounding factors to provide further insights into the BMI-IQ relationship.