Associations of Obesity with the Risk of Anal Fistula: A Mendelian Randomization Study

(1) Background: Obesity has become a worldwide pandemic, while anal fistulas represent a prevalent anorectal disorder that affects a significant number of individuals across the globe. However, the relationship between obesity and anal fistula remains unclear. (2) Methods: We assess obesity comprehensively through multiple indicators including body mass index (BMI), body fat percentage (BFP), waist circumference (WC), and waist-hip ratio (WHR). In order to evaluate the causal effects of obesity on the risk of anal fistula, two-sample Mendelian randomization (MR) analysis was completed using five methods: inverse variance weighting (IVW), MR-Egger, weighted median, simple mode and weighted mode. IVW method was used as the main method. (3) Results: IVW method found that there were positive effects of genetically determined BMI (OR: 1.001, 95%CI: 1.001-1.002, p = 0.022), BFP (OR: 1.001, 95%: 1.000-1.003, p = 0.035), WC (OR: 1.001, 95%CI: 1.000-1.003, p = 0.035), WHR (OR: 1.001, 95%CI: 1.000-1.003, p = 0.024) on the risk of anal fistula. The MR-Egger intercepts and MR-PRESSO method show no evidence for significant pleiotropy and heterogeneity. (4) Conclusion : Our MR study supports a causal role of obesity in increasing the risk of anal fistula. We emphasize that obese patients with anal fistula underscore the urgent need for attention to weight control.