Background Acute appendicitis (AA) is a common surgical emergency. Observational studies have reported associations between obesity-related anthropometric traits and AA, but these associations may be affected by confounding and reverse causation. We used Mendelian randomization (MR) to investigate the potential causal effects of body mass index (BMI) and waist-to-hip ratio (WHR) on AA risk.

Methods and Findings We obtained genome-wide association study (GWAS) summary statistics for BMI (ukb-b-19953), WHR (ieu-a-73), and AA (finn-b-K11_APPENDACUT) from the IEU Open GWAS database. We conducted single-variable MR (SVMR) and multivariable MR (MVMR) analyses. The primary estimator was inverse-variance weighted (IVW) MR, complemented by MR-Egger, weighted median, weighted mode, and simple mode methods. Instrument strength was assessed using the variance explained and F-statistics. Sensitivity analyses included Cochran’s Q for heterogeneity, MR-Egger intercept and MR-PRESSO global test for horizontal pleiotropy, leave-one-out analysis, and Steiger directionality testing. We mapped instrumental variants to cis-eQTL genes (eQTLGen) and performed GO and KEGG enrichment analyses.

In SVMR, genetically predicted BMI (OR 1.145, P = 0.0006) and WHR (OR 1.336, P = 0.0040) were associated with higher AA risk. Instruments were strong (BMI: ; mean/min F-statistic= 64.15/29.76; WHR: ; mean/min F-statistic= 48.41/29.75). Sensitivity analyses did not show strong evidence of heterogeneity or directional pleiotropy, and Steiger tests supported the hypothesized direction (exposure -> outcome). In MVMR including both traits, WHR remained associated with AA risk (OR 1.374, P = 0.0110), whereas BMI was not (P = 0.8000). Enrichment analyses suggested WHR-mapped genes were enriched in pathways related to adipocyte differentiation, while BMI-mapped genes were enriched in terms including nuclear envelope and endocytosis-related pathways.

Conclusions These MR analyses are consistent with a potential causal relationship between obesity-related traits and AA risk, with WHR showing an association independent of BMI in multivariable models. Further work in diverse populations and with additional sensitivity analyses is warranted to assess robustness to pleiotropy and generalizability.

The authors have declared no competing interest.

This study was supported by the Gansu Provincial Department of Science and Technology project on the role of WHR and BMI in the distribution of lymphocytes and cytokines in the pathogenesis of pediatric appendicitis, and by the Gansu Provincial Natural Science Foundation (grant number 25JRRA331). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Ethics Committee of Gansu Provincial Maternal and Child Health Care Hospital waived ethical approval for this work. This study used only publicly available, de-identified summary-level GWAS data from the IEU OpenGWAS database and FinnGen. No new participants were recruited, and no individual-level identifiable data were used. Ethical approval and informed consent had been obtained in the original studies by their respective ethics committees.

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