Context: There are only a few systematic reviews on the association of obesity with risk of inflammatory bowel disease (IBD) to date.

Objective: The current study was undertaken to systematically review prospective cohort studies on the association between body mass index (BMI) and risk of IBD. It was carried out according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.

Data sources: Relevant prospective cohort studies published from 1969 to July 2020 were searched through PubMed, MEDLINE, SCOPUS, EMBASE, and Google Scholar, using suitable keywords.

Data extraction: Hazard ratios (HRs) or relative risks (RRs) and 95% confidence intervals (CIs) for IBD or its subtypes across categories of BMI were extracted.

Data analysis: The log HRs/RRs, including standard errors, were calculated based on reported HRs or RRs and their 95% CIs, and overall effect size was calculated using a fixed-effects model. All statistical analyses were done using STATA version 14.0 (Stata Corp LP, College Station, TX, USA).

Conclusion: Overall, 9 studies were included. Combining findings from 5 studies, a statistically significant 21% lower risk of ulcerative colitis incidence was found in patients with obesity than in those with normal weight (RR, 0.79; 95% CI, 0.68-0.92; I2= 0.0%), but not with risk of ulcerative colitis exacerbation, as found by meta-analysis of 3 studies. Pooling data from 5 studies, no significant differences were seen in the risk of Crohn's disease incidence between patients in the highest range of BMI and those in the normal range. In addition, no significant nonlinear association was found between BMI and risk of Crohn's disease (P=0.94). A significant inverse association was found between obesity and total IBD incidence (RR, 0.76; 95% CI, 0.66-0.88; I2=93.2%), but not between increasing BMI and IBD exacerbation, or between increasing BMI and IBD incidence.