Abstract
Background and aims
The association between smoking and inflammatory bowel disease (IBD) relies on old meta-analyses including exclusively non-Jewish White populations. Uncertainty persists regarding the role of smoking in other ethnicities.
Methods
We systematically searched Medline/PubMed, Embase and Scopus for studies examining tobacco smoking and the risk of developing IBD, i.e., Crohn’s disease (CD) or ulcerative colitis (UC). Two authors independently extracted study data and assessed each study’s risk-of-bias. We examined heterogeneity and small-study effect, and calculated summary estimates using random-effects models. Stratified analyses and meta-regression were employed to study the association between study-level characteristics and effect estimates. The strength of epidemiological evidence was assessed through prespecified criteria.
Results
We synthesized 57 studies examining the smoking-related risk of developing CD and UC. Non-Jewish White smokers were at increased risk of CD (29 studies; RR: 1.95, 95% CI: 1.69‒2.24; moderate evidence). No association was observed in Asian, Jewish and Latin-American populations (11 studies; RR: 0.97; 95% CI: 0.83–1.13), with no evidence of heterogeneity across these ethnicities. Smokers were at reduced risk of UC (51 studies; RR: 0.55, 95% CI: 0.48–0.64; weak evidence) irrespectively of ethnicity; however, cohort studies, large studies and those recently published showed attenuated associations.
Conclusions
This meta-analysis did not identify any increased risk of CD in smokers in ethnicities other than non-Jewish Whites, and confirmed the protective effect of smoking on UC occurrence. Future research should characterize the genetic background of CD patients across different ethnicities to improve our understanding on the role of smoking in CD pathogenesis.
Meta-analysis of the role of oral contraceptive agents in inflammatory bowel disease.
