Smoking and inflammatory bowel disease: a meta-analysis

OBJECTIVE: To assess whether there is a true effect of smoking on the 2 most prevalent forms of inflammatory bowel disease (IBD): Crohn disease (CD) and ulcerative colitis (UC). METHODS: For this meta-analysis, we searched multiple health care databases, including MEDLINE and EMBASE (January 1980 to January 2006), to examine the relationship between smoking and IBD. Keywords searched included smoking, Inflammatory bowel disease, Crohn's disease, and ulcerative colitis. Data were abstracted using predefined inclusion and exclusion criteria. An odds ratio (OR) was recalculated for each study using the random-effects model, and a combined OR was calculated. RESULTS: A total of 245 articles were obtained through an electronic search of health care databases. Thirteen studies examined the relationship between UC and smoking, whereas 9 examined the relationship between CD and smoking. We found evidence of an association between current smoking and CD (OR, 1.76; 95% confidence interval [CI], 1.40-2.22) and former smoking and UC (OR, 1.79; 95% CI, 1.37-2.34). Current smoking had a protective effect on the development of UC when compared with controls (OR, 0.58; 95% CI, 0.45-0.75). CONCLUSION: This is the first meta-analysis, to our knowledge, to evaluate the relationship between smoking and IBD using accepted quality standards for meta-analysis reporting. Our meta-analyses confirm that smoking is an important environmental factor in IBD with differing effects in UC and CD. By using predefined inclusion criteria and testing for homogeneity, the current analysis provides an estimate of the effect of smoking on both these forms of IBD.