Researchers from England and Switzerland investigated whether smoking determines the type of inflammatory bowel disease (IBD) that develops in individuals with very similar genetic susceptibility.
Smoking tobacco is known to predispose to the development of Crohn's disease (CD), yet is associated with a reduced incidence of ulcerative colitis (UC).
Smoking habits and disease characteristics were analyzed in 242 IBD pedigrees (658 patients).
Within this group there were 339 affected sibling pairs, of whom 89 were discordant for smoking when diagnosed.
|Odds ratios for smokers with IBD:|
Smoking at diagnosis was associated with development of CD (odds ratio (OR) 3.55) in all of the familial patients, with increases when analyzed for ileocecal disease, fibrostenosis, and intestinal resection.
Smokers were also protected from UC (OR 0.28).
Of the 89 sibling pairs discordant for smoking at diagnosis, 23 were also discordant for disease type. In 21 of these pairs, CD occurred in the smoker and UC in the non-smoker (OR 10.5).
Author S. Bridger, of Guy's, King's, and St. Thomas's School of Medicine and Dentistry. London, England, said on behalf of the group, "Smoking is a strong environmental risk factor for CD and increases the likelihood of needing surgery.
"However, sib pairs who are discordant for both smoking and IBD type almost always show CD in the smoker and UC in the non-smoker.
"Therefore, in some cases tobacco consumption acts on IBD genetic predisposition to shift the phenotype from UC towards CD."
"The explanation of part of the apparent protective effect of smoking on sporadic UC may be that the form of IBD that develops in a proportion of smokers is not UC but CD," it was concluded.