By Kirsty Oswald
Smoking increases the risk for Crohn's disease (CD) but not ulcerative colitis (UC), the results of the largest prospective study of the link between inflammatory bowel disease (IBD) and the habit confirm.
The study also showed that, while there is no link between current smoking and UC risk, smoking cessation was associated with a 56% increase in risk for the disease compared with those that had never smoked (0.33 vs 0.15%).
"The distinctly different relationship between cigarette smoking and CD from that of smoking and UC underscores the complexity of the pathogenesis of these diseases," say Leslie Higuchi (Harvard Medical School, Boston, Massachusetts, USA) and colleagues.
The data were taken from 229,111 women enrolled in the Nurses' Health Study (NHS) since 1976 and the NHS II since 1989. Together, 54% of the participants had never smoked, 22% were former smokers, and 23% were current smokers. The participants updated their smoking status every other year, allowing the researchers to assess the effects of smoking cessation. Over 32 years of follow up, there were 336 incident cases of CD and 400 incident cases of UC.
The results, reported in the American Journal of Gastroenterology, echo those of previous meta-analyses. Current smoking increased the risk for CD by 90% compared with those who had never smoked (0.14 vs 0.12%) and former smokers had a 35% increased risk (0.23 vs 0.12%). The risk increased with cumulative smoking dose, measured in pack-years in both current and former smokers.
By contrast, the authors found no significant association between current smoking and risk for UC, which is consistent with several other studies that fail to support its putative protective effect. However, after 2-5 years of giving up smoking, a significant increase in risk became apparent in former smokers, such that they were three times as likely to develop UC compared with current smokers (one in 162,897 vs 915,207 person-years).
UC was also linked to cumulative smoking dose. Participants that quit after 5 pack-years or less had a similar risk for UC as those who had never smoked, compared with a 2.12-fold increase in risk in those with over 25 pack-years smoking.
The authors suggest that this could be due to suppression of UC risk in genetically predisposed individuals, where withdrawal of smoking may lead to UC onset or unmask symptoms. Smoking has also been reported to potentially affect the colonic mucus layer, modify cytokine production, and affect the microvasculature, which could influence UC or CD risk.
The authors conclude that more research is needed: "Further studies may clarify the mechanisms by which smoking and smoking cessation influence the risk of IBD."
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