Abstract
PURPOSE: Smoking may have a protective role in developing ulcerative colitis (UC) but have the opposite effect on Crohn's disease (CD). This study aimed to determine the risk of developing inflammatory bowel disease (IBD) according to smoking status and onset age of smoking. MATERIALS AND METHODS: We collected data on the smoking experiences of participants aged 20-39 years who underwent biannual examinations provided by the Korean National Health Screening Program from 2009 to 2012. IBD diagnosis was identified using the National Health Insurance Service. The risk of IBD according to smoking status and onset age of smoking was analyzed after adjusting for major clinical variables. RESULTS: During a median 10.59-year follow-up, the risk of UC in ex-smokers was significantly higher than that in non-smokers, and the earlier ex-smokers started smoking, the higher risk of UC [ex-smokers whose onset age of smoking was <20 years, adjusted hazard ratio (aHR) 1.928, 95% confidence interval (CI)=1.649-2.255; 20-24 years, aHR 1.728, 95% CI=1.541-1.939; 25-29 years, aHR 1.676, 95% CI=1.489-1.887; ≥30 years, aHR 1.226, 95% CI=1.010-1.486]. The risk of UC was significantly lower in current smokers whose onset age of smoking was 25-29 years than in non-smokers (aHR 0.825, 95% CI=0.709-0.959). The risk of CD did not differ according to smoking status and onset age of smoking. CONCLUSION: Ex-smokers who started smoking at a young age have a high risk of UC, even after adjusting for the smoking amount.