Environmental factors in inflammatory bowel disease: A co-twin control study of a Swedish-Danish twin population

Genetics and environmental factors are implicated in the etiology of inflammatory bowel disease (IBD). We studied environmental factors in a population-based Swedish-Danish twin cohort using the co-twin control method. A questionnaire was sent to 317 twin pairs regarding markers of exposures in the following areas: infections/colonization and diet as well as smoking, appendectomy, and oral contraceptives. Odds ratios (OR) were calculated by conditional logistic regression. When confounding appeared plausible, multivariate conditional logistic regression was added. The questions were also divided into topic groups, and adjustment was made for multiple testing within each of the groups. The response rate to the questionnaire was 83%. In consideration of the study design, only discordant pairs were included (Crohn's disease [CD], n = 102; ulcerative colitis [UC], n > = 125). Recurrent gastrointestinal infections were associated with both UC (OR, 8.0; 95% confidence interval [CI], 1.0–64) and CD (OR, 5.5; 95% CI, 1.2–25). Hospitalization for gastrointestinal infections was associated with CD (OR, 12; 95% CI, 1.6–92). Smoking was inversely associated with UC (OR, 0.4; 95% CI, 0.2–0.9) and associated with CD (OR, 2.9; 95% CI, 1.2–7.1). The observed associations indicate that markers of possible infectious events may influence the risk of IBD. Some of these effects might be mediated by long-term changes in gut flora or alterations in reactivity to the flora. The influence of smoking in IBD was confirmed.