title: Environmental factors in inflammatory bowel disease: a co-twin control study of a Swedish-Danish twin population.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of Gastroenterology, Department of Internal Medicine, Orebro University Hospital, Orebro, Sweden. jonas.halfvarson@orebroll.se
authors.forename: Jonas
authors.initials: J
authors.lastname: Halfvarson
authors.forename: Tine
authors.initials: T
authors.lastname: Jess
authors.forename: Anders
authors.initials: A
authors.lastname: Magnuson
authors.forename: Scott M
authors.initials: SM
authors.lastname: Montgomery
authors.forename: Marianne
authors.initials: M
authors.lastname: Orholm
authors.forename: Curt
authors.initials: C
authors.lastname: Tysk
authors.forename: Vibeke
authors.initials: V
authors.lastname: Binder
authors.forename: Gunnar
authors.initials: G
authors.lastname: Järnerot
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001062:Appendectomy / Q000706:statistics & numerical data; D001424:Bacterial Infections / Q000453:epidemiology; D016022:Case-Control Studies; D003093:Colitis, Ulcerative / Q000453:epidemiology / Q000209:etiology; D003276:Contraceptives, Oral / Q000009:adverse effects; D003424:Crohn Disease / Q000453:epidemiology / Q000209:etiology; D003718:Denmark / Q000453:epidemiology; D004032:Diet / Q000009:adverse effects; D004200:Diseases in Twins / Q000453:epidemiology / Q000209:etiology; D005260:Female; D006801:Humans; D015212:Inflammatory Bowel Diseases / Q000453:epidemiology* / Q000209:etiology*; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D011159:Population Surveillance; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects; D013548:Sweden / Q000453:epidemiology
chemical_list: D003276:Contraceptives, Oral