Environmental risk factors in inflammatory bowel diseases. Investigating the hygiene hypothesis: A Spanish case–control study

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Abstract
Objective. Environmental factors have been implicated in the etiology of inflammatory bowel disease (IBD), but evidence for the hygiene hypothesis is unclear. We investigated the relationship between early-life infection-related exposures and risk of IBD. Patients and methods. A hospital-based case–control study was carried out. A total of 124 cases of Crohn's disease (CD) and 146 of ulcerative colitis (UC) were compared with 235 and 278 well-matched control subjects, respectively. A multi-item questionnaire on familial history of IBD, childhood circumstances and familial socioeconomic status was carried out. Results. In a multivariate model, living in urban areas (odds ratio (OR) 4.58 (95% CI 2.17–10)), high educational level (OR 1.83 (95% CI 14–2.95)) and social status (OR 1.68 (95% CI 1.2–2.35)) were risk factors for CD, whereas childhood respiratory infections (OR 0.35 (95% CI 0.23–0.52)) and gastroenteritis (OR 0.55 (95% CI 0.36–0.85)) were protective factors. Living in urban areas (OR 4.6 (95% CI 2.29–9.9)), a high educational level (OR 10.3 (95% CI 2.54–42.1)) and social status (OR 2.042 (95% CI 1.31–3.17)) were also risk factors for UC, whereas respiratory infections (OR 0.42 (95% CI 0.29–0.6)) and gastroenteritis (OR: 0.6 (95% CI 0.42–0.86)) were protective factors. Appendectomy (OR 0.173 (95% CI 0.06–0.52)) and current smoking (OR 0.75 (95% CI 0.59–0.96)) were also protective for UC. Conclusion. These results further support the hypothesis that better living conditions during childhood are associated with an increased risk for IBD, and reinforce the negative association between smoking and appendectomy and the risk of UC.

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