Significant changes have been observed in the epidemiology of inflammatory bowel diseases (IBD) in the last two decades. Traditionally, the incidence of IBD was higher in the developed, industrialized countries, in contrast, nowadays it became more prevalent in the previously low incidence areas. In particular, the incidence of ulcerative colitis (UC) is similar to that observed in North America and Western Europe, while the incidence of Crohn's disease (CD) in developing countries is still low, suggesting that the environmental factors may act faster or differently in UC than in CD. In Europe, the North to South gradient disappeared, and also the West to East gradient is diminishing. Smoking and appendectomy may be considered as important environmental factors in both UC and CD, however, with opposite effects. In addition, the use of oral contraceptives is associated to disease susceptibility in both diseases. The role of diet, perinatal events, stress and nonsteroidal anti-inflammatory drugs in the pathogenesis is still controversial.
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