Environmental influences on the onset and clinical course of Crohn's disease-part 1: an overview of external risk factors.

The pathogenesis of Crohn's disease (CD) involves host, genetic, and environmental factors. These factors result in disturbances in the innate and adaptive immune systems and composition of the intestinal microbiota. Epidemiologic and migration studies support an environmental component in the development of CD. Environmental risk factors include childhood hygiene, air pollution, breastfeeding, smoking, diet, stress, exercise, seasonal variation, and appendectomy. This review, part 1 of a 2-part series, provides an overview of these external contributors to the development or exacerbation of CD. Part 2, which will be published in a subsequent issue, will discuss the influences of infections, vaccinations, and medications (including antibiotics, nonsteroidal anti-inflammatory agents, and oral contraceptives) on CD.