Epidemiologic studies of inflammatory bowel disease indicate that these disorders occur in susceptible individuals, possibly due to genetic abnormalities, resulting in disturbance of bowel wall function. Environmental factor(s) exerting a primary effect at an early age, in most patients, cause the disease to manifest usually in teenagers or young adults. Secondary environmental factors may alter disease expression, severity, or influence relapse. Studies in identical twins provide the best evidence for the role of genetic factors in the cause of Crohn's disease. Changing trends in disease incidence (seen in many countries) provide strong support for environmental factors as playing a major influence in disease expression. We now require further laboratory and epidemiologic studies in family members at risk of disease, in migrants who move from low incidence areas to high incidence areas, and in area where Crohn's disease particularly is starting to appear. The clinician can play an important role by performing careful observations in patients with a view to uncovering new clues to disease etiology.
PIP: The epidemiology of the 2 main types of inflammatory bowel disease, Crohn's disease and ulcerative colitis, is updated in this review. The incidence of these diseases is about 3-20 new cases/100,000/year. Ulcerative colitis usually has a higher incidence, but Crohn's disease incidence is rising fast in the U.S., U.K., Denmark and Israel, catching up with the rates of ulcerative colitis. Incidence is moderate in Central Europe and Australia, and low in South America, Asia, and Africa. There are no proven links to climate, environmental factors, genetic markers, or transmissible agents; although infections, fiber, sugar and milk have all been proposed as possible causes. Epidemiological studies suggest that ulcerative colitis is more common in non-smokers, while Crohn's disease is 2-4 times more common in smokers. Symptom remission in Crohn's disease has been reported in women who stop taking oral contraceptive, while, on the contrary, both diseases have been reported are more prevalent in non-pill users. In any event the relative risk is less than 2, a weak association.