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Genetics and environmental factors play etiologic roles in ulcerative colitis (UC). Prior research suggests that patients who undergo appendectomy have less risk for subsequent UC than do people with intact appendices. However, whether appendectomy itself or the indication for appendectomy (i.e., appendicitis or mesenteric lymphadenitis) is responsible for this lower risk is unclear.
During 3 decades of follow-up among more than 700,000 Swedish and Danish patients who had undergone appendectomy, 1192 patients developed UC. Appendectomy for appendicitis or mesenteric lymphadenitis before age 20 was associated with significantly lower risk for UC during adulthood (relative risks, 0.45 and 0.65, respectively, vs. the general population). Howeve…