BOSTON (AP) - Young people who get appendicitis are largely protected in future years from colitis, another common ailment, a study shows.

The finding, which was being reported Thursday in The New England Journal of Medicine, may help clarify the unknown cause of ulcerative colitis.

It was known that patients with a history of appendectomy are less likely to have ulcerative colitis, an aggravated bowel condition. But scientists were unsure if it is the appendicitis or removal of the appendix that reduces the risk.

A group of Swedish researchers set out to find the answer. They studied the histories of 212,963 patients who underwent appendectomy and an equal number of other patients for comparison.

It turned out that patients who had appendectomies, but not appendicitis, had the same risk of colitis as patients with no history of appendectomy. Doctors have traditionally removed the appendix during surgery for suspected appendicitis, though tissue samples sometimes show later that the patient did not have appendicitis.

By contrast, the researchers found that people who had appendicitis before the age of 20 are less than half as likely to develop colitis.

Similarly, patients with unnecessary appendectomies for an intestinal inflammation known as mesenteric lymphadenitis are about half as likely to come down with colitis later.

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