In the linked observational study (doi:10.1136/bmj.b716), Frisch and colleagues present new data on the striking inverse association between ulcerative colitis and appendicectomy, which was first described more than 20 years ago.1

Despite the great progress in molecular genetics in the past few years, the cause of inflammatory bowel disease is still unclear. In genetically susceptible people, antigens or microbial agents lead to an inappropriate mucosal immune response, which may be triggered by environmental factors. The search for relevant environmental factors, which was prompted by the observation that the incidence of inflammatory bowel disease is increasing,2 has been going on for decades. Consequently, many consumer items available in the modern Western world have been examined—such as sugar, margarine, cornflakes, toothpaste, and coca cola—and recently even fridges and television sets.3

Despite a mass of epidemiological studies, only three associations with potentially modifiable lifestyle related factors …