Epidemiology of inflammatory bowel disease--methodological considerations.

The causes and mechanisms of action of inflammatory bowel disease have, so far, eluded discovery. Epidemiological studies have shown that ulcerative colitis tends to level off, whereas Crohn's disease tends to increase. Some of these changes may be due to diagnostic practices and increasing awareness of the disease and Crohn's colitis. The disease varies according to geographical location and a distribution along a north-south axis has been suggested. The differences may be due to study design, or may reflect differences in lifestyle, diet or be due to genetic predisposition triggered by environmental factors. Epidemiological studies designed to investigate such interactions may provide clues to its aetiology. Inflammatory bowel disease could, therefore, serve as a model for the importance of epidemiology when to test or reject the hypothesis of aetiology.