Environmental factors affecting inflammatory bowel disease

What are the most significant environmental factors affecting the risk and course of inflammatory bowel disease (IBD)?

John Marshall: It is a very good question, and I have to say that we do not have all the answers. There is a lot of research going on around the world to try to answer those questions.

I think certainly we know that IBD is not common equally in different parts of the world and in different social settings, which gives us some clues about what exposures are important. We know, for example, that IBD is much more common in Western nations and has become much more common over time. In North America and Western Europe, for example, IBD appeared in the middle of the 20th century; it was very rare before that. We do not know [for sure] but we think some of the reasons for that relate to changes in diet and perhaps hygiene. There is a very rich hygiene hypothesis that if our immune systems do not see antigens at a very young age, we overreact to them at an older age and that can manifest as IBD as well as lots of other allergic disorders. So certainly: hygiene may play a role, but what else is in our Western, more affluent lifestyle that leads to IBD is not necessarily very clear.

One very important environmental exposure is smoking. We certainly know that for Crohn disease it is a risk factor for development of the disorder, it is a risk factor for a more aggressive disease behavior. Patients with Crohn disease who have surgery and smoke are at higher risk of recurrence after surgery. So if patients with Crohn disease are looking for one behavior they can adopt to have a better disease course, it is to not smoke.

In ulcerative colitis, the opposite may be true: in some patients, ulcerative colitis seems to respond to smoking in a favorable way. I am not sure that that response offsets the negative effects of smoking, but it is very common for ulcerative colitis to appear shortly after someone quits smoking. I think [patients] should still quit smoking; I think there is a lot of benefit to be had. In fairness, there have been clinical trials looking at nicotine as therapy for ulcerative colitis. But certainly, for Crohn disease that relation is very tight, and... do not smoke!