What Causes Inflammatory Bowel Disease?

The cause of IBD has been linked to genetics and environment

Washing strawberries in a strainer
Could IBD be caused by our obsession with cleanliness?. Jen Brister/Stocksy United

Scientists are not certain what causes inflammatory bowel disease (IBD), which consists of ulcerative colitisCrohn's disease, and indeterminate colitis. It's considered an "idiopathic" disease, or a disease with an unknown cause. However, there are several theories about the origins of IBD, as well as the conditions that may contribute to its development.

We don't yet know exactly what causes IBD, however, we do have some clues, and several theories.

It's generally thought that there is a genetic part to IBD, and that something (or more than one something) in our environment affects the genes that are associated with IBD. This might "trigger" the IBD, and be the start of symptoms. While researchers have learned a lot about the genes that may be associated with IBD, it's the trigger part that is difficult to pin down. It's now also thought that there could be hundreds of different types of IBD, although we still segment them into the 3 categories in order to treat them with medications. There could, in fact, also be hundreds or thousands of triggers. Some research has pointed at a few possibilities for the reason some people who have IBD genes develop IBD and other people do not.

Genetics and IBD

Years ago, it was thought that IBD might run in families, but the link seemed tenuous because it was not a direct parent-to-child situation, as is the case with some inherited conditions.

With the discovery of now hundred of genes that may be associated with IBD, it has become apparent that there is a hereditary component to IBD. First-degree relatives of people who have IBD are significantly more likely to also have the disease. However, the majority of people who have IBD have no family history, so not everyone has a family member who has the disease.

So while IBD clearly does run in families, it is not the only factor to be taken into consideration when looking at the possible causes of IBD. There has to be something else that's making some people with the same genes develop IBD, while others do not.

IBD's Inflammatory Response

IBD is frequently called autoimmune disease, a disease that is triggered by the immune system, but it is more accurate to say that it is an immune-mediated response. Seasonal flare-ups -- during the spring or autumn -- sometimes occur in people with IBD. One theory is that this is an IgE-mediated allergic response.

The allergic response sets off a chain of events resulting in an excess of eosinophils (cells that try to fight the allergic response) in the body. These eosinophils release four toxic compounds, three of which are found in statistically significant amounts in the stool of IBD patients. This leads some researchers to conclude that an allergic response may have a role in the development of IBD.


Another strong area of research is the role that cytokines play in the development of IBD.

Cells called tumor necrosis factor (TNF, or also sometimes called tumor necrosis factor-alpha) are responsible for regulating the immune response, among other functions. TNF is found in higher quantities in the stool of people with IBD than it is in people who do not have IBD (via a fecal calprotectin test). Starting with the approval of Remicade in 1998, several anti-TNF drugs (commonly called biologics) have been developed to treat IBD. The success of these medications lends some weight behind the idea that TNF plays some role in the cause of IBD or the inflammation associated with IBD flare-ups.

Environmental Factors in IBD

There are some clear trends in the epidemiology of IBD that may point to one or more environmental causes. IBD tends to occur most often in developed countries and amongst those with higher socioeconomic status. IBD also tends to occur more often in the urban areas of developed countries. These factors have led researchers to think that there may be some connection between IBD and the lifestyles or environment of people living in developed countries, although no one knows yet what this could be.

One suggested theory is that industrialized nations are "too clean", and because children and adolescents are exposed to fewer bacteria, their immune systems may be inadequate, which leads to autoimmune disease.

The Bottom Line

We don't know exactly what causes IBD, but we do know that it's not caused by diet or stress. There is clearly a genetic component that runs in families, but it's that second part, something around us that's "triggering" the genes that's proving difficult to find. The good news is that we know so much more now about IBD than we did even just a decade ago. More research is being done, and scientists are getting closer and closer to understanding how we can treat these diseases more effectively and prevent them in future generations.


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Crohn's and Colitis Foundation of America. "About the Epidemiology of IBD." CCFA.org 2009.

National Institute of Diabetes and Digestive and Kidney Diseases. "Ulcerative Colitis." National Institutes of Health Feb 2006.

Peterson CG, Sangfelt P, Wagner M, Hansson T, Lettesjö H, Carlson M. "Fecal levels of leukocyte markers reflect disease activity in patients with ulcerative colitis." Scand J Clin Lab Invest 2007; 67:810-820.

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