What Causes Inflammatory Bowel Disease (IBD)?

by Constance Pietrzak, M.D. Health Professional

IBD is a disease process that is characterized by long-term, or chronic, inflammation of the lining of the gut and affects nearly 1.4 million people in the United States. The two main types of IBD are ulcerative colitis (UC) and Crohn’s disease (CD), and they have differing and some overlapping clinical characteristics.

UC is characterized by chronic inflammation of the colon lining only, extending from the anus without interruptions to any point in the colon. The inflammation involves only a portion of the wall thickness. In CD, inflammation involves the full thickness of the wall of the gut lining, and it can occur anywhere throughout the gastrointestinal tract from the mouth to the anus. It can occur in a patchy distribution with areas of normal gut lining intervening with inflamed areas. Despite the significant knowledge we have about the clinical aspects of these diseases, the exact cause remains unclear. It is likely that the cause for IBD is multifactorial, including genetic, environmental and lifestyle factors.

What are the risk factors?** Age/gender**

While there is a slight increased predominance of CD in females, both men and women are at similar risk for developing IBD. Onset typically occurs in adolescence; however, there is a bimodal distribution with a peak age of onset at 15-25 years of age and again at 50-80 years of age. The cause for this second peak remains unclear.


Both types of IBD are more common in Jews than non-Jews. It is less common in Hispanics than in Caucasians. It is thought that any racial and ethnic differences may be related to environmental, lifestyle and genetic differences.


Genetic influence on IBD is a topic of great interest in the gastrointestinal community. Studies show that approximately 10 to 25 percent of individuals with IBD have a first-degree relative with the disease. Also, those family members typically are found to have the same type of IBD. This association has been seen more commonly in CD than UC. Compared to the general population, individuals with first-degree family members who have IBD are 3 to 20 times more likely to develop the disease. IBD has been linked to various genes. One gene is known as CARD15/NOD2 and is present in duplicate form in 17 percent of people with CD. It is associated with disease and structure formation of the portion of small intestines that connects to the large intestines called the terminal ileum. Other genetic associations include the IBD5 gene on chromosome 5 and the IL23R gene on chromosome 1p31, which codes for factors that promote inflammation.


Smoking has different effects on CD and UC. UC is primarily a disease of non-smokers. About 13 percent of patients with UC are either current or former smokers. Smoking appears to be protective of the development of UC. There is an association with smoking cessation and the development of UC, with a slight increased risk in developing the disease in former smokers and an increase in disease activity. In contrast, smoking is associated with an increased risk for developing CD. Smoking in CD also increases disease activity and recurrence.


Those with IBD do not have any specific dietary restrictions. However, there are a few food antigens/allergens that are thought to trigger an immune response resulting in the development of IBD. Studies attempting to identify these specific foods are inconsistent. Hypersensitivity to cow’s milk in infancy has been thought to play a role in the development of IBD, especially UC. Refined sugar intake is associated with developing CD. Increased dietary intake of total fat, animal fat, milk protein and polyunsaturated fatty acids may play a role in increasing the incidence of IBD. Again, major conclusions on diet cannot be drawn from these inconsistent data.

Increased physical activity has been associated with a decreased risk of CD, although this data is limited. While it is thought that a large amount of intra-abdominal fat may contribute to inflammation of the gut lining, it is not clear if obesity has any association with development of IBD or disease activity.

Gut microbiome

The gut contains millions of bacteria, which is referred to as the gut microbiome. Changes to the microbiome have been implicated in various disease processes, including IBD. An imbalance in the bacteria of the gut, or dysbiosis, may contribute to the development of IBD. The exact mechanism for this is unknown. The microbiome is currently a hot topic, and studies are largely limited at this time.

IBD is a complicated disease of the gastrointestinal tract and the cause remains poorly understood. The cause for IBD is likely multifactorial, and one single cause has not been identified. It is important to eat well, exercise, avoid smoking and follow up regularly with your doctor to maintain good health.

Constance Pietrzak, M.D.
Meet Our Writer
Constance Pietrzak, M.D.

Constance Pietrzak, M.S., M.D., is a gastroenterologist with Advocate Medical Group in Chicago. Through her work with HealthCentral, she strives to expand knowledge on gastroesophageal reflux disease (GERD) and inflammatory bowel disease (IBD). Follow Constance on Twitter for timely updates on IBD, and more.