The cause of IBS remains unknown. Researchers do know that the smooth muscles in the walls of the colon of people with IBS may react differently to stimuli than those of people without IBS. These muscles contract and relax to move substances along inside the colon. But if you have IBS, these contractions may be stronger (called spasms) or last longer than normal, causing stool to move through too quickly. This does not allow enough time for water to be absorbed from the stool and results in the frequent watery stools that characterize diarrhea. Alternatively, these contractions may be too infrequent, slowing the passage of stool through the colon. This provides too much time for water to be absorbed from the stool, thus stools become hard and dry and bowel movements become less frequent (constipation).

No one knows exactly why the colon smooth muscle of people with IBS reacts differently than the same muscles in people without IBS, but there are several reasonable possibilities. Some researchers believe that the nerves that control sensations and muscle contractions in the bowel may be impaired somehow in people with IBS, causing them to respond abnormally to passage of food through the colon. Or, these nerves may have a heightened sensitivity to the stretching of the colon that occurs with gas. Other researchers think that the central nervous system (the brain and spinal cord), which can exert control over the muscles in the colon through the autonomic nervous system, may be the root of the problem. Finally, some researchers believe that hormones may be involved.

People with IBS may have adverse reactions to things that other people find innocuous. These “triggers” of symptoms may cause the muscles of the colon to behave abnormally in people with IBS and include:

• Foods: Certain foods may bring on or worsen symptoms of IBS including chocolate, milk, alcohol, carbonated beverages, and some fruits and vegetables.
• Stress: In most people with IBS, emotional or life stress appears to worsen IBS symptoms or make them more frequent. Stress does not seem to cause IBS, only to worsen its symptoms.
• Other illnesses: Sometimes another illness, especially infectious diseases of the gastrointestinal tract can trigger IBS.