Causes of Irritable Bowel Syndrome

The colon or large intestine is a muscular tube about 6 feet long. It connects the small intestine with the rectum and anus (see image below).

The colon absorbs water and salts from digested food after it has traveled from the stomach through the small intestine. The muscles of the colon contract (tighten or squeeze) and gradually move the material toward the rectum. Strong contractions then lead to a bowel movement. Colon contractions are controlled by nerves, hormones, and by electrical activity in the muscles.

Researchers have found that, for unknown reasons, the colons of people with IBS are more sensitive than usual, and react to things that would not bother other people. For example, the muscles may contract too much after eating. These contractions can cause cramping and diarrhea during or shortly after a meal. The nerves can be overly sensitive to the stretching of the bowel (because of gas, for example), causing cramping or pain. Diet and stress play a role in IBS for many people, causing symptoms or making them worse.

In women, IBS symptoms may be worse during their menstrual periods, so hormone changes may be involved. Sometimes IBS symptoms appear after another illness.