Some evidence indicates that the immune system, which fights infection, is also involved. Irritable Bowel Syndrome symptoms result from the following: The normal motility of the colon may not work properly. It can be spasmodic or can even stop temporarily. Spasms are sudden strong muscle contractions that come and go.

The lining of the colon (epithelium), which is affected by the immune and nervous systems, regulates the passage of fluids in and out of the colon. In Irritable Bowel Syndrome, the epithelium appears to work properly. However, fast movement of the colon's contents can overcome the absorptive capacity of the colon. The result is too much fluid in the stool. In other patients, colonic movement is too slow, too much fluid is absorbed, and constipation develops.

The colon responds strongly to stimuli (for example, foods or stress) that would not bother most people.

In people with Irritable Bowel Syndrome, stress and emotions can strongly affect the colon. It has many nerves that connect it to the brain. Like the heart and the lungs, the colon is partly controlled by the autonomic nervous system, which has been proven to respond to stress. For example, when you are frightened, your heart beats faster, your blood pressure may go up, or you may gasp. The colon responds to stress also. It may contract too much or too little. It may absorb too much water or too little.

Research has shown that very mild or hidden (occult) celiac disease is present in a smaller group of people with symptoms that mimic Irritable Bowel Syndrome. People with celiac disease cannot digest gluten, which is present in wheat, rye, barley, and possibly oats. Foods containing gluten are toxic to these people, and their immune system responds by damaging the small intestine. A blood test can determine whether celiac disease is present. (For information about celiac disease, see the Celiac Disease fact sheet from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).)

The following have been associated with a worsening of Irritable Bowel Syndrome symptoms:

-Large meals
-Bloating from gas in the colon
-Medicines
-Wheat, rye, barley, chocolate, milk products, or alcohol
-Drinks with caffeine, such as coffee, tea, or colas
-Stress, conflict, or emotional upsets

Researchers have also found that women with Irritable Bowel Syndrome may have more symptoms during their menstrual periods, suggesting that reproductive hormones can exacerbate Irritable Bowel Syndrome problems.