It's not known exactly what causes irritable bowel syndrome. The walls of the intestines are lined with layers of muscle that contract and relax in a coordinated rhythm as they move food from your stomach through your intestinal tract to your rectum. If you have irritable bowel syndrome, the contractions may be stronger and last longer than normal. Food is forced through your intestines more quickly, causing gas, bloating and diarrhea.

In some cases, the opposite occurs. Food passage slows, and stools become hard and dry. Abnormalities in your nervous system or colon also may play a role, causing you to experience greater than normal discomfort when your intestinal wall stretches from gas.

There are a number of other factors that may play a role in IBS. For example, people with IBS may have abnormal serotonin levels. Serotonin is a chemical messenger that's normally associated with brain function, but it also plays a role in normal digestive system function. It's also possible that people with IBS don't have the right balance of good bacteria in the intestine.

Triggers affect some people, not others

For reasons that still aren't clear, if you have IBS you probably react strongly to stimuli that don't bother other people. Triggers for IBS can range from gas or pressure on your intestines to certain foods, medications or emotions. For example:

  • Foods. Many people find that their signs and symptoms worsen when they eat certain foods. For instance, chocolate, milk and alcohol might cause constipation or diarrhea. Carbonated beverages and some fruits and vegetables may lead to bloating and discomfort in some people with IBS. The role of food allergy or intolerance in irritable bowel syndrome has yet to be clearly understood.

    If you experience cramping and bloating mainly after eating dairy products, food with caffeine, or sugar-free gum or candies, the problem may not be irritable bowel syndrome. Instead, your body may not be able to tolerate the sugar (lactose) in dairy products, caffeine or the artificial sweetener sorbitol.
  • Stress. If you're like most people with IBS, you probably find that your signs and symptoms are worse or more frequent during stressful events, such as a change in your daily routine. But while stress may aggravate symptoms, it doesn't cause them.
  • Hormones. Because women are more likely to have IBS, researchers believe that hormonal changes play a role in this condition. Many women find that signs and symptoms are worse during or around their menstrual periods.
Jul. 29, 2011