The exact cause of irritable bowel syndrome (IBS) is unknown.
Factors that appear to play a role include:
Muscle contractions in the intestines. The walls of the intestines are lined with muscle layers that contract as they move food through the digestive tract. Contractions that are stronger and last longer than normal can cause gas, bloating, and diarrhea. Weak intestinal contractions can slow the passage of food, leading to hard, dry stools.
Nervous system. Nerve abnormalities in your digestive system can lead to more discomfort than normal when your abdomen expands due to gas or stool. Poorly coordinated signals between the brain and gut can cause the body to overreact to normal changes in digestion, resulting in pain, diarrhea, or constipation.
Inflammation of the intestines. Some people with IBS have an increased number of immune system cells in their intestines. This immune system response is associated with pain and diarrhea.
Severe infection. Irritable bowel syndrome (IBS) can develop after a severe bout of diarrhea (gastritis) caused by bacteria or a virus. IBS can also be associated with an excess of bacteria in the intestines (bacterial overgrowth).
Changes in the bacteria in the gut (microflora). The microflora is the "good" bacteria residing in the gut, which plays a key role in your health. Research suggests that the microflora of people with IBS may differ from the microflora of healthy people.
