As part of the digestive process, the intestines move food through the intestinal tract by muscular bowel contractions called peristalsis. Irritable bowel syndrome occurs when peristalsis develops inconsistencies such as abnormal muscle movement or spasm of the lower part of the colon.
Sometimes the spasm delays the bowel movement, causing constipation. At other times it may lead to more rapid passage of the stool, resulting in diarrhoea, or small stools that are not well formed. Although the causes of IBS are not yet fully understood, the nervous system is believed to play a role by interfering with normal peristaltic movement.
Other factors that have a role are food allergies and intolerances. Commonly problematic foods include meat, dairy products, fatty foods, chocolate, alcohol, carbonated beverages and artificial sweeteners. Gluten from grains such as wheat and barley may also trigger or aggravate symptoms for some sufferers.
IBS is sometimes triggered by a bacterial or parasitic infection of the bowel and its subsequent treatment with antibiotics. The disruption of the ‘friendly’ bacteria in the bowel that is caused by both the infection and the treatment may be involved in this phenomenon.