Cause of Irritable Bowel Syndrome

Irritable bowel syndrome is believed to be due to  the abnormal function (dysfunction) of the muscles of the organs of the  gastrointestinal tract or the nerves controlling the organs. The nervous control  of the gastrointestinal tract, however, is complex. A system of nerves runs the  entire length of the gastrointestinal tract from the esophagus to the anus in  the muscular walls of the organs. These nerves communicate with other nerves  that travel to and from the spinal cord. Nerves within the spinal cord, in turn,  travel to and from the brain. (The gastrointestinal tract is exceeded in the  numbers of nerves it contains only by the spinal cord and brain.) Thus, the  abnormal function of the nervous system in IBS may occur in a gastrointestinal  muscular organ, the spinal cord, or the brain.

The nervous system that  controls the gastrointestinal organs, as with most other organs, contains both  sensory and motor nerves. The sensory nerves continuously sense what is  happening within the organ and relay this information to nerves in the organ's  wall. From there, information can be relayed to the spinal cord and brain. The  information is received and processed in the organ's wall, the spinal cord, or  the brain. Then, based on this sensory input and the way the input is processed,  commands (responses) are sent to the organ over the motor nerves. Two of the  most common motor responses in the intestine are contraction or relaxation of  the muscle of the organ and secretion of fluid and/or mucus into the organ.

As already mentioned, abnormal function of the nerves of the  gastrointestinal organs, at least theoretically, might occur in the organ,  spinal cord, or brain. Moreover, the abnormalities might occur in the sensory  nerves, the motor nerves, or at processing centers in the intestine, spinal  cord, or brain. Some researchers argue that the cause of functional diseases is  abnormalities in the function of the sensory nerves. For example, normal  activities, such as stretching of the small intestine by food, may give rise to  abnormal sensory signals that are sent to the spinal cord and brain, where they  are perceived as pain.

Other researchers argue that the cause of  functional diseases is abnormalities in the function of the motor nerves. For  example, abnormal commands through the motor nerves might produce a painful  spasm (contraction) of the muscles. Still others argue that abnormally  functioning processing centers are responsible for functional diseases because  they misinterpret normal sensations or send abnormal commands to the organ. In  fact, some functional diseases may be due to sensory dysfunction, motor  dysfunction, or both sensory and motor dysfunction. Still others may be due to  abnormalities within the processing centers One area that is receiving a great  deal of scientific attention is the potential role of gas produced by intestinal  bacteria in patients with IBS. Studies have demonstrated that patients with IBS  produce larger amounts of gas than individuals without IBS, and the gas may be  retained longer in the small intestine. Among patients with IBS, abdominal size  increases over the day, reaching a maximum in the evening and returning to  baseline by the following morning. In individuals without IBS, there is no  increase in abdominal size during the day.

There has been a great deal  of controversy over the role that poor digestion and/or absorption of dietary  sugars may play in aggravating the symptoms of IBS. Poor digestion of lactose,  the sugar in milk, is very common as is poor absorption of fructose, a sweetener  found in many processed foods. Poor digestion or absorption of these sugars  could aggravate the symptoms of IBS since unabsorbed sugars often cause  increased formation of gas.

Although these abnormalities in production  and transport of gas could give rise to some of the symptoms of IBS, much more  work will need to be done before the role of intestinal gas in IBS is  clear.

Dietary fat in healthy individuals causes food as well as gas to  move more slowly through the stomach and small intestine. Some patients with IBS  may even respond to dietary fat in an exaggerated fashion with greater slowing.  Thus, dietary fat could--and probably does--aggravate the symptoms of IBS.