Irritable Bowel Syndrome (cont.)
Bhupinder Anand, MD
William C. Shiel Jr., MD, FACP, FACR
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
In this Article
What causes irritable bowel syndrome (IBS)?
As described previously, IBS 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 small and large colon as well as the other organs shown in the illustration on the previous page. 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. (As an organ system, the gastrointestinal tract is exceeded only by the spinal cord and brain in the numbers of nerves it contains.) 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 is 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 through 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, may occur in the organ, spinal cord, or brain. Moreover, the abnormalities may 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 an abnormality of 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 an abnormality of the function of the motor nerves. For example, abnormal commands through the motor nerves may 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. 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 some patients with IBS produce larger amounts of gas than individuals without IBS, and the gas may be retained longer in the small intestine. In patients with IBS, the abdominal size increases during 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 the abdominal size during the day.
There is much 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 may 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.
Medically Reviewed by a Doctor on 10/15/2014
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