Irritable Bowel Syndrome (cont.)
What causes 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 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. (An organ system 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. 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. 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.
Next: What are IBS symptoms? »
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