Irritable Bowel Syndrome, or IBS, describes a pattern of recurrent bouts of abdominal
pain and abnormal bowel motility causing things like constipation or diarrhea, or a mixture
of the two, and often times the abdominal pain improves after a bowel movement.
Although it sounds similar, IBS is different from inflammatory bowel disease or IBD, which
involves some of the same IBS symptoms, but also includes inflammation, ulcers, or other
damage to the bowel, whereas IBS does not involve these, and instead can be thought
of as a functional disorder.
Right now, the underlying biological mechanisms that produce the symptoms of irritable bowel
syndrome aren’t well understood, so most research is focused on these key symptoms:
abdominal pain and abnormal bowel motility.
With regard to abdominal pain, a lot of people with irritable bowel syndrome have “visceral
hypersensitivity,” which means that the sensory nerve endings in the intestinal wall
have an abnormally strong response to stimuli like stretching during and after after a meal.
This visceral hypersensitivity might explain why people with the disease experience recurrent
abdominal pain.
With regard to abnormal bowel motility, the underlying mechanism is a little less clear.
One clue is that eating foods that contain short chain carbohydrates such as lactose
and fructose often trigger the symptoms.
One possible explanation is that unabsorbed short-chain carbohydrates act as solutes that
draw water across the gastrointestinal wall and into the lumen.
In addition to triggering visceral hypersensitivity which causes pain, that excess water can also
cause smooth muscle lining the intestines to spasm, and create diarrhea if the excess
water’s not reabsorbed back into the body.
To make matters worse, the unabsorbed short-chain carbohydrates are often metabolized by gastrointestinal
bacterial flora which produce gas that could trigger more bloating, spasm, or pain.
Now, there are also epidemiologic clues as well.
For example, in North America, irritable bowel syndrome is most common among middle aged
women, but in other parts of the world, it affects both sexes equally.
Also, one important risk factor is having a bout of acute gastroenteritis, for example
from norovirus or rotavirus, but also another risk factor is being stressed, and both of
these seem to be triggers for developing irritable bowel syndrome in many individuals.
Since the mechanism behind irritable bowel syndrome is still unclear, most treatments
target the symptoms.
This includes things like diet modification, like avoiding certain foods like apples, beans,
and cauliflower, all of which have short-chain carbohydrates.
In addition, to treat constipation, soluble fiber, stool softeners, and osmotic laxatives
can help, whereas for gastrointestinal spasms and pain, anti-diarrheals like serotonin antagonists
and antimuscarinic medications can help.
Finally, managing things like stress and controlling feelings of anxiety and depression can all
help symptoms irritable bowel syndrome symptoms.
Okay, as a quick recap—patients with irritable bowel syndrome have recurrent abdominal pain
with constipation or diarrhea, and the pain is usually relieved after bowel movements.
The underlying pathophysiology isn’t well understood, but seems to be related to visceral
hypersensitivity and abnormal bowel motility, and triggers or risk factors include short-chain
carbohydrates, bouts of acute gastroenteritis, and stress.