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.