Irritable bowel syndrome (IBS) is probably not a single disease, but rather a set of symptoms that stem from a variety of causes. It may be generally described as a disorder in the functioning of the gastrointestinal tract. Some experts suspect that IBS involves disturbances in the nerves or muscles in the gut. Others believe that abnormal processing of gut sensations in the brain may hold the key, at least in some cases.
Several studies have demonstrated that a bout of infectious gastroenteritis (stomach or bowel inflammation) may increase the risk of developing symptoms by as much as seven to 14 times. What's more, emotional upset or stress may heighten this risk even further. One study reported that among people hospitalized with gastroenteritis, those who had experienced a distressing life event (such as divorce or the death of a family member) during the previous year were significantly more likely to develop IBS than those who'd had an uneventful year.
Another explanation for IBS is the overgrowth of bacteria in the small intestine. This overgrowth may cause the feeling of bloating and the passing of excess gas which are common symptoms of IBS. Researchers have found some evidence of small intestinal bacterial overgrowth in IBS patients, and it appears that bacterial overgrowth may contribute to many common symptoms of IBS, including bloating and distension, diarrhea, constipation, and heightened sensitivity to pain. Treatment with antibiotics may improve some of these symptoms. Treatment with probiotics, live microbes intended to confer health benefits, has also been proposed, but more data are needed to confirm this strategy.
Because the spasmodic pain associated with IBS seems to emanate from the colon, researchers have concentrated on this part of the gastrointestinal (GI) tract, searching for any irregularities. The findings, thus far, have been inconsistent. Some researchers have found that the colon muscle of a person with IBS begins to spasm after only mild stimulation. The colon seems to be more sensitive than usual, so it responds strongly to stimuli that wouldn't affect other people. Sometimes, the spasms lead to diarrhea; other times, to constipation. But some studies show that most of the time, colonic motor activity is no different for IBS patients than for anyone else.