What causes irritable bowel syndrome and how do you know if you have it?

A. Irritable bowel syndrome (IBS) is very common — it’s generally estimated that up to 20 percent of the population has complaints that meet the criteria for IBS. It’s less a specific disease than a set of symptoms: abdominal pain or discomfort that’s accompanied by either constipation or diarrhea, or both in alternation. People with IBS usually experience some kind of pain in their lower abdomen, but the type of pain and its location can vary. Braden Kuo, a gastroenterologist who directs the GI Motility Lab at Massachusetts General Hospital, says that it’s more than a passing bout of stomach upset, which we all experience from time to time. In IBS, symptoms generally persist for three months or more.

Kuo says that IBS is often a diagnosis of exclusion, meaning that your doctor first rules out other medical conditions. If you have symptoms like blood in the stool, recent unexplained weight loss, or worsening pain, it could be a sign of something more serious. IBS should also not be confused with inflammatory bowel disease (IBD), a much rarer inflammatory disease that can have life-threatening complications. IBS is a chronic condition that doesn’t pose serious health risk but can make quality of life suffer.

There is no single cause of IBS. “It’s extremely heterogeneous,’’ Kuo says. However, he adds, “Ultimately, we believe IBS is a disorder of the nerve endings in the gut.’’ These are responsible for sensing conditions of the gut and controlling its movements. For various reasons, the nerve endings become hypersensitive, causing inappropriate contractions in the gut. The condition seems to involve a feedback loop between the gut and brain, Kuo says. Stress is known to aggravate IBS, and brain imaging studies have found that people with IBS react differently to pain than those without the condition.

Many people can gain some control over the symptoms. Drug treatments can alleviate the diarrhea, constipation, or chronic pain involved in IBS. Modifying the diet can also offer relief, but Kuo says it’s a matter of trial and error for patients to find what works for them. Mind-body therapies may also help: A recent study found that mindfulness meditation can improve symptoms of IBS for some people.

Correction:Because of a reporting error, this story previously misidentified inflammatory bowel disease, a relatively rare, potentially life-threatening condition.