What causes irritable bowel syndrome (IBS)?

Carmen Patrick Mohan, MD
Internal Medicine
Irritable bowel syndrome is characterized by chronic abdominal pain and changes in bowel habits (constipation or diarrhea) without a clear cause. The disorder appears to be related to the interaction of multiple factors and not all of the factors are well established. No two patients with IBS are exactly alike. In some patients, irregular gut motility appears to be the primary problem. Other patients have what is termed “visceral hypersensitivity” -- they are highly sensitive to the sensations produced by the gut such as bloating. For patients who have diarrhea-predominant IBS, a poorly understood inflammatory response may be the underlying problem and an infection may have precipitated it. The medical community is still investigating causes of IBS. Genetic traits, alterations in gut microbiology, and food sensitivities are all still being explored. Regardless of the cause, there are several different kinds of treatment for IBS. See you doctor if you feel you might have IBS.
Nina Singh, MD

Irritable Bowel Syndrome (IBS) – also known as spastic colon, spastic colitis, mucous colitis and nervous or functional bowel – is an alteration with the way the nervous system and the gut interact.   Often people have significant abdominal pain and cramping. Some people can have diarrhea while others have constipation, and some people can alternate between the two. Very often, stress can trigger an exacerbation of irritable bowel syndrome.  Certain foods such as coffee, alcohol, spices, raw fruits, vegetables and milk, as well as infections, illnesses, and woman’s menstrual cycle can also be associated with flare-ups.

Lawrence S. Friedman, MD
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.
Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)

Millions of women suffer from frequent stomach aches, bloating, constipation and diarrhea. In this video, Dr. Oz explains what causes these problems and the keys to relief. 

Ms. Ashley Koff, RD
Nutrition & Dietetics
Irritable bowel syndrome (IBS) is one of the most common ailments of the bowel (intestines). What can be frustrating about IBS is that it's not linked to any structural defects. In other words, it's a functional disorder. If you are personally familiar with IBS, then you know very well how much it can hamper quality of life. Many studies have concluded that the constant intake of food additives and the ingestion of pesticides, chemicals, and dyes can cause irritation to the intestinal tract and/or an imbalance of the intestinal bacteria, resulting in inflammation or symptoms of IBS.
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William B. Salt II., MD

IBS is one of many functional symptom syndromes, composed of medically unexplained symptoms, which are “caused” by dysfunction of the mind/brain—body connection.

To explain the unexplainable and cause, look at the terms used here and then “see the big picture.”


Functional refers to how the body works.

Symptoms of IBS include abdominal pain or discomfort relieved after having a bowel movement and/or associated with diarrhea, constipation, or both.

Symptom Syndromes are collections of medically unexplained symptoms. They are also known as functional somatic syndromes and chronic multisymptom illnesses. Nearly every specialty defines at least one syndrome. Examples include RHEUMATOLOGY (fibromyalgia), UROLOGY (interstitial cystitis/painful bladder and chronic prostatitis/painful prostate), and GASTROENTEROLOGY (irritable bowel syndrome and over 30 other functional gastrointestinal disorders, or FGIDs).

Medically Unexplained Symptoms (MUS) cannot be explained by medical tests, such as x-rays, endoscopies, and blood tests, because they are caused by dysfunction.

Dysfunction is disturbance or “malfunction” of how the body works.

Mind/Brain-Body Connection refers to how the mind/brain and body communicate and talk with one another.

MUS and symptom syndromes frequently overlap with one another and are commonly associated with and often attributed to stress, depression, anxiety, and/or panic. Medical and scientific research is showing how the mind/brain and body communicate and both how and why symptoms occur. One of the most important discoveries is that the central mind/brain can become sensitized to peripheral body pain and symptom signals. So these symptom syndromes are now being called, central sensitivity syndromes.

A new book, Still Hurting? FIND HEALTH!, written by this author with Thomas L Hudson MDiv JD, (StillHurtingFINDHEALTH.com), proposes a new unifying and holistic medical model of MUS and their related symptom syndromes as chronic disease, explains both how and why they occur, and shows what people can do to help themselves and work effectively with their caregivers.


The cause of MUS and pain can be understood as disease/dysfunction, regardless of whether the symptoms are widespread (e.g., the pain and fatigue of fibromyalgia) or localized to a specific area of the body (e.g., the abdominal pain and bowel dysfunction of irritable bowel syndrome).

Patsy Catsos
Nutrition & Dietetics
The origins of irritable bowel syndrome (IBS) are not well understood by clinicians and scientists. Over the years, our view of IBS has changed and evolved and will probably continue to do so. At the present time, it seems likely that IBS arises from some combination of the following:
  • Abnormal contraction and relaxation of the intestinal muscles that move food through the gastrointestinal system, known as a motility disorder. Motility disorders may cause food to move through the gastrointestinal (GI) tract too slowly or quickly, and can sometimes cause intestinal muscle spasms to occur.
  • Low pain threshold for distention in the intestines of affected individuals, known as visceral hypersensitivity. This means that a buildup of gas or fluids in the large intestine may hurt the IBS sufferer but not cause a painful sensation to someone who does not have IBS.
  • Miscommunication between the gut and the brain, possibly relating to a nervous system dysfunction.
Stress and inflammation may influence these three proposed origins of IBS in ways that are unclear at this time.
Michael T. Murray, ND
Naturopathic Medicine
There appear to be four main causes of irritable bowel syndrome (IBS): stress, insufficient intake of dietary fiber, food allergies, and meals too high in sugar. Stress increases the motility (the rhythmic contractions of the intestine that propel food through the digestive tract) of the colon and leads to abdominal pain and irregular bowel function. Stress can also cause spasm in the bowel, leading to constipation or diarrhea. Insufficient intake of dietary fiber diminishes the ability of the colon to propel food through the digestive tract. Food allergies also cause irritation and inflammation in the digestive tract, causing irregular bowel function. Food allergy as a cause of IBS has been recognized since the early 1900s. More recent studies have shown that the majority of patients with IBS (approximately two-thirds) have at least one food allergy, and some have multiple food allergies. The most common allergens indicated to cause IBS are dairy products, at 40 to 45 percent, and grains, at 40 to 60 percent. Many patients have noted marked clinical improvement when using elimination diets.
A diet high in refined sugar may be the key factor that makes IBS far more common in the United States than in other countries. Meals high in refined sugar can contribute to IBS by decreasing intestinal motility. Eating a diet high in sugar and refined carbohydrates usually means a diet low in fiber. Also, when blood sugar levels rise too rapidly, the normal rhythmic contractions of the gastrointestinal tract slow down and in some portions stop altogether. Also, excessive sugar and carbohydrates change the environment for organisms in the digestive tract, favoring organisms that can cause abnormal bowel function. 
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.