What causes IBS?

Until now, causes of IBS have not been well understood.  Historically, researchers have believed a combination of physical and psychological health problems could lead to IBS.  Food sensitivity has long been suspected but, until the advances we have made, efforts to accurately identify specific foods have been unsuccessful.  In two studies published in peer-reviewed dermatology journals (Study 1, Study 2) it was shown that food allergies in many individuals with IBS or IBS-like symptoms can be identified by simple, painless skin patch testing, and avoidance of those foods may improve or eliminate their IBS symptoms.  The responsible food does not need to be  new; it may be a food eaten for a long time but to which the person with IBS eventually becomes allergic, and it is often not obvious.  An expert consensus panel concluded in 2010 that other types of food allergy testing using skin prick and scratch testing or measuring blood antibody levels are not useful for IBS  (Boyce JA, Assa’ad A,, Burks AW, et al. Guidelines for the Diagnosis and Management of Food Allergy in the United States:  Summary of the NIAID-Sponsored Expert Panel Report.  J Allergy Clin Immunol 2010;126:1105-18).
Other possible causes that have been suspected include the following*:
Brain-gut signal problems – Signals between the brain and nerves of the small and large intestines, also called the gut, control how the intestines work. Problems with brain-gut signals may cause IBS symptoms, such as changes in bowel habits and pain or discomfort.
Gastrointestinal motor problems – Normal motility, or movement, may not be present in the colon of a person who has IBS. Slow motility can lead to constipation and fast motility can lead to diarrhea. Spasms, or sudden strong muscle contractions that come and go, can cause abdominal pain. Some people with IBS also experience hyperreactivity, which is an excessive increase in contractions of the bowel in response to stress or eating.
Hypersensitivity – People with IBS have a lower pain threshold to stretching of the bowel caused by gas or stool compared with people who do not have IBS. The brain may process pain signals from the bowel differently in people with IBS.
Mental health problems – Mental health, or psychological, problems such as panic disorder, anxiety, depression, and post-traumatic stress disorder are common in people with IBS. The link between these disorders and development of IBS is unclear. Gastrointestinal disorders, including IBS, are often found in people who have reported past physical or sexual abuse. Researchers believe people who have been abused tend to express psychological stress through physical symptoms.
Bacterial gastroenteritis – Some people who have bacterial gastroenteritis—an infection or irritation of the stomach and intestines caused by bacteria—develop IBS. Researchers do not know why gastroenteritis leads to IBS in some people and not others, though psychological problems and abnormalities of the lining of the gastrointestinal tract may be factors.
Small intestinal bacterial overgrowth (SIBO) – Normally, few bacteria live in the small intestine. SIBO is an increase in the number of bacteria or a change in the type of bacteria in the small intestine. These bacteria can produce excess gas and may also cause diarrhea and weight loss. Some researchers believe that SIBO may lead to IBS, and some studies have shown antibiotics to be effective in treating IBS. However, more research is needed to prove a link between SIBO and IBS.
Body chemicals – People with IBS have altered levels of neurotransmitters, which are chemicals in the body that transmit nerve signals, and gastrointestinal hormones, though the role these chemicals play in developing IBS is unclear. Younger women with IBS often have more symptoms during their menstrual periods. Post-menopausal women have fewer symptoms compared with women who are still menstruating. These findings suggest that reproductive hormones can worsen IBS problems.
Genetics – Whether IBS has a genetic cause, meaning it runs in families, is unclear. Studies have shown that IBS is more common in people with family members who have a history of gastrointestinal problems. However, the cause could be environmental or the result of heightened awareness of gastrointestinal symptoms.
*Bullet point information compliments of National Digestive Diseases Information Clearinghouse