Irritable Bowel Syndrome (IBS) is the most common gastrointestinal diagnosis, where prevalence in some populations is reported to be as high as 30%. It is estimated to affect 20% of Canadians. Functional bowel complaints, including IBS, account for 50% of all visits to gastroenterologists. Symptoms of IBS include abdominal pain, bloating, and altered bowel habits such as diarrhea, constipation, or alternating between the two stool consistency extremes.

The symptoms of IBS are similar to those experienced by patients with Small Intestinal Bacterial Overgrowth (SIBO). There are mechanisms in the body to help prevent naturally occurring colon bacteria from backing-up into the small intestine. Why these systems break down sometimes is unknown. A specialized breath test exists to assess this overgrowth, called a lactulose hydrogen breath test, or LHBT for short.

Could people diagnosed with irritable bowel syndrome really have SIBO? Researchers in California wanted to test whether overgrowth is associated with IBS and whether treatment of overgrowth reduces intestinal complaints.

A study undertaken at Cedars-Sinai Medical Center used 448 subjects who were referred by their doctors for detection of SIBO. After completing a questionnaire, the researchers determined that 202 subjects could be considered as having irritable bowel syndrome according to standard symptom criteria (see sidebar). Of these, 157 (78%) were positive for bacteria overgrowth using the LHBT.

The subjects’ doctors then prescribed a 10-day course of antibiotics (e.g. Neomycin, ciprofloxacin, flagyl, or doxycyline) to eradicate their bacterial overgrowth. Of the 157 initially qualifying subjects, 47 were referred back by their doctors for a follow-up LHBT and were given a second questionnaire – without being given the results of their LHBT. Of these 47 subjects, 25 achieved complete eradication, and 22 incomplete eradication of their SIBO. Antibiotic treatment significantly reduced hydrogen production in all 47 subjects, with greater reduction in hydrogen production seen in those subjects whose SIBO was completely eradicated.

This study found that the 25 subjects (48%) who were referred back and achieved successful eradication of SIBO, no longer met the Rome criteria for IBS. No difference was seen if SIBO eradication was unsuccessful.

Therefore, the researchers conclude from this small study that subjects with IBS appear to have a high prevalence of SIBO and treatment with antibiotics appears to significantly improve symptoms.

It is important to stress that antibiotics may only help those IBS patients who were positive for SIBO with a lactulose hydrogen breath test. Generally, IBS is not treated with antibiotics. In reflecting on the prevalence of IBS in society to be as high as 30%, it would be irresponsible to treat IBS patients with antibiotics without first confirming bacterial overgrowth. The researchers point out that “Controlled studies are needed to further investigate this phenomenon”.

First published in the Inside Tract® newsletter issue 129 – January/February 2002
American Journal of Gastroenterology 2000;95:3503-3506