What Is SIBO and What Does It Have to Do With IBS?

SIBO, the acronym for a condition called small intestinal bacterial overgrowth, is being researched as a possible cause of irritable bowel syndrome (IBS). Like many things related to IBS, the issue of SIBO being the underlying problem is complicated and marked by some controversy in the world of IBS research. This overview of SIBO as it relates to IBS can help you decide if this is something you should speak to your healthcare provider about.

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What Is SIBO?

SIBO is the accumulation of an excessive amount of gut bacteria in the small intestine. Any condition which impairs the normal transit or motion of the small intestine can predispose it to SIBO. Crohn's disease and previous abdominal surgery are among the risk factors for the development of SIBO.

How Is SIBO Diagnosed?

Due to the difficulty of doing direct biopsies of the small intestine to assess for the presence of bacteria, a test commonly known as the hydrogen breath test (HBT) is used. Patients are given a solution, such as lactulose, to drink, and then a breath test is given to assess for the presence of gas, such as hydrogen or methane.

In a healthy individual, one would not expect to see any hydrogen or methane in the breath until two hours have passed, the approximate time it would take for the lactulose to travel to the large intestine where it would be acted upon by bacteria, thus releasing the gas. A positive gas result seen within 90 minutes of drinking the solution provides evidence of bacteria higher up in the digestive tract, i.e. at the level of the small intestine.

The SIBO Theory for IBS

The fact that bloating is a ubiquitous symptom for IBS sufferers, regardless of whether constipation or diarrhea as a predominant symptom, has led researchers to look for an underlying common problem. In addition, although IBS patients often point to specific foods as causing symptoms, no clear-cut research supports this.

Evidence that SIBO may be an underlying cause for IBS comes from two main findings. The first is that some researchers have found that significantly more IBS patients have a positive HBT than do unaffected people, possibly indicating SIBO as a problem. The second is the research finding that many patients see a significant decrease in IBS symptoms following a trial of specific antibiotics. These antibiotics are not absorbed in the stomach and thus are available to act on any bacteria that may be lurking in the small intestine.

The SIBO theory seeks to explain why bacteria end up in the wrong place. The small intestine has a natural "cleansing wave" -- movement of muscles in the lining that serves to empty the small intestine at regular intervals. It is thought that the impairment of this muscle movement may result in the retention of bacteria.

One theory is that a bout of gastroenteritis may damage the muscles responsible for this cleansing action, a possibility that might explain the phenomenon of post-infectious IBS. It is also thought that stress can slow down the action of these muscles, thus explaining the relationship between stress and IBS.

The SIBO theory attempts to account for the fact that IBS can manifest itself as either diarrhea or constipation. The thinking is that different types of bacteria and the gasses that they produce have different effects on gut motility. Some studies have found that patients who demonstrate a higher amount of methane are more likely to experience constipation, while diarrhea-predominant patients demonstrate a higher level of hydrogen.

It is also theorized that SIBO may be the true underlying cause of fructose and other sugar intolerance.

The Controversy

Although the SIBO theory appears to tie up IBS in a nice neat package, many researchers are not convinced. There are several major criticisms of the theory. A major critique is that the HBT is not seen as being a reliable measure due to a high error rate.

Of greater importance is the fact that the high rates of SIBO and the success of antibiotics as a treatment seen in studies conducted by SIBO theorists have not always been replicated by other researchers. Concern also exists regarding the long-term use of antibiotics, especially given the fact that IBS is a condition with a chronic course.

The Bottom Line

As you can see, the relationship between SIBO and IBS remains murky. The general consensus appears to be that SIBO may be the underlying problem for a sub-set of IBS patients and that one particular type of antibiotic, Rifaximin, has the most research support for its effectiveness in terms of reducing bloating and diarrhea. Hopefully, continued research will clarify the issue, as well as come up with safe and effective treatments.

Should you talk to your healthcare provider about taking an HBT for SIBO? Given the fact that SIBO does appear to be an issue for some IBS patients and the fact that an antibiotic may bring symptom relief, SIBO may certainly be worth further investigation, particularly if bloating is a predominant part of your symptom picture.

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  • American College of Gastroenterology IBS Task Force An Evidence-Based Position Statement on the Management of Irritable Bowel Syndrome. American Journal of Gastroenterology. 2009:S1-S35.

  • Lin, H. Small Intestinal Bacterial Overgrowth: A Framework for Understanding Irritable Bowel Syndrome. The Journal of the American Medical Association. 2004 292:852-858.

  • Pimentel, M. A New IBS Solution. Health Point Press. 2006.

  • Quigley, E. Gut Bacteria and Irritable Bowel Syndrome. International Foundation for Functional Gastrointestinal Disorders Fact Sheet.

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.