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The Complex Relationship Between IBS and SIBO
By Thomas G. Guilliams, PhD

The symptoms of SIBO (e.g., bloating, abdominal distension/ pain/discomfort, diarrhea, fatigue, weakness, etc.) overlap considerably with those of irritable bowel syndrome (IBS), and many clinicians consider these conditions to be commonly associated.1 However, epidemiological research shows that the relationship between the two conditions is not well-understood, is controversial, and varies considerably depending upon the diagnostic criteria used to define SIBO (and IBS). In fact, the large variance in the frequency of SIBO in IBS subjects mirrors that found in healthy controls.

Ghoshal et al. reported in their meta-analysis that the frequency of SIBO amongst those with IBS ranged from 4% to 78%, whereas it ranged from 1% to 40% in healthy controls.2 Another systematic review and meta-analysis including 12 studies (N = 1,921 patients with IBS) reported the discrepancy in the prevalence of SIBO in IBS subjects varies by test method.3 Namely, the average prevalence of SIBO measured via lactulose breath test was 54%, and measured via the glucose breath test averaged only 31%; as compared to a mean prevalence of just 4% for jejunal aspirate enumeration. Based on the wide discrepancy of a SIBO diagnosis in IBS subjects based on test method, the Rome Foundation does not recommend routine testing for SIBO in IBS patients.4 Several large systematic reviews and meta-analyses have been recently published to explore the relationship between SIBO and IBS.

One review evaluating 50 studies, reaffirmed the heterogeneity of this relationship based primarily on how the included studies tested for SIBO (e.g., small intestinal fluid aspirate and culture [N = 5], LBT [N = 24], GBT [N = 21]), the thresholds used to define a positive test, and even which Rome criteria was used to define IBS.5 Overall, amongst these 50 studies, the pooled prevalence of SIBO in subjects diagnosed with IBS was 38% (95% CI: 32-44), which was higher in studies diagnosing SIBO using either breath tests (40%, 95% CI: 33-46) versus small intestinal aspirate and culture (19%, 95% CI: 8-30). Among subjects with IBS, female gender (OR: 1.6, 95% CI: 1.1-2.3), IBS-D subtype (OR: 1.7, 95% CI: 1.3-2.3), and older age (OR: standard mean difference: 3.1 years, 95% CI: 0.9-5.4) were associated with increased odds of SIBO, though PPI use did not increase the odds of SIBO in those with IBS (OR: 1.1, 95% CI: 0.7-1.7).

When the threshold for small intestinal fluid aspirate and culture was set at >105 CFU/mL, the prevalence of SIBO in IBS subjects was 13%, compared to a prevalence of 28% when the threshold was set at >103 CFU/mL (P = 0.02). Since these studies spanned many years there were several different diagnostic criteria used for IBS, including Rome I, Rome II, Rome III, Manning’s criteria, or other physician-specified diagnosis. SIBO was more prevalent in IBS subjects meeting the Rome I criteria (72%, 95% CI: 44- 91) compared to Rome II (40%, 95% CI: 27-54), Rome III (35%, 95% CI: 28-43), or other criteria (30%, 95% CI: 22-38).

Therefore, while there appears to be a fairly strong association between these two diagnoses, it is still difficult to determine the prevalence of SIBO in populations of subjects with IBS. Members of the Rome IV group have noted that the status of SIBO in IBS subjects is highly controversial and explain that the diagnostic test modality and the diagnostic criteria contribute to the variance in the prevalence of SIBO in IBS. A more recent meta-analysis using a more stringent criteria for including trials (25 trials) showed a similar prevalence of SIBO in IBS subjects of 31% (OR = 3.7).6

† While beyond the scope of this mini-review, the recent discoveries linking a large subset of IBS-D subjects with a previous gastrointestinal infection may have implications for connecting SIBO and IBS. Subjects with “post-infectious IBS” appear to have antibodies to a bacterial toxin (cytolethal distending toxin [CDT]), suggesting a previous infection, and often antibodies to vinculin, suggesting an autoreactive immune response to this important gastrointestinal cytoskeletal protein. While the formal link between post-infectious IBS and SIBO is still lacking, changes in motility are proposed as a possible link between these two phenomena.



1. Takakura W, Pimentel M. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome – An Update. Frontiers in Psychiatry.2020;11(664).

2. Ghoshal UC, Shukla R, Ghoshal U. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy. Gut Liver. 2017;11(2):196-208.

3. Ford AC, Spiegel BM, Talley NJ, Moayyedi P. Small intestinal bacterial overgrowth in irritable bowel syndrome: systematic review and meta-analysis. Clin Gastroenterol Hepatol. 2009;7(12):1279-1286.

4. Barbara G, Feinle-Bisset C, Ghoshal UC, et al. The Intestinal Microenvironment and Functional Gastrointestinal Disorders. Gastroenterology. 2016.

5. Chen B, Kim JJ, Zhang Y, Du L, Dai N. Prevalence and predictors of small intestinal bacterial overgrowth in irritable bowel syndrome: a systematic review and meta-analysis. J Gastroenterol. 2018.

6. Shah A, Talley NJ, Jones M, et al. Small Intestinal Bacterial Overgrowth in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis of Case-Control Studies. Am J Gastroenterol. 2020;115(2):190-201.

Thomas G. Guilliams, PhD

Thomas G. Guilliams, PhD (Tom) earned his doctorate in molecular immunology from the Medical College of Wisconsin in Milwaukee. For the past two decades, he has spent his time investigating the mechanisms and actions of lifestyle and nutrient-based therapies, and is an expert in the therapeutic uses of dietary supplements. Tom serves as an adjunct assistant professor at the University of Wisconsin School of Pharmacy and was the VP of Science for Ortho Molecular Products for 24 years (he now serves them as a consultant). Since 2014 he has been writing a series of teaching manuals (Road Maps) that outline and evaluate the evidence for the principles and protocols that are fundamental to the functional and integrative medical community.  He is the founder and director of the Point Institute, an independent research and publishing organization that facilitates the distribution of his many publications. A frequent guest-speaker, Dr. Guilliams provides training to a variety of health care disciplines in the use of lifestyle and natural medicines. He lives in the woods outside of Stevens Point, Wisconsin with his wife and children.

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