Bacterial Overgrowth Hits 1 in 5 IBD Patients, Study Shows

If you have inflammatory bowel disease (IBD) and still feel bloated, gassy, or off even when your Crohn's disease or ulcerative colitis (UC) seems otherwise under control, a second, overlapping gut problem may be part of the picture. A systematic review and meta-analysis pooling data from 11 studies found that small intestinal bacterial overgrowth (SIBO), an excess of bacteria in the small intestine, a section of the gut that normally hosts relatively few microbes, shows up far more often in people with IBD than in people without it. The gap was not small: the odds of having SIBO were more than 9 times higher in people with IBD than in controls.

Key takeaways

• A meta-analysis of 11 studies (1,175 people with IBD, 407 controls) found SIBO in 22.3% of IBD patients, all diagnosed by breath test.
• The odds of SIBO were 9.51 times higher in IBD overall, and higher still in Crohn's disease (nearly 11 times) than in ulcerative colitis (about 8 times).
• Fibrostenosing Crohn's disease and prior bowel surgery, especially removal of the ileocecal valve, raised the odds further.
• The case-control design cannot show whether SIBO causes IBD symptoms or results from the gut changes IBD itself causes.

SIBO Turns Up in 22% of IBD Patients, Meta-Analysis Finds

A systematic review and meta-analysis published in Alimentary Pharmacology & Therapeutics found that small intestinal bacterial overgrowth (SIBO), diagnosed in every included study by breath test (a noninvasive test that measures hydrogen or methane gas after drinking a sugar solution, which spikes early if bacteria have overpopulated the small intestine), was present in 22.3% of people with inflammatory bowel disease. That is roughly 1 in 5. Compared with people who did not have IBD, the odds of having SIBO were 9.51 times higher in the IBD group. The researchers, writing under lead author Shah and colleagues, described the increase as substantial.

Crohn's Disease and Ulcerative Colitis Patients Both Face Higher Odds

IBD is not one disease. It covers Crohn's disease, which can inflame any part of the digestive tract, and ulcerative colitis, which is confined to the colon. The Centers for Disease Control and Prevention estimates that as many as 3.1 million adults in the United States live with one of the two. The meta-analysis found elevated SIBO odds in both: 10.86 times higher in Crohn's disease and 7.96 times higher in ulcerative colitis, compared with non-IBD controls. Anyone managing either condition, and still dealing with bloating, gas, diarrhea, or unexplained weight change after their IBD flare has been treated, is exactly who this finding concerns.

How Researchers Pooled 11 Studies and 1,175 Patients

The team searched the medical literature through May 2018 and combined 11 case-control studies covering 1,175 adults with IBD and 407 controls without it. Every included study used a breath test to diagnose SIBO, which the researchers noted keeps the comparison consistent across studies that might otherwise use different diagnostic methods. Within the Crohn's disease studies, the researchers also looked at who was most likely to have SIBO. Two subgroups stood out: patients with fibrostenosing disease (a form of Crohn's marked by scar tissue narrowing the intestine) had 7.47 times higher odds of SIBO, and patients with prior bowel surgery had 2.38 times higher odds, an effect the researchers linked especially to removal of the ileocecal valve, the one-way valve between the small and large intestine that normally helps keep colon bacteria out of the small bowel.

What Persistent Gut Symptoms in IBD Could Mean for You

If you have Crohn's disease or ulcerative colitis and still have bloating, excess gas, or diarrhea after inflammation has been brought under control, this study is a reason to raise SIBO with your gastroenterologist rather than assume every symptom is a flare. That is especially worth asking about if you have fibrostenosing Crohn's disease or have had bowel surgery, particularly at the ileocecal valve, since those groups showed the highest odds in this analysis. A breath test is the noninvasive way SIBO was diagnosed in every study reviewed here. What this research does not do is establish that treating SIBO improves IBD outcomes, or endorse any particular antibiotic, probiotic, or diet as a fix. No treatment approach was tested in this meta-analysis at all; it measured prevalence, not treatment.

Why This Study Can't Prove SIBO Causes IBD Symptoms

The biggest limitation is built into the study design itself. Because every one of the 11 underlying studies was case-control, comparing people who already had IBD (with or without SIBO) rather than following people over time, the meta-analysis cannot determine which comes first. SIBO could be contributing to IBD symptoms, or it could simply be a downstream consequence of the altered gut anatomy, motility, and inflammation that IBD itself causes, especially after surgery removes the ileocecal valve's natural barrier. The researchers also noted that in individual studies, having disease in both the small and large bowel, rather than active disease itself, seemed more tied to SIBO risk, though that pattern was not part of the pooled statistical analysis. The study was published in Alimentary Pharmacology & Therapeutics in 2019.

Next Steps: Confirming Whether Treating SIBO Helps

What is missing is the trial that would actually change practice: a prospective study testing whether identifying and treating SIBO in IBD patients (with antibiotics, dietary changes, or otherwise) improves their symptoms or outcomes. Existing clinical guidance on diagnosing and treating SIBO already exists for the general population, but this meta-analysis does not tell us whether IBD patients need a different threshold or approach. Until that research exists, the practical takeaway stays modest: know that the risk is elevated, and bring it up with a doctor rather than self-diagnosing or self-treating.

Common Questions

Is SIBO the same thing as IBD? No. SIBO is an overgrowth of bacteria in the small intestine, while IBD (Crohn's disease and ulcerative colitis) is a separate, chronic inflammatory condition. This study found the two frequently occur together, not that they are the same problem.

Can having IBD cause SIBO, or does SIBO cause IBD flares? This meta-analysis cannot say. Its case-control design shows the two are strongly associated but cannot determine which one comes first or whether one causes the other.

Who is most likely to have both conditions? People with fibrostenosing Crohn's disease and people who have had bowel surgery, especially removal of the ileocecal valve, showed the highest odds of SIBO in this analysis.

How is SIBO diagnosed? Every study in this meta-analysis used a breath test, which measures hydrogen or methane gas exhaled after drinking a sugar solution to detect bacterial overgrowth in the small intestine.

References

1. Ayesha Shah, Mark Morrison, Daniel Burger, et al. (2019). Systematic review with meta‐analysis: the prevalence of small intestinal bacterial overgrowth in inflammatory bowel disease. Alimentary Pharmacology & Therapeutics. https://doi.org/10.1111/apt.15133
2. IBD Facts and Stats | CDC. cdc.gov. https://www.cdc.gov/inflammatory-bowel-disease/php/facts-stats/index.html
3. Kathy Bull-Henry (2020). Continuing Medical Education Questions: February 2020: ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology. https://doi.org/10.14309/ajg.0000000000000526