SIBO and IBS: What Is Small Intestinal Bacterial Overgrowth, and How Are They Connected?

If you deal with constant bloating, gas, and unpredictable trips to the bathroom, you may have been told you have irritable bowel syndrome (IBS). But there is another, often-overlooked culprit that could be behind those symptoms: small intestinal bacterial overgrowth, or SIBO. The two conditions overlap so much that many people diagnosed with IBS actually have SIBO that has gone unnoticed. Here is what SIBO is, how it connects to IBS, and what you can do about it.

What Is SIBO?

SIBO stands for small intestinal bacterial overgrowth. It happens when bacteria that normally belong in your large intestine grow in excessive numbers—or the wrong types show up—in your small intestine, where bacteria should be fairly scarce.

Those extra bacteria ferment the food you eat before your body can properly absorb it. That fermentation creates gas and kicks off a chain of uncomfortable digestive symptoms. You might also hear about a close cousin called intestinal methanogen overgrowth (IMO), which involves an overgrowth of methane-producing microbes and is often lumped in with SIBO.

What Are the Symptoms?

SIBO symptoms look a lot like other tummy troubles, which is part of what makes it tricky to spot. The most common ones include :

• Bloating and a swollen, distended belly
• Lots of gas and belching
• Belly pain or discomfort
• Diarrhea, constipation, or a mix of both
• Fatigue and feeling run-down

Because the bacteria get in the way of nutrient absorption, long-running SIBO can also lead to deficiencies (like low vitamin B12) and unintended weight loss.

What Causes SIBO?

SIBO usually develops when something throws off the systems that normally keep your gut bacteria in check—things like stomach acid, gut movement, and the valve between your small and large intestine. Common triggers include :

• Past abdominal or gut surgery
• A sluggish gut that moves food along too slowly
• Structural issues like scarring or narrowed sections
• Certain chronic conditions, such as diabetes or scleroderma
• Long-term use of acid-reducing medications

That last one matters more than you might think. In one study, bacterial overgrowth showed up in 53% of people taking a common acid-blocking drug, compared with just 17% on a different medication. Less stomach acid means one of your gut’s natural defenses is turned down.

How Is SIBO Diagnosed?

The most common way to check for SIBO is a simple, noninvasive breath test. You drink a sugary solution, and your breath is measured for the hydrogen and methane gases that gut bacteria give off when they ferment that sugar. A bigger-than-normal rise in those gases points to SIBO. There is also a more involved option—sampling fluid directly from the small intestine—but it is invasive and used far less often. One thing to keep in mind: breath tests are not perfect and can sometimes give false results, so they work best alongside your symptoms and history.

How Is SIBO Linked to IBS?

This is where SIBO and IBS get tangled together. IBS is a very common gut condition—affecting an estimated 10% to 15% of people in North America—that causes belly pain along with changes in bowel habits. For years it was thought to be mostly stress-related, but we now know it is far more complicated, and an imbalance of gut bacteria (including SIBO) is one piece of the puzzle.

The symptoms are nearly identical

Bloating, gas, pain, diarrhea, and constipation are hallmarks of both conditions. That overlap means SIBO can masquerade as IBS—or show up right alongside it—so you often cannot tell them apart on symptoms alone.

SIBO is more common in people with IBS

Studies consistently find SIBO more often in people with IBS than in those without. One large review found SIBO in about 35.5% of people with IBS, compared with 29.7% of those without. The link also seems to vary by IBS type—one study found SIBO in over half of people with diarrhea-predominant IBS.

The evidence is promising, but not the whole story

It is worth being realistic here. The connection is real, but the research has limits, partly because breath tests are not perfectly reliable and studies define things differently. Experts continue to debate exactly how big a role SIBO plays. The takeaway: SIBO likely drives symptoms for a meaningful share of people with IBS, but it is not the single cause of IBS.

How Is SIBO Treated?

The goal of treatment is to calm your symptoms by clearing out the overgrowth. Most plans combine a few approaches:

Antibiotics

Antibiotics are the main treatment. Rifaximin, which stays mostly in the gut, is the best-studied option and clears SIBO in roughly 7 out of 10 people. Other antibiotics like metronidazole and ciprofloxacin can work too.

Diet

Changing what you eat can support treatment. A low-FODMAP diet—which cuts back on certain fermentable carbs—is commonly used to starve the troublesome bacteria of their favorite fuel. Many SIBO eating plans focus on limiting sugars, some fruits and starchy vegetables, dairy, and grains.

Probiotics and fixing the root cause

Some probiotic strains may help. In one trial, people with both IBS and SIBO saw their symptom scores drop by more than 70% with probiotics. However be careful of starting with probiotics without first getting rid of the overgrowth of bacteria as this may lead to more bloating symptoms.

A Functional Medicine Approach to Gut Health

The Bottom Line

SIBO and IBS are separate conditions with a close, overlapping relationship. SIBO can cause symptoms that look almost exactly like IBS, and it shows up more often in people who have IBS. While the science is still evolving, finding and treating SIBO can offer real relief for many people whose IBS has not improved with the usual approaches. If your digestive symptoms just will not quit, talk to a gastroenterologist about whether a SIBO breath test makes sense for you. Getting the right diagnosis is the first step toward feeling better.

This article is for educational purposes only and is not a substitute for professional medical advice. Always talk to a qualified healthcare provider about diagnosis and treatment.

Frequently Asked Questions

Can SIBO cause IBS? 

SIBO can drive IBS-like symptoms for many people, and the two share nearly identical symptoms. But SIBO is not considered the single cause of IBS, which has many contributing factors.

How do you test for SIBO? 

Usually with a simple breath test that measures hydrogen and methane after you drink a sugary solution.

At Restore Integrative Medical Clinic in Central, we offer a range of functional gut and SIBO testing to identify what’s the root cause of SIBO. The SIBO test is a 3 hr breath test that involves checking for hydrogen and methane gas that are caused by SIBO bacteria.

What is the best treatment for SIBO? 

Antibiotics (especially rifaximin) are the mainstay, often paired with a low-FODMAP diet and steps to address the underlying cause.

Ways to Enhance your Gut Microbiome

Does SIBO come back?

 Yes, recurrence is common, which is why treating the root cause and following up matter so much.

Contributed by Dr Ho See Yunn

Chief Doctor of Restore INtegrative Medical Clinic

Longevity Physician

Functional Medicine Practitioner

Begin your gut healing journey with our functional medicine approach in Central, Hong Kong

Investing in your gut health is investing in your future self. Whether you’re seeking more energy, hormonal balance, or a proactive plan for healthy ageing, the team at Restore Integrative Medical Clinic is here to guide you.

Book your personalised functional gut consultation today and discover what’s possible when your care is built around you. Restore Integrative Medical Clinic offers a range of gut programs to treat your long underlying gut issues from a root cause approach.

Restore Integrative Medical Clinic — Room 1602, 16/F, 1 Duddell Street, Central, Hong Kong · +852 2331 8816, www.restoreintegrativemedical.com

 Ref: 

• Small Intestinal Bacterial Overgrowth: A Comprehensive Review, Gastroenterology & Hepatology (PMC), https://pmc.ncbi.nlm.nih.gov/articles/PMC3099351/

• Cleveland Clinic, Small Intestinal Bacterial Overgrowth (SIBO), https://my.clevelandclinic.org/health/diseases/21820-small-intestinal-bacterial-overgrowth-sibo

• Shukla R, Ghoshal U, SIBO and IBS: A Bridge Between Functional and Organic Dichotomy, Gut and Liver (PMC), https://pmc.ncbi.nlm.nih.gov/articles/PMC5347643/

• Diagnosis and treatment of small intestinal bacterial overgrowth, Arquivos de Gastroenterologia (PMC), https://pmc.ncbi.nlm.nih.gov/articles/PMC12043196/

• Leventogiannis K et al., Effect of Four Probiotics on Symptoms of IBS, Probiotics and Antimicrobial Proteins (Springer), http://link.springer.com/10.1007/s12602-018-9401-3

• Quigley EMM et al., Critical appraisal of the SIBO hypothesis and breath testing, ESNM/ANMS (PMC), https://pmc.ncbi.nlm.nih.gov/articles/PMC11268457/

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