IBS or SIBO? Why the Two Get Confused, and What the Research Actually Shows
If you've been told you have irritable bowel syndrome but it never quite felt like a satisfying answer, you're far from alone. IBS and its subtypes: Irritable bowel syndrome-constipation (IBS-C), Irritable bowel syndrome-diarrhea (IBS-D) and Irritable bowel syndrome-mixed (IBS-M), there is also IBS casued by medications. By definition a diagnosis of exclusion: a label given once other conditions have been ruled out, based on a pattern of symptoms rather than a clear biological marker. Small intestinal bacterial overgrowth (SIBO) has become a popular alternative explanation in recent years, and there's genuine science behind why the two get tangled up. There's also genuine, ongoing debate within gastroenterology about how reliable SIBO testing actually is, which is worth understanding before you spend money chasing a label.
How IBS is actually diagnosed
IBS is diagnosed using the Rome IV criteria, a symptom-based framework requiring recurrent abdominal pain associated with changes in bowel habit, in the absence of an identifiable structural or biochemical cause. There's no blood test or imaging scan that confirms IBS. This is precisely why so many people find the diagnosis unsatisfying: it tells you what you have based on a pattern, not why you have it.
Where SIBO enters the picture
SIBO refers to an abnormally high concentration of bacteria in the small intestine, an area that's normally far less densely populated with microbes than the colon. When this overgrowth happens those bacteria ferment carbohydrates earlier in the digestive process than they should, producing excess gas, bloating and altered bowel habits that look remarkably similar to IBS symptoms. The overlap is genuinely substantial, which is exactly why SIBO is so often raised as "what's really going on" for people with an existing IBS label.
The part most people aren't told: SIBO testing is contested
Here's where the research gets genuinely important and it's not the simple, settled story many wellness sources present. SIBO is typically diagnosed using a breath test, measuring hydrogen and methane gas after drinking a lactulose or glucose solution. A 2024 study comparing both test types in the same cohort of 287 IBS patients (diagnosed using Rome IV criteria) found that the lactulose breath test returned a SIBO-positive result in 47% of patients, while the glucose breath test, in the same population, returned a positive result in only 4.5% (Mion et al., 2024). That's roughly a tenfold difference depending purely on which substrate was used, in the same people, at the same time.
This isn't a minor technical footnote. A formal clinical practice update published in 2024 in Neurogastroenterology & Motility, written by a panel of gastroenterologists from Mayo Clinic, McMaster University, Houston Methodist and the University of Gothenburg and formally endorsed by both the European and American neurogastroenterology societies, took a deliberately critical look at the SIBO-IBS hypothesis itself. Among their findings: the lactulose breath test, one of the two most commonly used SIBO tests, appears in some cases to simply be detecting how quickly a test substance moves through your gut (transit time) rather than actually detecting bacterial overgrowth in the small intestine at all (Kashyap et al., 2024, citing earlier scintigraphy research by Yu, Cheeseman & Vanner). In plain terms, a meaningful share of "positive" SIBO results may reflect a fast gut rather than an overgrown one.
So does this mean SIBO isn't real?
No, and that's an important distinction. SIBO is a well-recognised, genuine condition, particularly in people with predisposing factors like prior gut surgery, scleroderma or conditions that slow gut motility. The issue isn't whether SIBO exists, it clearly does. The issue is that breath testing as currently used has real limitations and a positive result doesn't automatically mean bacterial overgrowth is the actual driver of someone's symptoms. Research has also found that SIBO breath test results don't reliably predict how severe someone's gastrointestinal symptoms are; factors like anxiety, depression and inflammatory markers were found to correlate more strongly with symptom severity than the breath test result itself (Spanish gastroenterology research, 2025).
What this actually means for you
If you've been chasing a SIBO diagnosis as the missing piece of your IBS puzzle, it's worth knowing that a positive or negative breath test is one data point among several, not a definitive verdict. A thorough clinical picture, looking at symptom pattern, diet history, stress load, motility and the broader gut-brain relationship tends to be far more useful than treating a single breath test number as the whole story.
Frequently Asked Questions
Is SIBO the real cause of most IBS?
Not necessarily. There's genuine overlap between SIBO and IBS symptoms, but SIBO breath testing has well-documented reliability issues, and a formal 2024 clinical practice update from leading gastroenterology societies specifically cautioned against over-relying on these tests to explain IBS symptoms.
Why did my lactulose and glucose SIBO breath tests give different results?
This is a known issue. A 2024 study found the lactulose breath test returned a positive SIBO result roughly ten times more often than the glucose test in the same patients, partly because lactulose can be fermented once it reaches the colon, which can be misread as small intestinal overgrowth.
If a SIBO breath test is positive, does that mean it's definitely causing my symptoms?
Not automatically. Research has found that breath test results don't reliably predict how severe someone's symptoms are with psychological and inflammatory factors playing a significant independent role.
What should I do if my IBS diagnosis hasn't helped explain my symptoms?
A deeper clinical work-up looking at gut motility, the microbiome, food triggers and the stress-gut connection together rather than chasing a single test result, tends to give a far clearer and more actionable picture.
Want a clearer picture of what's actually behind your symptoms?
If "IBS" or a SIBO test result hasn't given you a satisfying explanation, let's look at the full clinical picture together. Book your free 15-minute Discovery Call (https://earthflow-health.au4.cliniko.com/bookings) and we'll work through what's actually going on.
References
1. Mion, F., Subtil, F., Machon, C., Roman, S., & Mialon, A. (2024). The prevalence of small intestine bacterial overgrowth in irritable bowel syndrome is much higher with lactulose than glucose breath test: Results of a retrospective monocentric study. Clinics and Research in Hepatology and Gastroenterology, 48(9), 102482. https://www.sciencedirect.com/science/article/abs/pii/S2210740124002031
2. Kashyap, P., Moayyedi, P., Quigley, E. M. M., Simren, M., & Vanner, S. (2024). Critical appraisal of the SIBO hypothesis and breath testing: A clinical practice update endorsed by ESNM and ANMS. Neurogastroenterology & Motility, 36(6), e14817. https://pmc.ncbi.nlm.nih.gov/articles/PMC11268457/
3. SIBO breath test does not predict symptom severity in disorders of gut-brain interaction: role of anxiety, depression, and inflammatory biomarkers. https://pubmed.ncbi.nlm.nih.gov/40276984/