SIBO Explained: Small Intestinal Bacterial Overgrowth, Its Symptoms, and Why It Keeps Coming Back
If you bloat within an hour of eating, feel distended by the evening even when you’ve eaten carefully, and have been told you have IBS without ever feeling like that fully explained it, SIBO is worth understanding. It’s one of the most talked-about gut conditions, and also one of the most misunderstood, particularly when it comes to why it so often returns after treatment.
This article covers what SIBO actually is, the symptom pattern that points towards it, why a gut microbiome test can’t diagnose it, and why lasting improvement depends on finding why it developed in the first place.
What is SIBO?
SIBO stands for small intestinal bacterial overgrowth. The name is precise and worth unpacking, because the location is the whole point.
Your large intestine (the colon) is meant to be densely populated with bacteria, that’s normal and healthy. Your small intestine, by contrast, is meant to be relatively sparse. It’s where most of your food is digested and absorbed, and it works best without a large bacterial population competing for that food.
SIBO is what happens when bacteria overgrow in the small intestine, where they shouldn’t be in large numbers. They start fermenting your food before you’ve properly absorbed it, producing gas and a cascade of symptoms in the process. Importantly, these don’t have to be “bad” bacteria, they can be ordinary, even beneficial ones that have simply set up in the wrong place and the wrong numbers.
The symptoms and patterns that points to the small intestine
SIBO symptoms overlap heavily with IBS, which is part of why it’s so often missed. But there’s a pattern that tends to point upstream, to the small intestine specifically:
Bloating that comes on soon after eating, often within an hour, rather than building slowly over the day
Visible distension, the classic “looked fine in the morning, look pregnant by evening” experience
Excessive gas and belching
Abdominal discomfort or pain
A change in bowel habit, and the direction is a clue (more on that below)
Nausea or early fullness
Some people also notice more systemic effects: fatigue, or signs of poor nutrient absorption, because overgrown bacteria can interfere with how you take up nutrients from food.
The direction of your bowel habit can hint at what’s being produced. Overgrowth that produces more hydrogen tends to be associated with looser stools, while overgrowth dominated by methane-producing organisms is more often linked with constipation (this methane pattern is now sometimes described as its own entity). It’s not a perfect rule, but it helps shape the picture.
Why a gut microbiome test can’t diagnose SIBO
This is the misunderstanding I most want to clear up, and it follows directly from the location point above.
A gut microbiome test analyses a stool sample, which reflects the large intestine, at the end of the digestive tract. SIBO is a problem in the small intestine, much further up. They are genuinely different environments, and a stool test simply isn’t looking in the right place. So while microbiome testing is useful for other questions, it cannot confirm or rule out SIBO. (I’ve explained more about what gut microbiome testing can and can’t tell you separately.)
This matters because people sometimes arrive with a microbiome report believing it has settled the SIBO question one way or the other, when in fact it can’t.
What causes SIBO?
Here is the single most important idea in this whole article: SIBO is almost always secondary to something else. It’s not usually a random infection you catch, it’s a consequence of an underlying issue that has allowed bacteria to accumulate where they shouldn’t. Finding that underlying issue is the difference between temporary relief and lasting change.
The common drivers include:
Impaired gut motility. Between meals, the small intestine performs a kind of “housekeeping” sweep that moves bacteria along and stops them settling. When that sweeping action is impaired, bacteria linger and overgrow. This is one of the most common drivers, and it can follow an episode of food poisoning or gastroenteritis, which is one reason SIBO so often begins after a bout of acute gut illness.
Reduced digestive defences. Stomach acid, bile, and digestive enzymes all help keep bacterial numbers in check. When any of these are low (through medications, or other conditions) that protective effect drops.
Structural factors. Previous abdominal surgery, adhesions, or anatomical variations can create pockets where bacteria accumulate or slow the normal flow.
Underlying medical conditions. Several conditions affecting motility or the gut more broadly can predispose to SIBO, which is part of why medical assessment matters.
How is SIBO tested?
The most common non-invasive test is breath testing. After drinking a sugar solution, you provide breath samples over a few hours, and the test measures hydrogen and methane gases (produced by bacteria) as a marker of overgrowth.
Breath testing is useful, but it’s worth knowing it’s an imperfect tool. Preparation affects the result, false positives and negatives both occur, and interpretation is genuinely debated among experts. The research “gold standard”, sampling fluid directly from the small intestine, is invasive and rarely done in practice. Because of all this, breath testing is best read alongside your symptom pattern and history rather than as a stand-alone verdict, and sometimes a carefully monitored treatment trial is part of the process.
Why SIBO so often comes back
This is where a lot of people get stuck, sometimes cycling through round after round of treatment, increasingly restrictive diets, and growing frustration.
The reason is straightforward once you see it: if treatment only reduces the bacteria but never addresses why they overgrew, the same conditions that allowed it the first time are still there. The overgrowth returns because its cause was never resolved. Clearing the bacteria without supporting motility or addressing the underlying driver is like bailing out a boat without finding the leak.
It also helps to reframe the goal. The aim isn’t to sterilise the small intestine, some bacteria there is normal. The aim is to bring numbers back into a healthy range and to restore the conditions that keep them there.
How is SIBO managed?
There’s no single protocol that suits everyone, but good management generally works on several fronts at once rather than just one:
Addressing the driver, particularly motility, since this is so often the key to preventing relapse.
Reducing the overgrowth through antimicrobial treatment, which may be herbal or pharmaceutical. Where a prescription medication is appropriate, that’s a decision made with your doctor, and it’s one of the reasons I work as part of a medical team rather than in isolation.
Managing symptoms with diet, carefully, lower-fermentation eating approaches can ease symptoms in the short term, but they are a way to create breathing room, not a cure. I’m cautious about long-term restrictive diets here, because they narrow nutrition and can make the gut environment less resilient over time. The aim is to use diet to settle things while the real work is done, then to widen it again.
Supporting relapse prevention, including motility support and addressing the modifiable factors that contributed in the first place.
The thread through all of this is the same one that runs through my whole approach: find the driver, treat the cause, and resist the urge to simply restrict harder.
When to see your doctor
Some features always warrant medical assessment rather than dietary management alone — including unintentional weight loss, signs of anaemia or nutrient deficiency, blood in the stool, or severe or escalating symptoms. Conditions such as coeliac disease and inflammatory bowel disease can produce overlapping symptoms and need to be excluded. And any prescription treatment is, of course, a medical decision. Good care for SIBO is collaborative, which is exactly how I prefer to work.
A clearer way through
If you’ve been going in circles with SIBO, treatment that works for a while and then fades, a diet that keeps shrinking, and no clear sense of why it started. The missing piece is usually the driver, not another round of the same thing.
That whole-picture, find-the-cause approach is what I focus on, working alongside the doctors at the clinic where appropriate. If you’d like help making sense of your symptoms and building a plan that addresses the root of the problem, you’re welcome to book a consultation or get in touch with any questions first.
Sophie Thelosen is a Clinical Nutritionist registered with the Australian Traditional Medicine Society (ATMS), practising within the integrative team at Mosman Integrative Medicine alongside Dr Mark Donohoe and Dr Isobel Marr. She works with people experiencing gut issues, food and supplement sensitivities, histamine intolerance, and chronic fatigue, in person in Sydney and via telehealth across Australia.
This article is general information and is not a substitute for individualised advice. If you have ongoing or severe symptoms, please consult an appropriate health professional.