f you've spent months tweaking your diet, cutting out foods one by one, and still can't work out why you're bloated by 2pm every day, SIBO is one of the more commonly missed explanations. It stands for small intestinal bacterial overgrowth, and unlike a lot of gut buzzwords, it's a specific, testable thing with a specific test attached to it: the breath test.
This guide covers what SIBO actually is, the symptoms that tend to point toward it, how the breath test works, what hydrogen and methane results mean, and how to prepare so your result is actually usable.
What is SIBO?
Your small intestine is meant to carry relatively few bacteria compared to your large intestine, which is a densely populated ecosystem by design. SIBO happens when bacteria that normally belong further down the digestive tract migrate into the small intestine and build up there. Once they're feeding on the same carbohydrates you are, before your body has had a chance to absorb them, you get fermentation, gas, and the bloating and discomfort that comes with it.
It's considered a common, and commonly overlooked, contributor to IBS-type symptoms. It's also been raised as a possible factor in some cases of fibromyalgia and chronic fatigue syndrome. A 2025 cohort found a notably higher rate of SIBO among people with fibromyalgia than among people without it, though causality hasn't been established, so this is still an area of ongoing research rather than a settled fact [2][4][5].
Symptoms that point toward SIBO
SIBO symptoms overlap heavily with IBS, which is part of why it goes undiagnosed for so long. The more commonly reported ones include:
- Diarrhoea, constipation, or an alternating pattern of both
- Excess gas or a feeling of fullness after eating
- Bloating, especially one that builds through the day rather than being there first thing in the morning
- Nausea
- Fatigue
- Nutrient malabsorption over time, since an overgrowth can interfere with how well you absorb what you eat
Bloating is the symptom most people notice first and the one that usually sends them looking for an explanation, but it isn't necessarily the most telling one. In people without other risk factors for SIBO, gas and diarrhoea have shown a stronger association with a positive breath test than bloating does [7]. None of these symptoms are unique to SIBO on their own, which is exactly why testing, rather than guessing from symptoms alone, is the useful next step.
How SIBO is tested: the breath test, explained
A SIBO breath test works on a simple principle: your own cells can't produce hydrogen or methane gas, but bacteria can, as a byproduct of fermenting the carbohydrate you've been given to drink. If your breath samples show a spike in hydrogen or methane after that drink, it suggests bacteria are fermenting it somewhere they shouldn't be, namely your small intestine, before it's had the chance to move further down your gut [1].
The test itself is a collection kit you use at home, not a self-contained test that reads its own result. You drink a sugar solution (the substrate), then collect a breath sample into a tube at set intervals, typically every 30 minutes, over about 3 hours. You then mail your samples back using the prepaid packaging provided, and a laboratory analyses each one for hydrogen and methane, comparing it back to your baseline reading at time zero.
Hydrogen vs methane: why both matter
This is the part the search queries behind this piece are actually asking about, so it's worth being specific.
Hydrogen is produced by most of the bacteria implicated in SIBO. A rise in hydrogen by the 90-minute mark is generally the marker used to flag bacterial overgrowth in the upper part of the small intestine. Most labs work from a reference cutoff of around 20 parts per million above your baseline reading by that point, though the exact figure your lab uses can vary slightly [1][2][3].
Methane is produced by a different group of organisms, archaea rather than bacteria, and overgrowth of these organisms is technically called intestinal methanogen overgrowth (IMO) rather than SIBO, though the two are usually discussed together and tested for at the same time [1][8]. A methane reading of around 10 parts per million or higher at any point in the test is generally treated as positive. Methane-dominant patterns have been linked to constipation-type symptoms in some studies, but the evidence here is mixed and more contentious than for hydrogen, with several more recent studies failing to confirm the association, so it's best treated as a pattern worth raising with your practitioner rather than a settled rule [3][6]. Either way, a peak methane level at any point in the test is read differently to a hydrogen rise, which is why a test that measures both gases gives a fuller picture than one that measures only one.
This is also why the substrate you're given matters. A glucose challenge tends to pick up overgrowth further up the small intestine, closer to the stomach, because glucose is absorbed quickly and rarely reaches the far end of the small bowel in a healthy gut. A lactulose challenge isn't absorbed the same way, so it travels further, which makes it useful for picking up overgrowth lower down. It's sometimes described as a way of gauging oro-caecal transit time (OCTT), how long it takes food to travel from your mouth to the start of your large intestine, but breath testing on its own isn't a validated way to measure OCTT. What matters practically is closer to the reverse: if your gut moves food through unusually fast, a lactulose rise can turn up early and look like SIBO when it's really just quick transit, which is one of the reasons results are best interpreted by someone experienced in reading them rather than from the raw numbers alone [3]. Some protocols pair lactulose with fructose instead of glucose, which is a relevant option if fructose malabsorption is also part of the picture you're investigating.
At i-screen, the two breath test options reflect this: a SIBO Breath Test (Glucose + Lactulose), which runs both substrates so it can flag overgrowth at the upper and lower end of the small intestine in one kit, and a SIBO Breath Test (Lactulose + Fructose) for a lactulose-plus-fructose approach. Both measure hydrogen and methane across the full sampling window rather than a single gas at a single timepoint.
Preparing for your breath test
Breath test results are sensitive to what's in your gut in the days before testing, which is exactly why prep instructions matter more here than for most at-home kits. Full instructions come with your kit, but the broad principles are:
- Avoid antibiotics for around 4 weeks beforehand where possible, since they can suppress the bacterial activity the test is trying to detect [2]
- Avoid promotility agents and laxatives for about a week beforehand, for the same reason [2]
- Stop probiotics and fermented foods in the lead-up, for the same reason
- Follow the low-fermentation prep diet the kit specifies for the day or two before testing, since high-fibre or high-sugar foods can produce a false positive by feeding fermentation that has nothing to do with SIBO
- Fast overnight before the test itself
- Avoid smoking and vigorous exercise on the test day, since both can shift your breathing pattern and affect the readings
Skipping prep is one of the more common reasons a breath test comes back murky or hard to interpret, so it's worth the few days of adjustment.
What your result means
A positive result, meaning a hydrogen or methane rise that crosses the reference threshold your lab uses, is a marker relevant to bacterial overgrowth in the small intestine. It's not, on its own, a diagnosis, and it doesn't tell you why the overgrowth happened. SIBO is often a downstream effect of something else, such as slow gut motility, a structural issue, or previous use of medications that affect stomach acid, so a positive result is usually the start of a conversation with your doctor or practitioner about what's driving it, not the end of one.
A negative result is reassuring but not absolute. Breath testing has known limitations, including false negatives in people whose gut bacteria don't produce much hydrogen at all, which is one of the reasons methane is measured alongside it rather than relying on hydrogen alone [1][2].
Either way, results are best read in the context of your symptoms and history with a practitioner, whether that's your GP, a gastroenterologist, or an integrative practitioner you're already working with. If you're investigating bloating and gut symptoms more broadly rather than SIBO specifically, our gut health and digestion testing page runs through the fuller range, including stool-based options like the Microbiome Check and Complete Microbiome Mapping (GI Map), which look at gut bacteria from a different angle to a breath test and can be useful alongside it rather than instead of it.
SIBO vs other gut culprits
SIBO isn't the only overlooked explanation for chronic bloating. H. pylori, a stomach bacterium linked to ulcers and reflux, produces a different symptom pattern (more upper-abdominal discomfort and reflux than bloating and gas) but gets confused with SIBO often enough that it's worth ruling in or out separately rather than assuming. If your symptoms are more clearly tied to specific foods rather than a general pattern of bloating regardless of what you eat, that's a different investigation again, and our food intolerance testing is the more relevant starting point.
None of these tests need a GP referral to order. You can go straight to testing, then take a clear result into the conversation with whoever is helping you make sense of it.
This information is not intended to be a substitute for professional medical advice. Always seek the guidance of your doctor or other qualified health professional with any questions you may have regarding your health or a medical condition. i-screen provides wellness and educational services only. Our services do not diagnose, treat, cure, or prevent any disease or medical condition.
