The GI-MAP is a stool test — it measures what's happening in the colon. SIBO is a small intestine problem, and no stool test can find it. If upper GI symptoms persist despite thorough gut work, the investigation that's been missing is a breath test for bacterial overgrowth in the small bowel.
Understanding SIBO starts with understanding where in the gastrointestinal tract the problem occurs — because location is everything in gut investigation.
The small intestine is approximately six metres long. Its job is absorption — nutrients, minerals, water, and most of what you eat crosses the intestinal wall here and enters the bloodstream. The small intestine is supposed to be relatively sparsely populated with bacteria: a small resident community that helps with digestion but doesn't dominate the environment.
The colon — the large intestine — is where the bacterial ecosystem is dense. Trillions of organisms fermenting fibre, producing short-chain fatty acids, competing for resources. This is the environment the GI-MAP stool test measures: the microbial community at the end of the digestive tract, excreted in faeces.
SIBO is what happens when colonic bacteria migrate upstream into the small intestine, or when the small intestinal bacterial community grows beyond its normal density. The result is fermentation occurring in the wrong location — in the tube where absorption happens, rather than the tube where fermentation is supposed to happen.
A client can have a perfectly adequate GI-MAP result — no significant pathogens, reasonable microbiome diversity, normal immune markers — and still have SIBO driving significant symptoms. The GI-MAP is not a test of small intestinal function. It was not designed to be. This is not a limitation of the GI-MAP — it is a limitation of using any single test to investigate the entire gastrointestinal tract. The small intestine requires its own investigation.
SIBO breath testing measures the gases produced by bacteria fermenting a sugar substrate in the small intestine. The gases are absorbed through the intestinal wall, carried in the bloodstream to the lungs, and exhaled — where they are measured in breath samples taken at timed intervals over three hours. Different bacteria produce different gases, and the gas profile identifies both the presence of SIBO and the type of organisms driving it.
Produced by gram-negative bacteria fermenting carbohydrates. The most common SIBO pattern. Symptoms typically include bloating, flatulence, loose stools or diarrhoea, and upper abdominal discomfort. Hydrogen-dominant SIBO often responds well to dietary intervention and targeted antimicrobial protocols. The ileocaecal valve incompetence and impaired migrating motor complex (MMC) are the most common structural drivers.
Produced by archaea — methanogens — rather than true bacteria, which is why methane-dominant overgrowth is now classified as Intestinal Methanogen Overgrowth (IMO) rather than SIBO strictly defined. Methane is associated with constipation, bloating, and slower gut transit — the opposite motility pattern from hydrogen-dominant SIBO. Methane-positive clients often have a history of constipation-predominant IBS and respond poorly to standard SIBO protocols designed for hydrogen-type.
The most recently characterised SIBO gas type — produced by sulphur-reducing bacteria. Associated with the "rotten egg" flatulence pattern, diarrhoea-predominant symptoms, and significant gut inflammation. H₂S-producing organisms include Desulfovibrio species. Previously invisible on standard breath tests — newer breath test technology now measures H₂S alongside hydrogen and methane, giving a complete picture of all three gas types in a single test.
SIBO doesn't have a pathognomonic symptom — a single finding that confirms the diagnosis without testing. What it has is a cluster of clinical signals that, when several are present together, make SIBO a high-probability differential that warrants breath testing. The most clinically specific signal is one that practitioners and clients rarely consider as diagnostic information.
| Clinical signal | Why it points to SIBO | Specificity |
|---|---|---|
| Probiotics make symptoms worse | Probiotics add bacteria to a gut that already has too many bacteria in the wrong location. In SIBO, adding more organisms to the small intestine — even beneficial ones — feeds the overgrowth and worsens fermentation, bloating, and symptoms. This is the most counterintuitive but most clinically specific SIBO signal. | HIGH |
| Bloating within 90 minutes of eating | Small intestinal fermentation is faster than colonic fermentation — gas production begins almost immediately after food reaches the bacteria in the small intestine. Bloating within 90 minutes of a meal, particularly one containing fermentable carbohydrates, reflects small intestinal rather than colonic fermentation. | HIGH |
| Symptoms worse with fibre and fermentable foods | FODMAPs, fibre, and fermentable carbohydrates are the substrate that SIBO bacteria feed on. Symptom worsening with increased fibre intake, or significant improvement on a low-FODMAP or elemental diet, is consistent with bacterial fermentation in the small intestine rather than colonic dysbiosis. | HIGH |
| Upper rather than lower abdominal bloating | SIBO fermentation occurs in the small intestine, which occupies the central and upper abdomen. Colonic bloating tends to be lower and more lateral. Upper abdominal bloating, distension in the periumbilical region, or the sensation of a "food baby" immediately after eating points to the small intestine as the source. | MODERATE |
| Early satiety | Bacterial overgrowth in the small intestine causes distension and gas production that creates a sensation of fullness disproportionate to the amount eaten. Early satiety — feeling full after small amounts — is a common SIBO symptom, particularly with methane-dominant SIBO where motility is already slowed. | MODERATE |
| Histamine intolerance | Many SIBO-associated bacteria — particularly gram-negative organisms — produce histamine as a metabolic byproduct. Histamine intolerance symptoms (flushing, headaches, skin reactions, palpitations with certain foods) that don't have another clear explanation are associated with SIBO, particularly hydrogen-dominant type. | MODERATE |
| Restless legs syndrome | The association between SIBO and RLS is one of the more surprising clinical findings — small studies have shown high rates of SIBO in RLS patients, and SIBO treatment improved symptoms in some cases. The mechanism may involve iron and folate malabsorption secondary to SIBO-mediated intestinal damage. | MODERATE |
| IBS diagnosis without further investigation | SIBO is estimated to be present in 30–85% of IBS patients in different studies — the range reflects diagnostic methodology differences, but the association is consistent. An IBS diagnosis made on symptom criteria alone, without breath testing, may be missing SIBO as the underlying mechanism for what is labelled a functional disorder. | MODERATE |
| Nutritional deficiencies despite adequate diet | SIBO damages the intestinal epithelium and competes with the host for nutrients — bacteria in the small intestine consume B12, iron, and fat-soluble vitamins before the host can absorb them. Iron-deficiency anaemia, B12 deficiency, or fat-soluble vitamin depletion without a clear dietary explanation warrants investigation of small intestinal absorption capacity. | MODERATE |
These tests are complementary — they investigate different anatomical locations and answer different clinical questions. Understanding what each does and doesn't tell you is the basis for knowing when to order both.
When both small intestinal and colonic investigation is warranted — which is the case for most clients with persistent and unexplained gut symptoms — Nordic Laboratories offer a discounted combination of SIBO breath testing with the GI-MAP plus Zonulin stool test. This is the most clinically complete gut investigation available as a package, covering the entire gastrointestinal tract from a functional perspective.
SIBO breath test accuracy depends heavily on preparation. The dietary preparation period before the test significantly affects both sensitivity and specificity — poor preparation is the most common cause of false negative results and uninterpretable breath curves.
For 24 hours before the test, a restricted preparatory diet eliminates most fermentable carbohydrates. The typical preparatory diet includes only: plain white rice, plain chicken or fish, eggs, clear broth, water, and black coffee or plain tea without milk. This depletes the fermentable substrate available to bacteria in the small intestine — so when the lactulose substrate is introduced during the test, any gas production is more clearly attributable to SIBO rather than background dietary fermentation. Specific preparation instructions are provided with the kit.
Nothing by mouth for at least 8–12 hours before beginning the test, except water. No smoking (affects breath hydrogen), no vigorous exercise the morning of the test (increases breath hydrogen), no antibiotics for at least 4 weeks before testing. If a recent course of antibiotics is unavoidable, the test should be delayed.
A breath test done without the preparatory diet is significantly more likely to produce an uninterpretable result or a false negative. The 24-hour preparation is the part of the process most often skipped or imperfectly followed — and the part that most directly determines whether the test result is clinically useful. The extra 24 hours of dietary restriction is a worthwhile investment in an accurate result.
The SIBO breath test produces a graph — breath hydrogen and methane levels (in parts per million) plotted against time over the three-hour collection period. Interpreting the graph requires understanding the pattern, not just the peak numbers.
The most widely used diagnostic criteria (North American Consensus, 2017) define a positive SIBO breath test as a rise in hydrogen of 20 ppm or more above baseline within the first 90 minutes of the test — the window during which lactulose reaches the small intestine before transit carries it into the colon. For methane, a level of 10 ppm or above at any point in the test is considered positive by most protocols, because methanogens can produce methane even at relatively low numbers.
A breath test that shows minimal gas production throughout — a flat line — requires careful interpretation. It may mean there is genuinely no overgrowth. It may mean preparation was inadequate and bacteria were starved before the test. Or it may mean the client has hydrogen sulphide-dominant SIBO, which produces H₂S rather than H₂ — and H₂S cannot be detected by standard breath tests. H₂S-capable breath testing resolves this third scenario.
A sharp rise in hydrogen or methane after 90–120 minutes typically represents the lactulose substrate reaching the colon — where colonic bacteria ferment it and produce gas. This is a normal finding that confirms gut transit time rather than SIBO. Distinguishing an early small intestinal rise from a late colonic rise is the key interpretive skill — and the clinical context (symptoms, transit history) informs that interpretation alongside the graph.
SIBO treatment is not the same as colonic dysbiosis treatment. The primary interventions are antimicrobial protocols targeting the small intestine (herbal antimicrobials or rifaximin for hydrogen-dominant; neomycin plus rifaximin or specific herbal combinations for methane/IMO), a diet that removes fermentable substrate during treatment (elemental diet, specific carbohydrate diet, or low-FODMAP), and structural work to address the driver of SIBO — most commonly the migrating motor complex, ileocaecal valve function, or underlying motility issues. Treating SIBO without addressing the driver produces relapse. The SIBO breath test identifies the gas type — the gas type guides the protocol.
Book a consultation to discuss your gut history, symptom pattern, and previous test results. We'll determine whether SIBO breath testing, the GI-MAP plus SIBO combination, or a different investigation is the right next step for your specific clinical picture.
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