SIBO: when gut bacteria set up camp in the wrong neighborhood
Last updated September 3, 2026.
SIBO (small intestinal bacterial overgrowth) is what happens when bacteria that belong in the colon migrate up and colonize the small intestine, fermenting your food before you absorb it: bloating, gas, pain, and changed bowels that feel like IBS but have a findable, treatable mechanism. It hides behind an IBS label in a meaningful share of cases, and the key to beating it is treating both the bacteria and the reason they moved in.
What does it feel like?
Bloating that builds through the day and after meals (the tight-waistband-by-evening pattern), excessive gas and belching, upper abdominal discomfort, and bowel change: diarrhea, constipation (when the bugs produce methane), or an alternation of both. Because the bacteria eat first, there can be nutrient theft: B12 deficiency, iron deficiency, fat malabsorption (pale, floating, hard-to-flush stools), and weight loss in established cases. The overlap with IBS is so complete that SIBO is best thought of as a findable cause of some IBS.
Why does it happen?
The small bowel normally stays relatively bacteria-free thanks to its cleansing waves, stomach acid, and valves; SIBO follows when those defenses fail: gut-slowing conditions and drugs (diabetes, hypothyroidism, opioids, some IBS medicines), past gut surgery or structural blind spots, low stomach acid (long-term PPIs), previous food poisoning (post-infectious nerve damage is a recognized trigger of both SIBO and IBS), and conditions like celiac, Crohn's, and scleroderma. Age and repeated antibiotics play supporting roles.
How is it diagnosed and treated?
- The breath test: the standard: drink a sugar solution (glucose or lactulose) and breathe into tubes over two to three hours; the bacteria's hydrogen and methane show up on the curve before your own digestion could produce them.
- Antibiotics that stay in the gut: rifaximin is the usual first choice (poorly absorbed, targeted), with metronidazole or neomycin combinations for the methane type; courses typically run two weeks.
- Fix the driver: the make-or-break step: review gut-slowing medications, manage diabetes or thyroid disease, address surgical anatomy; without this, SIBO recurs in a large share of cases.
- Correct the deficits: B12, iron, vitamin D, and fat-soluble vitamins are checked and replaced.
- Diet as support: a short low-FODMAP stretch calms symptoms and some practitioners use specific carbohydrate approaches; diet alone rarely eradicates, and prolonged restriction is its own harm.
When is it an emergency?
SIBO is outpatient medicine. The symptoms around it that are not: unintentional weight loss, blood in the stool, persistent vomiting, severe worsening abdominal pain, fever, or anemia (these bypass the SIBO hypothesis and get investigated: celiac, IBD, and cancer screening questions), and dehydration from relentless diarrhea. Also: bloating with a new, fixed, one-sided abdominal mass or a new change in bowel habit over 50 deserves proper assessment rather than a breath test first. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Is SIBO just IBS with a fancier name?
The relationship is genuinely interesting: SIBO is a measurable mechanism (too many bacteria in the small bowel, proven on a breath test), while IBS is a symptom pattern, and studies find SIBO in a substantial minority of people labeled with IBS, especially the post-food-poisoning subtype. So for some IBS patients, SIBO is the findable, treatable cause underneath the label, and treating it improves them. But the overlap is not total: many IBS patients test negative, and breath tests have imperfect accuracy, so a negative test does not erase symptoms and a positive one does not explain everything. The useful frame: SIBO is a hypothesis worth testing in the right story, not a rebrand.
What is the breath test like, and how accurate is it?
Simple to take, tricky to interpret: after a restricted diet the day before and an overnight fast, you drink a sugar solution (glucose or lactulose) and breathe into collection tubes every 15-30 minutes for two to three hours; bacteria fermenting in your small bowel release hydrogen and methane earlier than colonic fermentation would, and the curve makes the diagnosis. The honest caveats: the test has meaningful false positives and negatives (transit speed varies, preparation matters, and methane producers can be missed by hydrogen-only protocols), so results are read alongside the story, and a typical post-infectious story with a positive test is a much stronger signal than either alone.
Does rifaximin cure it?
It clears the overgrowth in a good share of cases (trials show roughly 50-70% response in IBS-with-SIBO populations), often within the two-week course, with the methane-predominant type needing the neomycin combination for similar odds. The catch is recurrence: if the reason the bacteria moved in (slow motility, surgery, acid suppression) is still running, SIBO returns in a substantial minority within months, which is why the driver-fixing step is not optional garnish. Some patients need repeat or rotating courses, and some motility problems get prokinetic medication as prevention. The antibiotic is the reset button; the underlying work is what makes the reset stick.
Can diet fix SIBO without antibiotics?
Diet manages symptoms impressively and eradicates the overgrowth rarely: low-FODMAP eating removes the fermentable fuel the bacteria feast on, and most people feel the bloating ease within weeks, which is genuinely useful while testing and treatment are arranged. But starving the bacteria is not removing them, and the diet has its own costs (it is restrictive, it alters the gut microbiome with prolonged use, and it can feed disordered eating), so it is a tool, not the treatment. The sensible sequence: test, treat with antibiotics if positive, fix the driver, use short low-FODMAP stretches for symptom control, then re-expand the diet as far as tolerated.
Why did my food poisoning cause a year of gut problems?
There is a recognized mechanism: certain food-poisoning bacteria release a toxin that damages the gut's cleaning-wave nerves, and the body then makes antibodies that cross-react with a protein in those nerves (the anti-vinculin story). The cleansing waves that sweep bacteria out of the small bowel weaken, bacteria accumulate, and SIBO follows: this is the post-infectious pathway behind a meaningful share of both SIBO and IBS cases, and the abrupt-after-gastroenteritis onset is its signature. The hopeful part: the nerve injury often recovers slowly, and treating the overgrowth plus supporting motility while it does is exactly the plan your story calls for.
What deficiencies should I be checked for after a year of this?
The bacteria eating first means you absorbing second, and the pattern is specific: B12 (they consume it: deficiency causes anemia and nerve symptoms), iron (upper-gut inflammation impairs its uptake), fat-soluble vitamins A, D, E, K (bacterial interference with bile salts causes fat malabsorption: the pale floating stools clue), and sometimes folate paradoxically high (some bacteria manufacture it). Worth a blood panel after a symptomatic year, alongside the celiac screen that belongs in any malabsorption workup. Replacing the deficits treats the fatigue and fog that SIBO patients often carry, and rechecking them after eradication confirms the gut is absorbing properly again.