Coaching / Tools & guides / SIBO: small intestinal bacterial overgrowth

Guide · by Alexandra Lasconi

SIBO: small intestinal bacterial overgrowth

Your small intestine normally holds relatively few bacteria; most live in the large intestine. In SIBO, too many bacteria grow in the small intestine, ferment food before you can absorb it, and cause bloating, gas, pain and changes in your stool, sometimes with deficiencies. SIBO is usually a sign that something else is not working (the gut's cleaning waves, stomach acid, or the anatomy after surgery), so lasting relief comes from treating the overgrowth and the reason behind it.

Diagnosis and antibiotics belong with your doctor. See a doctor promptly for weight loss without trying, blood in your stool, anaemia, fever, vomiting or symptoms that wake you at night: these need other causes ruled out first.

What it is

Symptoms, and the two types: hydrogen and methane.

Who gets it

The conditions and medicines that raise the risk.

Testing

Breath tests: how they work and how to prepare.

Treatment and food

What works, what to expect, and how to eat.

01 · What SIBO is

Common symptoms

Bloating, the most common symptom, often worse as the day goes on.
Gas, belching, abdominal discomfort.
Diarrhoea, or constipation when methane-producing microbes dominate.
Fatigue, and in more severe cases weight loss and deficiencies (B12, iron, fat-soluble vitamins).

No single symptom is specific to SIBO; the same picture fits IBS, coeliac disease, lactose intolerance and others.

Hydrogen or methane

Hydrogen-type SIBO: bacteria ferment carbohydrates into hydrogen; often linked to diarrhoea.
Methane (intestinal methanogen overgrowth, IMO): archaea, mainly Methanobrevibacter smithii, turn hydrogen into methane, which slows the gut; strongly linked to constipation (about 3.5 times the odds).
The type matters because testing and treatment differ.

02 · Who gets SIBO

Three defences normally keep bacteria out of the small intestine: stomach acid, the "migrating motor complex" (cleaning waves that sweep the small intestine between meals), and the valve between the small and large intestine. SIBO develops when one of them fails.

Conditions linked to SIBO

Irritable bowel syndrome: in studies, a large share of people with IBS test positive; cause or effect is still debated.
Slow gut movement: diabetes, scleroderma, hypothyroidism, intestinal pseudo-obstruction.
Bowel surgery: removal of the ileocaecal valve, gastric bypass, gastrectomy, blind loops.
Crohn's disease, coeliac disease, cirrhosis, pancreatic insufficiency, small-bowel diverticula, some immune deficiencies.
Older age; SIBO is also reported more often in women.

Medicines

Proton pump inhibitors (omeprazole and similar): most studies link them to a higher risk, up to about three times; testing people without symptoms is not recommended. Never stop a prescribed PPI without your doctor.
Opioids and other medicines that slow the gut.

03 · Testing

The most used test is a breath test: you drink a sugar solution (75 g glucose or 10 g lactulose) and breathe into bags every 15 to 20 minutes for 2 to 3 hours. Bacteria that ferment the sugar early produce gases you breathe out.

Result	Positive when
Hydrogen (SIBO)	A rise of 20 ppm or more above baseline within 90 minutes.
Methane (IMO)	10 ppm or more at any point during the test.
Small-bowel aspirate (in endoscopy)	10³ or more bacteria (colony-forming units) per ml.

How to prepare. No antibiotics for 4 weeks before; no laxatives or prokinetics for 1 week, if your doctor agrees; a low-fermentation dinner the day before (plain rice, eggs, meat or fish, no beans, fibre or sweeteners); fast 8 to 12 hours; no smoking or exercise on the morning. Breath tests can give false results either way (lactulose can read early when the gut is fast; glucose can miss overgrowth further down), so your doctor reads them together with your symptoms.

04 · Treatment and food

Medical treatment

Antibiotics are the evidence-based treatment, prescribed by your doctor. Rifaximin, which stays mostly in the gut, cleared SIBO in about 7 in 10 people in a meta-analysis; methane overgrowth may need a combination.
Treat the cause: a slow gut, a medicine, blood sugar in diabetes, an anatomical problem. This is the main way to prevent it coming back.
SIBO often returns: in one study in about 13% of people at 3 months, 28% at 6 months and 44% at 9 months. A plan after treatment matters.

Eating with SIBO

Short-term, lower-fermentation eating can ease symptoms: fewer fermentable carbohydrates (low-FODMAP), sugar alcohols (sorbitol, xylitol), inulin and sucralose. The evidence is low and mostly borrowed from IBS, so use it for weeks, not months.
Reintroduce foods one at a time once symptoms settle: long restriction starves the healthy bacteria in the large intestine.
Space your meals 3 to 5 hours apart, without constant snacking: the gut's cleaning waves run only between meals. Simple and sensible, though not tested in trials.
Protein at every meal and enough calories, especially if you have lost weight.
Probiotics: no consistent evidence for SIBO. A gluten-free diet does not help SIBO unless you have coeliac disease.

Deficiency to check	Why
Vitamin B12	Bacteria in the small intestine use it up before you absorb it.
Iron and ferritin	Inflammation and poorer absorption.
Vitamins A, D and E	Bacteria break down bile acids, so fat and fat-soluble vitamins are absorbed less well.
Folate	Often normal or high: some bacteria make it. A high folate with a low B12 is a typical SIBO pattern.

05 · The plan

1. Rule out the restCoeliac disease, IBD, lactose intolerance, thyroid: with your doctor.

2. TestA breath test if your doctor suspects SIBO; B12, iron, vitamin D, folate.

3. Treat and find the causeAntibiotics as prescribed; address the gut speed, the medicine or the anatomy behind it.

4. RebuildA few weeks of lower-fermentation eating, then reintroduce, with the Gut rebuild guide.

Living with Crohn's or colitis? Read Crohn's disease and ulcerative colitis: what you can influence.

Sources

This page is nutrition coaching information, not medical advice. Your doctor diagnoses and treats SIBO; talk to them before changing medication, starting supplements or following a restrictive diet.

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