SIBO: definition, symptoms, and how to actually test for it

Definition

SIBO (small intestinal bacterial overgrowth) is a condition in which bacteria proliferate in the small intestine in numbers above what’s normally present there. The small intestine is normally a low-bacterial environment compared to the colon — SIBO disrupts that balance.

When bacteria in the small intestine ferment carbohydrates from food before they can be absorbed, they produce hydrogen, methane, and (in some variants) hydrogen sulfide. These gases produce the symptoms of SIBO.

Subtypes

By dominant gas:

• Hydrogen-dominant SIBO — typical bacterial overgrowth; often presents with diarrhea, bloating, abdominal pain
• Methane-dominant SIBO (IMO — intestinal methanogen overgrowth) — driven by methanogenic archaea; typically presents with constipation, bloating
• Hydrogen sulfide SIBO — rarer, recently testable; “rotten egg” gas; typically presents with diarrhea

Symptoms

The classic SIBO presentation:

• Bloating within 30-90 minutes of eating (faster onset than typical IBS-spectrum bloating)
• Carbohydrate-triggered symptoms (worse with grains, fruit, beans, sweetened foods)
• Excess gas
• Abdominal pain or cramping
• Altered bowel habits (diarrhea, constipation, or mixed depending on subtype)
• Sometimes nausea, fatigue, or brain fog

The early-onset bloating after eating is the most distinguishing feature — it’s bacteria fermenting carbs in the small intestine where the food is, rather than waiting for normal colonic transit.

Causes

SIBO usually develops from:

• Impaired small intestinal motility — most common driver; can result from past food poisoning, hypothyroidism, diabetes, certain medications
• Anatomical abnormalities — diverticula, strictures, surgical changes
• Low stomach acid — chronic PPI use reduces the acid barrier that limits bacterial migration
• Immunodeficiency — IgA deficiency, immunosuppression
• Pancreatic insufficiency

The motility piece matters because it’s the most common preventable driver — and the most common reason SIBO recurs after treatment.

Diagnosis

The hydrogen/methane breath test is the standard non-invasive diagnostic [^2]:

1. 8-12 hour fast; specific dietary prep day before
2. Baseline breath sample
3. Drink substrate (typically 10 g lactulose or 75 g glucose)
4. Breath samples every 15-20 minutes for 2-3 hours
5. Lab measures hydrogen and methane (ppm)

Positive criteria:

• Hydrogen rises ≥20 ppm above baseline within 90 minutes → SIBO
• Methane ≥10 ppm at any point → IMO

The breath test isn’t perfect — false negatives occur with rapid transit or hydrogen-sulfide variants — but it’s the best available non-invasive test. For deeper walkthrough see our SIBO test article.

Treatment

The three main paths:

Antibiotics:

• Rifaximin 550 mg three times daily for 14 days — first-line for hydrogen-dominant
• Rifaximin + neomycin (or metronidazole) — for methane-dominant
• Response rates around 60-70% [^1]

Herbal antimicrobials:

• Berberine, oregano oil, neem, allicin at standardized doses
• 4-8 week courses
• Some studies show comparable efficacy to rifaximin

Dietary support:

• Low-FODMAP diet during and after treatment reduces fermentable substrate
• Important: dietary changes alone don’t fix SIBO — they manage symptoms while bacteria are addressed

Recurrence

SIBO recurrence is common — 30-50% within a year of treatment. The reason is usually the underlying motility issue that allowed bacteria to migrate in the first place wasn’t addressed. Prevention strategies:

• Prokinetic medications or supplements to maintain small bowel motility
• Meal spacing (allowing the migrating motor complex to “sweep” between meals)
• Address underlying conditions (thyroid, blood sugar, gut motility issues)
• Gentle stress and vagal nerve support

Related entries

• FODMAP
• Visceral fat