Small Intestinal Bacterial Overgrowth (SIBO)

Why bacteria overgrow

Several protective mechanisms normally keep bacteria in the small intestine low:

• stomach acid, which kills swallowed bacteria
• the migrating motor complex, waves of gut movement between meals that sweep contents forward
• the ileocecal valve, which stops colon bacteria moving backward
• bile, pancreatic enzymes and the immune system

When these fail, bacteria multiply. They ferment carbohydrates into gas, deconjugate bile acids (causing fat malabsorption), consume vitamin B12 and damage the gut lining.

Causes and risk factors

Slow gut movement

• diabetes with nerve damage to the gut
• scleroderma
• gastroparesis and intestinal pseudo-obstruction
• hypothyroidism
• opioid medicines
• Parkinson's disease

Structural problems

• previous abdominal surgery, especially gastric bypass, bowel resections or removal of the ileocecal valve
• strictures and fistulas from Crohn's disease
• small bowel diverticula
• adhesions

Reduced defenses

• long-term proton pump inhibitor use (evidence is mixed)
• chronic pancreatitis and pancreatic insufficiency
• cirrhosis
• immune deficiencies
• older age

Associations

• irritable bowel syndrome: some people with IBS test positive for SIBO, though the relationship is debated
• celiac disease that does not respond to a gluten-free diet

Symptoms

• bloating and abdominal distention, often soon after eating
• excess gas and belching
• abdominal discomfort or pain
• diarrhea, or constipation in methane-predominant overgrowth
• nausea
• fatigue

In severe cases:

• fatty stools (steatorrhea)
• weight loss and malnutrition
• deficiencies of vitamin B12, iron and fat-soluble vitamins A, D, E and K
• anemia and nerve problems from B12 deficiency

Folate levels may be high because bacteria produce it.

Diagnosis

• breath testing: after drinking glucose or lactulose, hydrogen and methane in breath are measured every 15 to 20 minutes for 2 to 3 hours. A rise in hydrogen within 90 minutes suggests SIBO. High methane suggests intestinal methanogen overgrowth, linked to constipation
• small bowel aspirate and culture during endoscopy, more definitive but invasive and less available
• blood tests: B12, folate, iron studies, vitamin D, albumin
• tests for underlying causes: imaging for strictures, motility testing, diabetes and thyroid tests

Breath tests can give false results with fast gut transit, recent antibiotics or poor preparation, so results are interpreted with symptoms.

Treatment

Antibiotics

• rifaximin, a non-absorbed antibiotic, is the most studied, usually for 1 to 2 weeks
• alternatives include amoxicillin-clavulanate, ciprofloxacin, doxycycline, metronidazole or trimethoprim-sulfamethoxazole
• methane-predominant overgrowth may be treated with rifaximin combined with neomycin
• recurrence is common, and some people need repeated or rotating courses

Treating the cause

• stopping or reducing opioids
• better diabetes control
• prokinetic medicines to improve gut movement
• surgery for strictures or blind loops
• pancreatic enzyme replacement when needed

Nutrition

• replacing vitamin B12, iron and fat-soluble vitamins
• a low-FODMAP diet may reduce symptoms in the short term, ideally with a dietitian
• an elemental diet (a liquid formula of pre-digested nutrients) for a couple of weeks is an option in difficult cases

Probiotics have uncertain benefit in SIBO.

Outlook

Antibiotics improve symptoms in many people, but SIBO often returns unless the underlying cause is addressed. Long-term management focuses on gut motility, nutrition and treating relapses.

Frequently asked questions

What is SIBO?

SIBO is a condition in which too many bacteria grow in the small intestine. They ferment food early, causing bloating, gas, abdominal discomfort and diarrhea, and can interfere with absorption of nutrients such as vitamin B12, iron and fats.

What causes SIBO?

SIBO usually develops when the gut's normal defenses fail: slow gut movement from diabetes, scleroderma or opioids, previous bowel surgery, strictures or diverticula, low stomach acid, or pancreatic insufficiency. Identifying the cause helps prevent recurrence.

How is SIBO diagnosed?

The usual test is a breath test after drinking glucose or lactulose, measuring hydrogen and methane gases produced by bacteria. A sample of small bowel fluid taken during endoscopy can also be cultured. Blood tests check for nutrient deficiencies.

How is SIBO treated?

Treatment is usually a 1 to 2 week course of antibiotics such as rifaximin, plus treating the underlying cause and correcting nutritional deficiencies. Diet changes, such as a short-term low-FODMAP diet, can help symptoms. Repeat courses may be needed.

Can SIBO come back?

Yes. SIBO often recurs, especially when the underlying cause, such as slow gut motility or anatomical changes after surgery, persists. Treating the cause, using prokinetic medicines and periodic retreatment help manage recurrences.