Symptoms and conditions
What is SIBO? Small intestinal bacterial overgrowth explained
SIBO, or small intestinal bacterial overgrowth, means that too many bacteria are living in the small intestine, where there should be few. They ferment food before it is absorbed, causing bloating, gas, abdominal discomfort and loose stools. It is diagnosed with a breath test and treated with a course of antibiotics, together with attention to why it developed.
In short
- The small intestine normally holds few bacteria. In SIBO they multiply there.
- The main symptoms are bloating, excess gas and loose stools. An excess of methane-producing microbes is linked with constipation.
- SIBO is almost always the result of something else: slow gut movement, a structural change or low stomach acid.
- Antibiotics clear it. It often returns unless the underlying reason is addressed.
What are the symptoms?
- Bloating and visible distension, worse after meals
- Excess gas and belching
- Abdominal discomfort or cramps
- Loose stools, or constipation when methane is high
- In severe, long-standing cases: weight loss, oily stools, and low vitamin B12 or iron
These overlap with IBS and with food intolerances, which is why a test is useful.
Why do bacteria overgrow?
The small intestine is kept clear by stomach acid, bile, steady forward movement, and a valve between it and the colon. SIBO develops when one of these fails.
Slow movement
Diabetes, an underactive thyroid, strong painkillers and other medicines, and nerve or muscle disorders of the gut.
Structural changes
Previous abdominal surgery, narrowed segments, pouches in the small intestine, and bands of scar tissue.
Low stomach acid
Long-term acid-blockers and surgery on the stomach.
Other conditions
Long-standing disease of the pancreas, cirrhosis, coeliac disease, Crohn’s disease and lowered immunity.
How is SIBO diagnosed?
By a hydrogen and methane breath test. You drink a sugar solution, and breath samples are collected every 15 to 20 minutes for two to three hours.
- A rise in hydrogen of 20 parts per million or more within 90 minutes suggests bacterial overgrowth
- Methane of 10 parts per million or more indicates an excess of methane-producing microbes
The result is read together with your symptoms and history. Correct preparation, including a day of plain food and an overnight fast, is essential.
How is it treated?
A course of antibiotics
A gut-specific antibiotic for one to two weeks is the main treatment. A combination is used when methane is high.
Treat the reason
Blood sugar and thyroid function are corrected, medicines that slow the gut or suppress acid are reviewed, and any structural cause is assessed.
Restore the housekeeping wave
During fasting, a cleaning wave sweeps the small intestine every 90 minutes to two hours. Leaving three to four hours between meals, and not eating late at night, lets it work. A medicine that promotes gut movement is added for some people.
Adjust the diet
Reducing highly fermentable foods for a few weeks eases symptoms. It does not clear the overgrowth, and a very restrictive diet over the long term is not advised.
Correct deficiencies
Vitamin B12, iron and vitamin D are checked and replaced where low.
Does SIBO come back?
Often, yes, particularly when the underlying cause cannot be removed. This is why I spend as much effort on the cause as on the antibiotic. If symptoms return, the test is repeated before another course is given.
When should I see a doctor?
See a doctor for bloating or loose stools lasting more than four weeks, and promptly for weight loss, blood in the stool, fever, night-time symptoms or anaemia. These are not explained by SIBO alone.
Questions patients ask me
Is SIBO the same as IBS?
No, though they overlap. A proportion of people with IBS symptoms have a positive breath test, and their symptoms improve when the overgrowth is treated. Many people with IBS do not have SIBO.
Can SIBO be treated with diet or herbs alone?
Diet reduces symptoms but does not clear the bacteria. The evidence for herbal preparations is limited. Antibiotics are the treatment with the most evidence.
Should I take probiotics for SIBO?
The evidence is mixed, and some people feel more bloated with them. I do not advise them routinely for this condition.
Do acid-blockers cause SIBO?
Long-term use lowers stomach acid, one of the barriers against overgrowth, and is linked with a higher risk. That is one reason to take them only when there is a clear need.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- American College of Gastroenterology guideline on small intestinal bacterial overgrowth. Pimentel M et al. Am J Gastroenterol. 2020. PubMed 32023228
- North American consensus on hydrogen and methane breath testing. Rezaie A et al. Am J Gastroenterol. 2017. PubMed 28323273
- American College of Gastroenterology guideline on irritable bowel syndrome. Lacy BE et al. Am J Gastroenterol. 2021. doi:10.14309/ajg.0000000000001036