What is SIBO?

What SIBO actually is

Your large bowel is home to an enormous community of bacteria. Your small intestine is meant to be much quieter. In a healthy small intestine the count rarely goes above about 1,000 bacteria per millilitre of fluid.

That quiet is not an accident. A handful of defences keep it that way, and SIBO is what happens when they stop doing their job.

None of this is new. Some of the very first descriptions came from people who developed low vitamin B12 after stomach surgery left a loop of bowel where bacteria could settle. It was called blind loop syndrome then.

What has changed is how often it gets named. A 2025 review in a gastroenterology journal argued that relying on breath tests, particularly the lactulose version, has led to SIBO being overdiagnosed.

What causes SIBO

SIBO develops when one of the defences that normally keep bacteria out of the small intestine stops working, and in many people more than one is involved. These are the five, in the order food meets them, and what can weaken each.

StatPearls, the clinical reference, also lists liver cirrhosis and kidney disease. SIBO is reported more often in women and in older adults, although how common it really is remains unknown.

If a medicine you take is on that list, that is a conversation for the doctor who prescribed it. Please do not stop or change a prescribed medicine on the strength of a web page, including this one.

Symptoms of SIBO

The classic symptoms are:

• abdominal discomfort
• bloating
• wind
• ongoing loose or watery stools

When overgrowth is more severe, particularly after bowel surgery, poor fat absorption can cause weight loss and low levels of vitamins A, D and K. Low B12 is typical too, often alongside a folate level that reads normal or even high.

Methane tells a different story. Methane in the gut is made by a separate group of microbes called methanogens, which are archaea rather than bacteria, and the pattern is now often called intestinal methanogen overgrowth, or IMO. Most of the research on it looks at a possible link with constipation.

Here is the part that matters most. Not one of those symptoms belongs to SIBO alone. Healthdirect lists irritable bowel syndrome, constipation, coeliac disease and food intolerances as the regular causes of ongoing bloating, and SIBO is not on that list. I have written separately about how to tell SIBO from ordinary bloating.

How SIBO is investigated

The test most people meet is a breath test. You drink a sugar solution and collect breath samples at set intervals, and for SIBO the North American Consensus says two hours is enough. Bacteria fermenting that sugar make hydrogen, methanogens make methane, and some of both leaves the body in your breath.

Under that consensus, a rise in hydrogen of 20 parts per million or more within 90 minutes counts as positive, and methane of 10 parts per million or more counts as methane-positive. The consensus doses are 10 grams of lactulose or 75 grams of glucose.

The other method draws fluid from the small intestine during an endoscopy and cultures it. The same consensus panel agreed that current culture techniques are not satisfactory for assessing SIBO, which means there is no settled gold standard to check a breath test against.

That uncertainty shows in the numbers. When 139 people had both a glucose breath test and a culture, the breath test picked up 42% of the culture positives and correctly cleared 84% of the negatives. Which sugar is used changes the answer too: pooled across 25 studies, 62.3% of people with IBS symptoms tested positive on the lactulose breath test, and so did 33.5% of people without symptoms, a gap that was not statistically significant.

Australian guidance lands in the same place. In Australian Prescriber in June 2026, the authors noted that overgrowth occurs in a subset of people with irritable bowel syndrome, but that routine testing for it is not of clinical value. When irritable bowel syndrome is being considered, the same article names three simple screening tests: a full blood count, C-reactive protein and coeliac serology.

We do offer an at-home lactulose breath test, and I would rather you read all of this before buying one than after. A breath test earns its place when the ordinary explanations have been looked at and the answer would change what happens next. Our guide to how the breath test works covers the report itself.

If you do test, the consensus recommends no antibiotics for four weeks beforehand, and says it is not necessary to stop acid-suppressing medicines called proton pump inhibitors. Your prescriber has the final say on any medicine.

What happens after a positive result

Treating SIBO is a medical decision, made with your GP or gastroenterologist. In medical practice the main treatment is a course of antibiotics, and rifaximin is the most studied of them.

It often comes back. Around 45% of people have SIBO return after finishing antibiotics, more often in older adults and in people on long-term acid-suppressing medicines. Dealing with the underlying cause is what lowers the chance of it returning, which is why the useful question after a result is less “how do I clear it” and more “why was it there”.

Two more things are worth knowing. The evidence does not show that probiotics or a low FODMAP diet resolve SIBO, although some people do feel better on them. And an elemental diet, a liquid nutrition formula, is sometimes used when antibiotics are not tolerated or have not worked, but it is expensive and hard to stick to.

Wellbeing George does not diagnose SIBO and does not treat it. Where a clinical nutritionist fits is alongside that medical care, not instead of it: looking at the gaps SIBO can leave, such as iron, B12 and the fat-soluble vitamins, using bloods your GP orders; working out which foods are worth keeping while symptoms are loud, without cutting your diet down for months; and piecing together the history that explains why those defences slipped. Our page on how we work with bloating and gut symptoms lays out the rest.