What is SIBO?

First, a quick tour of your gut

Your digestive system is home to trillions of bacteria. This is completely normal and actually vital for your health. But where those bacteria live matters just as much as how many there are.

Think of your gut as having two main sections. The small intestine is the long, winding tube where most of your food gets broken down and absorbed into your body. The large intestine (your colon) comes after it — and that is where the bulk of your gut bacteria are supposed to live.

The small intestine is not meant to be densely populated with bacteria. It has its own system for keeping numbers low: stomach acid kills many bacteria before they get there, and wave-like muscle contractions regularly sweep the small intestine clean between meals.

SIBO happens when that system breaks down.

So what exactly is SIBO?

SIBO stands for Small Intestinal Bacterial Overgrowth. It means that too many bacteria — or the wrong types — have taken up residence in the small intestine. Once there, they start doing what bacteria do: fermenting the food passing through. That fermentation produces gas and disrupts digestion in ways that can make you feel quite unwell (Pimentel et al., 2020).

The symptoms most people experience include bloating (often worse after eating), abdominal pain or cramping, excess gas, diarrhea, or sometimes constipation. In people who have had SIBO for a long time without treatment, deficiencies in nutrients like vitamin B12, vitamin D, and iron can develop, because the bacteria interfere with absorption. Some people also experience fatigue and brain fog alongside the gut symptoms.

There is also a closely related condition called intestinal methanogen overgrowth (IMO), where instead of excess bacteria, there is an overgrowth of a different type of microorganism called archaea. IMO tends to cause constipation more than diarrhea, and is now recognised as a separate but related condition that needs its own approach.

How common is SIBO?

More common than doctors once thought — though the exact number is hard to pin down, because different tests give different results.

Among people who already have gut symptoms and get tested, roughly one in three test positive for SIBO (Efremova et al., 2023). In the general population, estimates range from about 3% to 22% — a wide range that reflects differences in how studies were designed and how SIBO was tested for, not uncertainty about whether the condition is real.

What is particularly striking is the overlap with irritable bowel syndrome (IBS). A large meta-analysis — a study that pooled the results of 25 smaller studies covering over 6,500 patients — found that people diagnosed with IBS are roughly four times more likely to test positive for SIBO than people without gut symptoms (Shah et al., 2020). This does not mean SIBO and IBS are the same thing, but it does suggest that in many people labelled with IBS, SIBO may be contributing to or driving their symptoms.

SIBO is also significantly more common in people with inflammatory bowel disease: around one in three IBD patients test positive (Shah et al., 2019), and in people with diabetes, approximately one in three also test positive — largely because diabetes can damage the nerves controlling gut movement (Feng & Li, 2022).

Why does SIBO develop?

There are several well-researched reasons why the small intestine can lose its ability to keep bacterial numbers in check.

Slow or disrupted gut movement

Your gut is supposed to move in regular, coordinated waves — not just when you are eating, but between meals too. These sweeping contractions act like a self-cleaning mechanism, pushing bacteria along and stopping them from settling. When gut movement slows down or becomes disorganised, bacteria have time to build up. This is why SIBO is much more common in people with conditions like diabetes, Parkinson's disease, scleroderma, and hypothyroidism, and in people who take opioid painkillers regularly. A study of patients with unexplained digestive symptoms found that impaired gut motility was one of the two strongest independent risk factors for developing SIBO (Jacobs et al., 2013).

Acid-suppressing medications

Stomach acid plays an important protective role: it kills most of the bacteria you swallow before they can travel further into your digestive system.

Proton pump inhibitors, or PPIs, are medications commonly prescribed for acid reflux and heartburn (omeprazole, lansoprazole, pantoprazole). They work well for their intended purpose, but multiple large studies have found that people who take them regularly are roughly twice as likely to develop SIBO (Lo & Chan, 2013; Su et al., 2018). More recent research also suggests that the longer you take them, the greater the risk (Maslennikov et al., 2025).

Structural changes in the gut

The shape and layout of the digestive tract matters. Narrowings, pouches, or scar tissue from previous surgery can create areas where bacteria pool and multiply instead of moving along. The ileocecal valve — the small valve separating the small and large intestine — is another key structure; when it has been removed or damaged, bacteria from the colon can more easily migrate upward.

Inflammatory bowel disease

People with Crohn's disease or ulcerative colitis face considerably higher risk of SIBO, and the relationship seems to run in both directions: the inflammation and structural changes caused by IBD make SIBO more likely, while SIBO may in turn worsen symptoms and inflammation. A 2024 meta-analysis found that IBD patients are over five times more likely to have SIBO than healthy controls (Feng et al, 2025).

Why does any of this matter?

SIBO is a condition that is frequently missed, or mistaken for something else. Its symptoms look a lot like IBS, food intolerances, and other common digestive diagnoses. Many people spend years being told they have a 'sensitive gut' or that their symptoms are stress-related, when an underlying bacterial imbalance in the small intestine may be playing a significant role.

No two people with SIBO are exactly the same — and understanding what caused it in you is often the key to knowing how to address it. A one-size-fits-all approach rarely works here.

Knowing what SIBO is, how common it is, and what puts people at risk is the first step to asking the right questions — and getting the right answers.

In the next articles in this series, we will look at how SIBO is tested for and what treatment looks like — because the path forward is different for everyone.