On Nutrition

With increasing interest in the gut microbiota and its role in health and disease, small intestinal bacterial overgrowth, or SIBO, is also getting more attention. And, like many things related to gut health, there’s both reliable and not-so-reliable information to be found online. Here’s what you need to know.

First, SIBO is the presence of excess bacteria in the small intestine. While it’s normal to have a large, robust population of bacteria and other microorganisms — the gut microbiota — in your large intestine, or colon, your small intestine is relatively sterile. So if you do have a lot of bacteria in there, and they’re fermenting the food you eat, problems ensue. The most common symptoms are bloating, abdominal discomfort or pain, gas and diarrhea. Many people with SIBO describe themselves as looking fine in the morning, then six months pregnant by the afternoon. Other symptoms include early satiety, nausea, fatigue or “brain fog” after meals, unintentional weight changes and constipation.

What can be confusing is that SIBO doesn’t just happen on its own — it’s a secondary diagnosis. In other words, people who have SIBO developed it because of another health issue, usually one that interferes with gut motility. Normally, a synchronized, wavelike contraction of the muscles in the gastrointestinal tract moves food and bacteria along to the colon. You could think of this process as a digestive housekeeper that operates between meals. However, in people with SIBO, these contractions may be weak or even absent, allowing bacteria to set up shop.

So what can interfere with gut motility? Conditions such as diabetes that cause peripheral neuropathy — damage to nerves located outside the brain and spinal cord. Some medications, such as opioids or anti-diarrheal medications. Previous surgeries involving the intestines. Celiac disease or Crohn’s disease — autoimmune conditions that cause inflammation — can increase SIBO risk. Because stomach acid helps prevent overgrowth of bacteria in the upper part of the small intestine, medications that treat acid reflux or stomach ulcers can also increase SIBO risk, too.  

Unfortunately, it’s hard to know how many people have SIBO, in part because it’s underdiagnosed. Some people with SIBO may be asymptomatic, others are misdiagnosed. I’ve had clients who were diagnosed with irritable bowel syndrome but had symptoms that made me suspect SIBO, usually constant bloating and early satiety. While exact estimates vary, rates of SIBO among people who have IBS are much higher than in the general population.

How SIBO is diagnosed

The gold standard diagnostic method is to gather a sample of fluid from the lower part of the small intestine via an endoscope, but there are pros and cons to this, with one of the cons being that it’s invasive. Breath testing — which involves drinking a special sugar solution then breathing into a device that can measure levels of hydrogen and methane — is more common but also has pros and cons. The pros are that it’s noninvasive and can provide clues to how much bacterial fermentation is happening in the small intestine — again, where we don’t want fermentation to be happening — and what type of bacteria is responsible. This can help tailor type of antibiotic treatment and predict response to treatment. The major con is that false negatives — the test results say someone doesn’t have SIBO but they really do — are not uncommon.

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It’s important not to try to self-diagnose SIBO, because many gastrointestinal conditions have shared symptoms, and some of those conditions are serious or even life-threatening, while others are not. On the flip side, sometimes people who suspect they have SIBO have to advocate for themselves. I’m thinking of one IBS client with symptoms suspicious for SIBO who had to push for breath testing, and did test positive for excess methane. Experts I’ve talked to said that SIBO awareness among gastroenterologists is improving, in part due to recent guidelines published by the American College of Gastroenterology, as well as North American consensus guidelines, both of which provide guidance on diagnosing SIBO.

How SIBO is treated

Treating SIBO starts with addressing the underlying cause and modifying it if possible. The next step is typically treatment with antibiotics. Research has found that antibiotics are more effective than placebos, in terms of reducing the number of bacteria and improving symptoms. Because antibiotics don’t address the underlying cause, relapse is common, so additional rounds of treatment with antibiotics may be necessary. Probiotics are not recommended, as there is little evidence that they help, and some evidence that they can make things worse.

What about diet, you ask? The bad news is that there is no “SIBO diet,” and anyone who says differently is misinformed. There isn’t much research on the role of what we eat in preventing or helping to treat SIBO, but what little there is — together with some anecdotal “evidence” — suggests that diet may play more of a role in helping prevent SIBO from recurring after it’s been treated with antibiotics.

Common strategies to help maintain remission include spacing meals more widely apart or taking medications to improve gut motility, although research in this area isn’t robust. The low-FODMAP diet, which is used primarily to reduce IBS symptoms, might also help with SIBO because it reduces fermentable carbohydrates, but the research on this is weak. Other diets that claim to help with SIBO generally also restrict fermentable carbohydrates. Because of the restrictive nature of any diet that might or might not help treat or prevent SIBO, these diets are not recommended for anyone with a history of eating disorders, for pregnant or breastfeeding women, for people who struggle with getting enough nutrients due to poor appetite or other reasons, or for people who would find following detailed dietary instructions challenging.