Abdominal pain. Bloating. Diarrhea.

These are classic symptoms that can suggest a number of diagnoses. One that’s getting increasing attention — notably among the public, but also among physicians — is small intestinal bacterial overgrowth, or SIBO.

The condition “has become in recent years a progressively credible explanation for a variety of gastrointestinal symptoms,” according to a 2018 article by Nitin K. Ahuja and his sister, Amisha Ahuja, in the journal Practical Gastroenterology.

Amisha Ahuja is a gastroenterology fellow at Temple University Hospital. Nitin Ahuja is a gastroenterologist and an assistant professor of clinical medicine at the University of Pennsylvania’s Perelman School of Medicine.

Nitin Ahuja is a gastroenterologist and an assistant professor of Clinical Medicine at the University of Pennsylvania's Perelman School of Medicine.
Courtesy of Penn Medicine
Nitin Ahuja is a gastroenterologist and an assistant professor of Clinical Medicine at the University of Pennsylvania's Perelman School of Medicine.

We spoke with him recently about SIBO.

What is SIBO?

It’s complicated, in that there’s a conventional understanding of how SIBO occurs and manifests, and some atypical presentations. I tend to think of it as a circle with a very clear center and some very hazy edges. A lot of people with otherwise ill-defined GI symptoms can find themselves at these hazy edges, wondering whether they have SIBO.

Everyone has bacteria in their small intestine. That’s normal. But when the bacteria exceed a certain concentration, they begin to outcompete the gut mucosa for the nutrients in the food we eat. If the bacteria can metabolize your food before you can, they are liable to create all sorts of metabolic by-products that are poorly digested by the body. So, small molecules that don’t get absorbed can draw water into the bowels. Gaseous by-products can lead to bloating and distension. That maldigestion can potentially lead to the hallmark symptoms — abdominal pain, bloating, and diarrhea.

There are many reasons bacteria in the small intestine might overgrow. If the small intestine is slowed by background disease or medications, or if there is scar tissue outside the intestinal wall, disruptions in motility can create pockets for overgrowth. If there are diverticula in the small intestine, bacteria can pool there, too.

How is it diagnosed?

Most commonly by a breath test. Patients ingest a sugar solution that is metabolized by bacteria into hydrogen and/or methane. When exhaled at certain time points, high amounts of these gases suggest that small intestinal bacteria are producing them. At our center, patients breathe into a bag every 15 minutes for 180 minutes total. The breath that is captured is hooked up to a machine that calculates how much hydrogen and methane are in the sample.

But there are a lot of imperfections associated with breath testing. The test makes physiological assumptions that aren’t necessarily true across the board, such as how long it takes the carbohydrate to get from the mouth to the end of the small intestine. That varies a lot from one person to the next. I often tell patients that breath tests are a little better than a coin flip.

A study at the University of Michigan conducted a few years ago looked at thousands of people who had undergone hydrogen breath testing. They compared patients who were positive to patients who were negative and found that no presenting complaints were predictive of breath test positivity. That finding leaves open the possibility that bacterial overgrowth could be a red herring, unrelated to the primary symptom of concern. Or it could be that SIBO has various manifestations symptomatically.

Why was this once a contested diagnosis? Why is it getting attention now?

To a degree, the categorical understanding of this disorder is not as clean as we might want it to be. We like to think of disease as existing in separate and distinct boxes. In my subspecialty of gastroenterology, diagnoses can be more like overlapping circles. With SIBO, there’s a spectrum of presentation from classic to atypical. It can be challenging to know when it’s justified to entertain the hypothesis vs. when we’re being lured into considering SIBO when maybe another explanation is more justified.

So SIBO is encumbered by imperfections in both clinical hallmarks and available testing. If SIBO had a more discrete clinical picture, and a more accurate test associated with it, it would be a much less controversial diagnosis.

If you search for SIBO online, you’ll find a lot of information, including content from alternative practitioners and personal health blogs based much more on anecdotal experience than formal research. People do tend to develop pretty strong feelings about SIBO. As interest in nutrition and the gut microbiome has grown, SIBO has become an especially compelling inroad to thinking about the role of digestion in health.

What is the treatment?

Antibiotics are the conventional approach, because they kill bacteria. In SIBO, it’s not a question of good vs. bad bacteria, but rather too much bacteria. Some patients can get fixated on the idea of bacterial overgrowth as an infection — it’s not.

In some patients, the treatment is benign enough that we can deploy it on a hunch and see if people get better. The treatment becomes the test, in essence.

But antibiotics do have some risk of harm — either on the basis of short-term side effects or long-term antimicrobial resistance. For patients with atypical histories or inconsistent response to antibiotics in the past, we might consider more formal testing. Or, recognizing that there are other competing items on the list of potential explanations for gastrointestinal symptoms, we can elect to explore those other possibilities instead.

So far, there are almost no data in the realm of diet and SIBO. But the tack that a lot of people take is to reduce the fermentative capacity of the diet. One paradigm to borrow from is the low FODMAP diet, which involves a reduced intake of fermentable carbohydrates — examples would be cruciferous vegetables like broccoli or cabbage. The idea is that you’re providing less of an opportunity for bacteria to break food down in an aberrant way. This diet has been reasonably well studied in the context of irritable bowel syndrome, but some people repurpose it for SIBO. Again, there aren’t good data on this.

What are the long-term prospects for patients?

It depends on what the underlying predisposition is to SIBO. Some people will take a single course of antibiotics and feel better indefinitely. Others have a defined reason for the overgrowth, and when you take away that predisposition alongside treating with antibiotics, SIBO is no longer a problem. Some people, like patients with scleroderma, have a predisposition to bacterial overgrowth that is hard to take away. In those cases, it’s reasonable to expect that SIBO will recur, and in those circumstances, it can be reasonable to treat with repeat courses of antibiotics.

My takeaway is that SIBO is a real entity. The main thing that makes it a fascinating diagnosis for me — and frustrating for some — is that these hazy borders allow a lot of people to see their symptoms in it. It’s sort of like a Rorschach blot, abstract and open to interpretation. My hope is that further research will clarify the picture.