What Is Small Intestinal Bacterial Overgrowth (SIBO)? 

Small intestinal bacterial overgrowth (SIBO) is an increasingly common digestive condition that occurs when excessive invasive bacteria are growing in your small intestine, a region typically low in bacteria.

Not only does this cause highly uncomfortable gastrointestinal symptoms, but SIBO can also lead to nutrient malabsorption and other complications if left untreated.

In this article, we’ll dive into what SIBO is, its symptoms and causes, and how to manage symptoms with food.

What Is SIBO? 

Small intestinal bacterial overgrowth (SIBO) is, as the name suggests, excess bacteria in the small intestine. 

Although people have recently come around to the idea that the trillions of bacteria we house in our guts are vital and beneficial, there is one caveat: most of the bacteria need to stay in your colon.

The small intestine does contain its own microbiome of about 1,000 microbial species, but problems arise when the bacteria in this region are growing too much, causing issues like uncomfortable digestive symptoms, poor nutrient absorption, and damage to the intestinal lining.¹

There are three segments of the small intestine: the duodenum (closest to the stomach), the jejunum (in the middle), and the ileum (closest to the colon).

The duodenum should have relatively few bacteria because acid from the stomach kills them off. Peristalsis—the movement of food through the intestines—also prevents bacteria from staying in the upper small intestine.

As you go further down, the jejunum and ileum naturally have more bacteria, but still vastly less than in the colon.

When the protective mechanisms of keeping bacteria counts low (namely peristalsis and stomach acid) begin to fail, small intestinal bacteria proliferate, and SIBO occurs. 

To get technical, SIBO is defined as “the presence of colon-specific bacteria in the small intestine equal to or greater than 10⁵ CFU/mL.” ²

The characteristic symptoms of SIBO manifest when inflammation from invasive bacterial strains occurs. Bacterial species commonly found in SIBO that produce inflammatory enzymes or endotoxins include Escherichia coli, Aeromonas, and Klebsiella.³

Symptoms of SIBO

The main symptoms of SIBO are gastrointestinal, including:

• Diarrhea
• Bloating
• Gas
• Abdominal pain 
• Feeling uncomfortably full after eating 
• Nausea
• Loss of appetite

As SIBO symptoms are almost identical to IBS, the conditions can often be misdiagnosed for one another. Additionally, many people have both SIBO and IBS—research has found that up to 78% of people with IBS also have SIBO.⁴

SIBO Causes 

There are many different reasons why SIBO develops, and it often occurs due to other medical conditions.

The leading causes of SIBO are irritable bowel syndrome (IBS), intestinal disorders that slow gut motility, and chronic pancreatitis, accounting for 80 to 90% of cases.³

Healthy gut motility and peristalsis are needed to push food and bacteria through the digestive tract. Conditions like gastroparesis, diabetes, scleroderma, and autonomic neuropathy can affect gut motility. 

Some other causes of SIBO include:³

• Hypochlorhydria (low stomach acid), which means not enough acid is present in the upper small intestine to kill off invasive bacteria 
• Structural abnormalities in the intestines, like intestinal diverticulosis, strictures, or abdominal surgery
• Medications, including antacids, proton pump inhibitors, and narcotic pain medications
• Excessive alcohol use
• Increased intestinal permeability (leaky gut)
• Gut dysbiosis, which is when the gut has more unhealthy bacteria than healthy 
• Dysfunction of the ileocecal valve (the valve connecting the small and large intestines), which leads to bacteria migrating back up from the large intestine into the small intestine
• Celiac disease and inflammatory bowel disease (IBD), which consists of Crohn’s disease or ulcerative colitis, increase the risk of SIBO.

Managing SIBO With Food

Your doctor might recommend antibiotics to kill off the unwanted bacteria in the small intestine. However, others opt for herbal antimicrobials like berberine, oregano oil, or allicin (a component of garlic) to kill the invasive species.⁵ 

Prokinetics (medications that stimulate intestinal movement) may also be prescribed to prevent the recurrence of SIBO by improving gut motility. Speak with your healthcare provider about the right course of medication for you.

In addition, making changes to your diet and lifestyle can help to manage SIBO. There is no single diet that has been proven to help manage or treat SIBO. 

A low-FODMAP diet may help reduce symptoms, but it does not help with one of the root causes of SIBO (dysbiosis), as a diet low in FODMAPs is also low in dietary fiber. Without enough prebiotic fibers, the gut microbiome can still be imbalanced and dysbiotic, contributing to SIBO occurrence. 

Your doctor or a Registered Dietitian can help you try an elimination diet to figure out which foods are contributing to your digestive symptoms while still getting in enough fiber to feed your good gut bacteria.

Common food triggers for SIBO include:

• High-fructose fruits like apples, pears, watermelon, and mangos
• High-fructose corn syrup
• Honey 
• Artificial sweeteners and sugar alcohols (like sorbitol, mannitol, and erythritol)
• Garlic, onions, and leeks
• Beans and legumes
• Wheat
• Dairy products
• Cruciferous vegetables like broccoli, cauliflower, Brussels sprouts, cabbage, and kale

How Do You Test for SIBO?

The most common test for SIBO is the hydrogen and methane test, which involves fasting then drinking a sugary solution (usually lactulose or glucose) and collecting breath samples every 20 minutes for 2 to 3 hours afterwards. 

The type of gas in your breath indicates if there are bacteria where they shouldn’t be. Elevated levels of hydrogen and methane gases suggest bacterial overgrowth in the small intestine, as the bacteria produce them when fermenting sugars.

A test is considered positive if ​​at least one of these conditions are met:

• Hydrogen levels rise at least 20 ppm above baseline within the first 90 minutes of the test. 
• Methane level at least 10 ppm at any time point during the test indicates IMO (intestinal methanogen overgrowth, or methane-producing bacteria)
• Hydrogen sulfide level at least 3 ppm at any time point during the test indicates hydrogen sulfide-associated SIBO

However, this method is not always accurate. An upside to this method is that it’s much less invasive and can be done either at your doctor’s office or at home (although it needs to be prescribed first by a healthcare professional).

Another testing option is jejunal aspirate, which is more invasive but also more accurate. It involves taking a fluid sample from the small intestine using an endoscope, culturing the sample, and seeing how many bacteria are present.

Small Intestinal Bacterial Overgrowth FAQs