What Is Small Intestinal Bacterial Overgrowth? Symptoms, Causes, Diagnosis, and Treatment

Small intestinal bacterial overgrowth (SIBO) is a condition that happens when there is too much bacteria, or the wrong kind of bacteria, in your small intestine.

SIBO will likely throw your gut microbiome, home to trillions of microorganisms in your digestive tract, out of whack. These organisms help our bodies digest food and absorb nutrients, and some evidence, per a June 2019 review in the journal Immunity, suggests they play a role in the immune system. But when certain types of these bacteria overpopulate and throw off the microbiome balance our body needs, they can become harmful.

An article in StatPearls describes how bacteria in the small intestine break down carbohydrates and turn them into gas and fatty acids, absorb some of the nutrients from our food, and feed on the bile salts that help us digest fat. According to Cleveland Clinic, if you have more bacteria than necessary in the small intestine, you may have a difficult time digesting fats and absorbing essential nutrients. SIBO can cause:

• Gastrointestinal symptoms like gas, abdominal pain, bloating, nausea, oily stools, and diarrhea
• Weight loss, since your body is not absorbing fats
• Malnutrition, which can eventually damage bones and the central nervous system

When to See a Doctor

SIBO’s signs and symptoms overlap with other gastrointestinal conditions, such as lactose intolerance, celiac disease, and IBS, notes the Mayo Clinic. See a doctor for an evaluation as early as possible to determine whether you have SIBO or something else. Make an appointment if you experience:

• Ongoing diarrhea
• Sudden weight loss
• Abdominal pain that lasts for more than a few days
• Bloating
• Excessive flatulence

IBS and SIBO: Know the Difference

In a review published in Frontiers in Psychiatry in 2020, researchers noted that SIBO is common in people with IBS and shares some symptoms, including:

• Abdominal pain
• Distention
• Diarrhea
• Bloating

But experts still don’t fully understand the relationship between the two disorders. “There is undoubtedly overlap between SIBO and IBS,” says Melissa Hershman, MD, an associate professor in the division of gastroenterology and hepatology at Oregon Health and Sciences University in Portland. “It is not clearly understood which one of these disorders is the driver of the other, and there is still some expert debate about whether these are truly coinciding or separate disorders.”

Luckily, doctors are equipped with some tools to differentiate the two diagnoses, such as a breath test that clarifies which type of bacteria are overgrowing in the small intestine by detecting certain gases. And experts can often tell the two disorders apart by how they manifest in the clinic. “While IBS is a disorder of pain or abdominal discomfort, in some way associated with defecation, patients with SIBO tend to have symptoms of delayed bloating with abdominal distention, and either diarrhea or constipation depending on the overgrowth subtype,” says Dr. Hershman.

According to StatPearls, certain bodily fluids and proteins help control the bacteria in the small intestine, such as gastric acid, bile, and enzymes. Since digested food and bile are consistently moving through the small intestine, it is not home to as many bacteria as the large intestine. If the process of digestion is interrupted or slowed, that can create a breeding ground for bacteria in the small intestine. Bacteria can also creep up into the small intestine from the large intestine.

Cleveland Clinic says certain medications, as well as your age and lifestyle, can contribute to SIBO. And according to Mayo Clinic, health conditions that slow down the digestive process or interfere with how bodily fluids function are also associated with a higher risk of SIBO, including:

• Irritable bowel syndrome
• Intestinal inflammation caused by radiation therapy
• Hypothyroidism
• Diabetes
• Scleroderma
• Complications from surgery on the abdomen, stomach, or intestines
• Immunodeficiencies
• Structural defects in the intestines that were present at birth
• Disorders that affect how much mucus you produce, like cystic fibrosis
• Pancreatic insufficiency

An SIBO diagnosis may involve a few different tests, since there is currently no standard approach to testing for SIBO. If you think you might have SIBO, your care team may recommend that you take one or more of the following tests.

• Carbohydrate breath test This fast, cost-effective test will evaluate how much hydrogen or methane is in your breath. Bacteria in our guts emit these gases, so this test helps doctors understand whether your gut microbiome is stable or not.
• Aspirate culture This test is more invasive, time-consuming, and pricey. During an endoscopy, the doctor will take a sample of your digestive juices from your small intestine to see how many bacteria are in your digestive tract. But this test can be unreliable, since bacteria can grow in patches.
• Blood test An analysis of a blood sample tells doctors about your nutrient levels and whether other nutritional markers are low.
• Imaging Once you are diagnosed with SIBO, your doctor may order imaging tests such as computerized tomography (CT) scanning, magnetic resonance imaging (MRI), or an upper endoscopy to pinpoint any structural defects in your intestines that may be causing your SIBO.

A few different health conditions have overlapping symptoms with SIBO, which means you may need to be tested to rule those illnesses out, as well. They include:

• Inflammatory bowel disease
• Celiac disease
• Crohn’s disease
• Lactose intolerance

Catching SIBO early is vital. The sooner you get treated for SIBO, the faster you can recover from its debilitating symptoms such as abdominal pain, diarrhea, nausea, or in extreme cases, weight loss, anemia, and vitamin deficiency.

Antibiotics The main treatment available for SIBO is antibiotics. While antibiotics are effective in killing bacteria, they can also wipe out gut-friendly bacteria. There’s also the risk that people can develop a resistance to them.

Hershman says the antibiotic rifamixin is “the mainstay backbone to therapy in both hydrogen and intestinal methane overgrowth types [of SIBO].” A review and meta-analysis published in Expert Review of Gastroenterology & Hepatology in November 2021 echoes that rifaximin is a safe and effective medication for treating SIBO. The clinical guidelines published in the American Journal of Gasteroenterology in February 2020 also include rifaximin as a research-supported approach to treating SIBO. And a study published in Evidence-Based Practice in November 2021 showed that rifaximin was effective in treating SIBO in patients with IBS.

But, Hershman says, “rifaximin is often not covered by insurance with costs upwards of $2,000, so providers sometimes trial other antibiotics with lower efficacy in this condition.”

Many people with SIBO will relapse. In a study published in the American Journal of Gastroenterology, 44 percent of people with SIBO experienced a relapse in their symptoms. If you relapse within three months of your first round of antibiotics, doctors will prescribe another round of either the same or a different antibiotic. If you relapse more than three months later, you may have to undergo more tests to confirm that your SIBO has returned.

If your SIBO does not improve after two rounds of antibiotics, your doctor might consider a different diagnosis.

After Antibiotics

Since antibiotics wipe out all bacteria in your GI system, many people think they need to replenish their gut microbiome by taking probiotics. But Hershman notes the science behind probiotics and prebiotics is still inconclusive. “The truth of the matter is we don’t understand them well enough,” she says. “Patients will start probiotics after treatment thinking they need to restore their gut somehow, but we give you antibiotics to help your gut reset itself. Rather than giving you a huge dose of bacteria, it’s much better to let that plethora and biodiversity of your normal gut flora repopulate on its own.”

Hershman also recommends that patients treated for SIBO avoid foods high in carbohydrates, fiber, and dairy. But, she says, “At this point, I don’t believe there is conclusive evidence to favor a particular diet in [treating] SIBO.” But these foods feed bacteria and can help them overpopulate, so she still recommends her patients avoid them.

Most dietary strategies aim to lower your intake of fermentable products by limiting sugar alcohols and fermentable sweeteners, fiber, and dairy. That approach has strong scientific support in treating IBS, says Hershman, but less so in SIBO specifically. “If used long-term, it may lead to a further reduction in intestinal biodiversity.”

Other Treatment Options

Hershman says that if you experience recurring SIBO due to a structural defect in your intestines, your doctor may need to perform surgery to fix the defect contributing to your SIBO. She says a team of specialists at a medical center with an integrative health model can help, too. These care frameworks incorporate gastroenterologists with registered dietitians and GI psychologists who treat all the components of SIBO.

Emerging research, such as a study published in the Journal of Alternative and Complementary Medicine in February 2021, indicates that Chinese herbal medicine may be an effective treatment for SIBO. But there have not yet been many large clinical trials to confirm this.

“There are a lot of aspects that go into dealing with abdominal pain symptoms that go well beyond just your gut,” says Hershman, “so thinking about this in a comprehensive and holistic approach is key to helping patients feel better.”

Preventing SIBO is really difficult. But there are some key steps you can take to help balance your gut microbiome.

• Avoid certain medications. If you have or have had SIBO, tell your medical care team so you can avoid certain medications that contribute to SIBO. This includes opioids, which slow down your digestion. In addition, “avoiding unnecessary antibiotics can help to maintain your normal biodiversity,” says Hershman.
• Make dietary changes and observe the results. While the research behind diets to manage SIBO remains inconclusive, Hershman says she advises her SIBO patients to avoid a diet high in carbs, fiber, and dairy. “Essentially, what we are trying to do is avoid excess undigested carb loads in the gut which allow bacteria to overpopulate,” she says. Alcohol can feed harmful bacteria in the gut, too, according to a review published in Alcohol Research in 2017. Make sure you limit how much alcohol you drink.
• Prevent gastrointestinal infections. Some GI infections are associated with SIBO, so preventing them is a great way to keep your gut balanced. Hershman says appropriate vaccinations (such as hepatitis A, among others), hand-washing, water filtering, and proper food handling can all help. She also recommends using prophylactic medications, like high-dose Pepto-Bismol, to prevent diarrhea when traveling.