Small intestinal bacterial overgrowth (SIBO) occurs when large numbers of bacteria colonize the small intestine. It can cause bloating, diarrhea, or constipation. Treatment options include antibiotics, fecal transplant, and dietary changes.

Bacteria are essential to the digestive system, in the form of the gut microbiome. This is mostly present in the large intestine, but the small intestine does not contain many bacteria.

Gastric secretions and the forward movement of food through the digestive system prevent too many bacteria from growing in the small intestine. However, if these functions do not work effectively, bacteria can multiply.

When the small intestine doesn’t empty effectively, excessive bacteria ferment all the food that comes through causing unpleasant symptoms for some people.

SIBO appears to be related to irritable bowel syndrome (IBS), among other conditions. People with IBS are more likely to have SIBO than the rest of the population. However, not everyone with an IBS diagnosis tests positive for SIBO.

In this article, we discuss the symptoms, risk factors, and complications of SIBO. We also look at the best diets to relieve the symptoms.

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The symptoms of SIBO can vary from person to person, depending on how mild or severe the overgrowth of bacteria is and whether someone has any coexisting conditions.

However, the symptoms may include:

The type of microbe that is overgrowing may also determine the symptoms. Research suggests that hydrogen-producing bacteria are more associated with diarrhea, while methane-producing species are more associated with constipation.

Doctors are not always sure what causes SIBO. So far, researchers have identified the following factors that may contribute to SIBO:

Low motility

Usually, the intestines push food and bacteria through the digestive tract, which prevents too much bacteria from accumulating in the small intestine. However, if someone has low motility, this mechanism slows down, allowing food to ferment in the small intestine.

Dysbiosis

Dysbiosis is when a person’s microbiome becomes imbalanced, containing too many harmful species of microbes or not enough beneficial species. Research into how different species of microorganisms influence digestion is still ongoing, but previous studies suggest that people with IBS often have less diversity, fewer beneficial species, and higher amounts of methane-producing species in their microbiome, which can slow motility.

Hypochlorhydria

This term describes low levels of stomach acid. When someone does not have sufficient stomach acid, it may be possible for bacteria to migrate further down the digestive tract than usual, as the environment is not acidic enough to kill them. Researchers believe that people who take proton pump inhibitors (PPIs), have autoimmune gastritis, or have undergone a gastrectomy have an increased risk of hypochlorhydria and SIBO. However, there is a lack of conclusive evidence on this, and low motility may be a more important risk factor.

Structural differences

Sometimes, SIBO occurs because a person has structural differences in the digestive tract. Examples include small bowel diverticulosis, fistulas, and a shortened colon. According to a 2018 study, people who have undergone a colectomy are more at risk of developing SIBO. The incidence of SIBO after gastric bypass surgeries is also very high.

Alcohol abuse

Excessive alcohol consumption can damage the microbiome, leading to dysbiosis, damage to the intestinal walls, and inflammation.

People with certain underlying conditions are more likely than other people to get SIBO. These people include those with:

SIBO is also more common in older adults, possibly due to slower digestive motility. There is a higher prevalence among females, too, but the reasons for this are unclear.

A small 2017 study found that people with obesity were 11 times more likely to have SIBO than those without obesity. It is unclear why this was the case, as the risk was higher in participants who had healthy digestive motility and pH levels. More research is necessary to understand the link.

A doctor will diagnose SIBO by asking about a person’s symptoms and medical history. They may feel the abdomen for signs of excess gas or bloating. If they suspect SIBO, they will recommend testing.

Breath testing

A lactulose breath test measures the concentration of hydrogen and methane in a person’s breath. The results of this test can confirm a SIBO diagnosis and reveal the extent of the overgrowth. They also show whether the overgrowth consists mainly of hydrogen- or methane-producing microbes.

A person has to fast for 24 hours before the test. They then drink a sugary solution containing lactulose, which is a sugar that only gut bacteria can break down.

When bacteria break the sugar down, they produce gases, which enter the bloodstream and travel to the lungs. A breath test measures these gases as a person exhales.

Glucose vs. lactulose breath testing

Some doctors perform glucose breath testing instead of lactulose breath testing. Some scientists have criticized this method, as the body can absorb glucose quickly, which means that it might not reach the area of overgrowth.

However, others argue that this rapid absorption can be an advantage, as it makes it less likely that the sugar will reach the colon, where it would generate a false positive result by feeding a large number of bacteria there.

Small bowel aspirate and culture

Small bowel aspirate tests are more invasive than breath tests. They involve a doctor performing an endoscopy, which means inserting a small, thin camera through the mouth and down into the stomach.

When the device reaches the duodenum, which is where the stomach joins up with the small intestine, doctors use the endoscope to take a tissue sample. They then send the sample to a laboratory, which analyzes the bacteria present.

It is worth noting that with small bowel aspirate testing, as well as breath testing, there is no universally accepted threshold for what constitutes a positive SIBO result. This makes diagnosis more difficult for those who have less conclusive test results.

Other tests

The existing tests for SIBO are not always precise, so a doctor may recommend other tests to get a better picture of someone’s digestive health. These could include:

  • blood tests to detect markers of autoimmunity or inflammation
  • a mobility test to determine whether someone has any structural problems in the small intestine
  • an intestinal permeability test, which assesses whether the intestinal lining is “leaky“
  • stool tests to analyze the gut microbiome

As SIBO can occur for complex reasons, it can be difficult to treat. Various treatment options are available, including antibiotics, fecal microbiota transplants, and dietary changes.

Antibiotics

This is the main treatment for SIBO. Doctors may use one type of antibiotic or a combination to remove the overgrowth.

A popular option is rifaximin (Xifaxan). Previous studies suggest that this antibiotic works best for people with hydrogen-dominant overgrowths. Neomycin, or a combination of rifaximin and neomycin, may be better for those with methane-dominant SIBO. However, research into the best ways of treating SIBO is still ongoing.

Recurrence rates after SIBO treatment are relatively high. For this reason, alongside taking antibiotics, it is essential to address the underlying cause of the SIBO to reduce the risk of the overgrowth coming back.

Depending on the root cause, this may mean taking medications to speed up motility, stopping PPI treatment, or treating other conditions that might be contributing.

Probiotics and fecal microbiota transplants

The role of probiotics in SIBO treatment is controversial. While some studies have shown that specific species can improve motility and reduce hydrogen in the breath, others have shown that probiotics may cause SIBO.

If a person has low motility and takes probiotics, it is possible that they may make an overgrowth more likely or worsen an existing one. The results can vary from person to person.

A fecal microbiota transplant (FMT) is a fairly new treatment that involves a doctor transplanting a donor’s gut flora into a patient via the rectum. Again, research has produced mixed results on using this procedure as a treatment for SIBO. Some case studies have noted that FMT seems to cause SIBO. Currently, the Food and Drug Administration (FDA) has not approved this treatment.

Diets cannot cure SIBO, but they can lessen the symptoms. What works can vary from person to person, but a popular option is the low FODMAP diet.

The low FODMAP diet limits the intake of fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. These are substances that the human body does not break down, which means that bacteria can feed on them. Limiting high FODMAP foods may reduce gas, bloating, constipation, and diarrhea.

Both the type and amount of foods a person eats are important in the low FODMAP diet.

Alternatively, an elemental diet is another option that some researchers believe is effective in lessening SIBO symptoms.

This liquid-only diet aims to provide a person with all the necessary nutrients in an easily digestible form while reducing the amount of food in the small intestine.

However, 2022 research notes that while diet may modify the microbiome of the small intestine in someone with SIBO, there is greater evidence for using pharmacological treatments such as antibiotics.

It is best to work with a dietitian to implement a change in diet. A professional can ensure that a person gets all the nutrients they need.

In cases where SIBO is severe or not well-controlled, a person may have significant diarrhea or constipation. These symptoms can lead to complications, such as:

These complications can affect many aspects of health, including a person’s energy levels, hormones, and mental health. Additionally, living with SIBO can be challenging, which may cause stress, anxiety, and low mood.

It is not always possible to prevent SIBO, but people can take steps to look after their gut health. These include:

  • Eating a nutritious and varied diet: Eating a wide variety of foods can help increase gut flora diversity, which may reduce the chances of dysbiosis. Fruits, vegetables, and whole grains are especially beneficial.
  • Stopping smoking: Cigarettes and other products that contain nicotine can change the composition of a person’s gut flora. This may play a role in dysbiosis.
  • Taking prokinetics: Prokinetics are a type of drug that speeds up digestive motility. They may reduce the risk of SIBO in people with an increased risk of developing it, such as those who have underlying conditions or take PPIs. A 2018 study found that people who took prokinetics and PPIs together were less likely to receive a SIBO diagnosis than those who took PPIs alone.
  • Treating hypochlorhydria: If a person has low stomach acid, addressing this issue may reduce the risk of SIBO, although more studies are needed to confirm this link. How a doctor treats hypochlorhydria will depend on the cause.
  • Managing other conditions: If a person has conditions that are associated with SIBO, such as hypothyroidism or diabetes, then effectively managing these conditions may reduce the impact they have on the digestive system.

Here are some answers to questions people often ask about SIBO.

What is SIBO poop like?

Stools may be watery, fatty, and foul-smelling. Fatty stools tend to float on water.

How do you fix SIBO?

Treatment is usually with antibiotics. Around 45% of people find SIBO returns after a full course of treatment. If this happens within 3 months, the doctor will prescribe a second course of antibiotics. If it returns at a later date, they may give antibiotics or they may look for other possible causes.

What are the symptoms of SIBO?

Symptoms typically include abdominal discomfort, bloating and flatulence, watery diarrhea, and fatty stools. Over time, weight loss and vitamin deficiencies can result.

What happens if you do not treat SIBO?

Complications of SIBO include weight loss and nutritional deficiencies. Eventually, it can lead to intestinal failure, where the gut no longer functions effectively.

What are the non-digestive symptoms of SIBO?

Non-digestive symptoms of SIBO may include fatigue and poor concentration. In more severe cases, nutritional deficiencies including vitamin B12 and vitamin D can occur.

SIBO occurs when bacteria from the large intestine migrate into the small intestine. It can cause symptoms such as bloating, diarrhea, and constipation. Doctors can diagnose SIBO by carrying out a lactulose breath test or a small bowel aspirate and culture test.

The treatment for SIBO usually involves taking one or more antibiotics to remove the overgrowth of bacteria. The effectiveness of other treatments, such as probiotics, is less clear. Newer options, such as FMT, are not fully understood and may carry risks.

People should speak with a gastroenterologist who knows about SIBO and related digestive disorders if they are experiencing persistent symptoms.