Small Intestinal Bacterial Overgrowth (SIBO) Overview

Small intestinal bacterial overgrowth syndrome (SIBO), also referred to as small bowel bacterial overgrowth (SBBO), is a condition wherein the small intestine (small bowel) becomes ripe with abnormally large amounts of bacteria.

Normally, the number of bacteria is highest in the colon (large intestine) and substantially lower in the small intestine. Also, the types of bacteria within the small intestine (the part of the gastrointestinal tract connecting the stomach to the colon) are different from those within the colon.

However with SIBO, the types and amount of bacteria resemble colonic bacteria rather than those normally present in the small intestine.

High concentrations of bacteria in the colon are necessary for optimum health. In the small intestine, however, high numbers of bacteria interfere with proper nutrition, and are connected to various other health problems.

How SIBO Works Against Your Good Health

A healthy small intestine is required for the proper absorption of nutrients; this means that desirable levels of bacteria are necessary.

But when the small intestine becomes overgrown with bacteria they begin to compete for nutrients, which results in the production of byproducts that can irritate, inflame, and damage the absorptive surface of the small intestine.

This damage disrupts the absorption of carbohydrates (sugars), fats, fat-soluble vitamins, and sometimes proteins, leading to malabsorption and malnutrition.

In the process, a lot of gas is produced, resulting in bloating, flatulence, diarrhea, and a great deal of discomfort.

In severe SIBO cases, weight loss and vitamin deficiencies can develop.

Common SIBO Symptoms

While symptoms tend to be patterned and chronic, they may also come and go. Sometimes they persist for years before signs of malnutrition are present or medical attention is sought.  

Severity depends upon the type of bacteria present as well as the extent of overgrowth, and not all the symptoms occur for everyone:

  • Abdominal bloating and/or belching soon after eating
  • Abdominal cramps, discomfort, or pain
  • Abdominal distension – looking pregnant after a meal or at the end of the day
  • Bulky, loose, or smelly stools
  • Loud rumbling noises from the belly after eating
  • Nausea, especially after meals
  • Weight loss

Some SIBO sufferers complain of chronic constipation. Others report symptoms seemingly unrelated to the gastrointestinal tract, such as body aches and/or fatigue. 

Severe Problems Created by SIBO

When the bacterial overgrowth is prolonged and severe, damage to the small intestine’s absorptive surface can interfere with the absorption of proteins, sugars, fats, and vitamins. 

There are a number of more serious issues that can occur:

  • Poor digestion of fats results in weight loss and steatorrhea (foul smell and a loose, floating, and greasy appearance).
  • Poor digestion of proteins results in increased susceptibility to infections,
    hair loss and weight loss.
  • Poor digestion of sugars leading to belching, bloating, cramping, gas, and watery diarrhea.
  • Vitamin B12 deficiency (consumed by the overpopulation of bacteria in the small intestine and can result in anemia or low blood counts).
  • Vitamin A deficiency leading to sensitivity to sunlight, night blindness, 
    and other vision problems. There may also be some skin issues and changes. a
  • Vitamin D deficiency that could lead to softening bones and joint pain.
  • Vitamin E deficiency might lead to problems with the nervous system.
  • Calcium / Magnesium deficiency may cause problems with muscle cramps.

SIBO Causes and Risk Factors

What causes SIBO? Risk factors tend to fall into three categories:

  1. Problems with motility/muscular activity; 
  2. Immune system disorders; and
  3. Conditions (often structural) that encourage colonic bacteria to move into the small intestine.

Healthy muscular activity of the small intestine is important not only for the proper digestion of food, but it also clears out bacteria and controls bacterial counts in the small intestine.

Muscular interference allows bacteria to remain in the intestine longer and multiply. Bacteria may also be allowed to spread backwards from the colon into the small intestine when normal motility is disrupted. 

SIBO can develop as a result of any condition disrupting motility/muscular activity throughout the small intestine. Some of the potential causes of SIBO: 

  • Diabetes damages nerves that control intestinal muscles.
  • Muscular and neurologic diseases can alter the activity of intestinal muscles.
  • Obstruction, partial or intermittent, of the small intestine interferes with food
    transport and bacteria through the small intestine. Causes of obstruction leading
    to SIBO include surgical scarring (adhesions), Crohn’s disease, and sometimes malignancy.
  • Out-pouchings of the small intestine called diverticula that allow bacteria to multiply.
  • Scleroderma damages the intestinal muscles directly; it slows the bowel leading
    to an increase in bacteria.
  • Surgery – Bacterial overgrowth can result following duodenum and/or stomach surgery or surgeries in which the ileocecal valve (junction between small and large intestine) is surgically resected.

Disorders of the immune system can foster bacterial overgrowth. Inherited immunodeficiency conditions and the use of immunosuppressant medications to treat various conditions can be linked to SIBO.

Conditions that cause colonic bacteria to enter the small intestine are also linked to SIBO. Diseases of the ileum (i.e. Crohn’s disease) may require removal of a patient’s ileocecal valve, which connects the small and large bowel, and an increased reflux of bacteria into the small bowel may result.

Following bariatric surgery, there may be an increase in the bacterial load of the small bowel. Use of proton pump inhibitor medications (for acid reflux or ulcers) decreases stomach acid, and can also lead to bacterial overgrowth.

SIBO can develop when food remains in the stomach too long. This state of stagnation disrupts gut flora balance, and causes bacterial overgrowth.

Patients on TPN (total parenteral nutrition) – intravenous feeding for malnourishment – are at increased risk for developing SIBO. 

Additionally, consuming large amounts of sugars (which bacteria feed on) can encourage opportunistic bacteria to relocate from the colon to the small intestine; there they will thrive, but you will not!

The IBS and SIBO Connection

There is much speculation regarding a possible relationship between SIBO and irritable bowel syndrome (IBS). The similarity between IBS and SIBO symptoms has led researchers to suggest that SIBO may be responsible for the symptoms experienced by some IBS patients.

IBS is difficult to diagnose and treat, and diagnosis is based on symptoms and testing to rule out other illnesses rather than any one definitive test.

Though research is only in the preliminary stages, scientists have noticed that significantly more IBS patients experience abnormal hydrogen breath test (HBT) results than unaffected individuals.

In addition, many patients report a significant decrease in IBS symptoms after being treated with specific antibiotics used for SIBO.

Diagnosing SIBO – The Hydrogen Breath Test (HBT)

Patients must fast for twelve hours prior to testing. On test day, they’re given a (pleasant-tasting) carbohydrate syrup solution such as glucose, lactulose, or sucrose to drink, and then asked to breathe into a balloon-type device every fifteen minutes over a two-hour period.

Bacteria feed off carbohydrates, emitting gases as byproducts that are absorbed into the bloodstream and released via the lungs. The test assesses the presence of gas in breath.

In a healthy individual, hydrogen or methane wouldn’t be present until at least two hours after swallowing the solution (once the digested food reaches the colon).

A positive gas result prior to two hours after swallowing the sugar solution is evidence of bacteria higher up in the digestive tract, at the level of the small intestine.

HBT testing for SIBO has several drawbacks:

  • Other conditions such as Celiac disease and pancreatic insufficiency can produce abnormal breath test results.
  • Individuals with delayed gastric emptying may require prolonged testing by several hours.
  • Crohn’s disease of the small intestine, small intestine strictures, or other anatomical abnormalities (like prior small bowel resections) can result in positive breath test and produce similar symptoms.

The one definitive way to determine whether symptoms are SIBO-related is to treat and eliminate harmful bacteria. If symptoms disappear, then SIBO has likely been responsible for the previous symptoms.

Treatment for SIBO

For less severe SIBO cases, some patients find a change in diet helpful. A SIBO diet requires restricting dietary carbohydrates and increasing fats. In order to maximize the potential health benefits of a low carbohydrate/high fat diet, a dietitian may be consulted.

As the body heals, the intestine may have difficulty digesting sugars, making the replenishment of minerals and vitamins essential. Additionally, some health care providers recommend B12 and iron supplementation.

A variety of antibiotics are also sometimes prescribed to treat SIBO:

  • Augmentin
  • Bactrim
  • Ciprofloxacin
  • Levofloxacin
  • Metronidazole
  • Neomycin
  • Rifaximin

Many health care providers recommend standard doses of antibiotics for one to two weeks. But there are certain drawbacks to antibiotic use. While treatment is often very effective, it doesn’t address the underlying disease causing SIBO. Symptoms often recur once a course of treatment has been completed, which necessitates prolonged and/or repeated treatment.

Though rarely dangerous, there are sometimes side effects such as diarrhea, nausea, and vomiting. Fungal infections of the digestive tract, mouth, and vagina are common with antibiotic usage because antibiotics destroy the good bacteria as well as the bad. Another disadvantage is that antibiotic-resistant bacteria may begin to emerge.

For these reasons, one treatment option is to prescribe a short course of antibiotics to begin treatment, followed by probiotic use over the long term to help replenish the good bacteria in the gut.

Probiotics are live bacteria, the most common of which are Lactobacilli and Bifidobacteria. A good treatment option for some bacterial overgrowth cases, probiotics replenish the gut’s good bacteria. Certain yogurts and fermented foods contain a variety of healthy living bacterial species as well.

Even after bacterial overgrowth has been successfully treated, damaged tissue can take several months or more to heal. Known for their healing properties, probiotics may be helpful throughout SIBO treatment. Be sure to use probiotics in moderation as high dosages can sometimes trigger diarrhea. 

Probiotics can be used alone, in combination with antibiotics, and for long-term maintenance/prevention of recurrence.

SIBO and Relationships to Other Diseases

  • In 2009, the Journal of Clinical Gastroenterology reported that recent studies suggest SIBO as a likely cause for non-responsive Celiac disease.
  • A large number of people who suffer from rosacea also have SIBO. When treated for SIBO, many patients reported that the rosacea disappeared.
  • Research has shown that 20-40% of diabetic patients who suffer with chronic diarrhea do so because they also have SIBO.

SIBO is thought to be an underlying root cause of – or at least is linked to – various frustrating and perplexing illnesses. A good theory since bacterial overgrowth can wreak a great deal of havoc on one’s entire digestive system.

So if you suffer from abdominal pain, bloating, gas, and diarrhea, it makes good sense to talk to your doctor about scheduling a breath test to check for SIBO.

Though testing is not an exact science at this point in time, antibiotic and probiotic treatment is inexpensive, relatively simple, and often extremely successful.

Julie C. Guider MyGoodGut

Medically reviewed by Julie Guider, M.D.

Dr. Julie Guider earned her medical degree from Louisiana State University School of Medicine. She completed residency in internal medicine at the University of Virginia. She completed her general gastroenterology and advanced endoscopy fellowships at University of Texas-Houston. She is a member of several national GI societies including the AGA, ACG, and ASGE as well as state and local medical societies.