What is SIBO (Small Intestinal Bacterial Overgrowth)?

overgrowth of bacteria in the small intestine

SIBO is a digestive condition caused by the overgrowth of bacteria in the small intestine.

Small Intestinal Bacteria Overgrowth, called SIBO, is a digestive disorder characterized by the overgrowth of bacteria in the small intestine. Because it is a relatively new condition, no set diagnostic criteria exist for SIBO as of yet.

Normally the small intestine does not contain very much bacteria. Generally the small intestine has less than 10,000 bacteria per ml of fluid. In SIBO, over the small intestine contains 100,000 bacteria per ml of fluid, or more. These kinds of bacteria usually reside in the large intestine but, for a variety of reasons, become trapped in the small intestine. Once trapped there, they feed and reproduce, wreaking havoc on the digestive system.

Common SIBO symptoms:

Generally, SIBO causes digestive symptoms such as:

  • gas
  • bloating
  • diarrhea
  • constipation
  • abdominal pain or cramping
  • food intolerances
  • excessive gurgling noises from the stomach
  • nausea
  • Vitamins or nutrient deficiencies

SIBO can also cause other systemic problems that do not result in acute gastrointestinal symptoms, like fatigue, aches, and weight loss or gain.

SIBO has numerous causes.

Until recent years, doctors thought that overgrowth of bacteria in the small intestine arose only from structural changes to the intestine such as scar tissue from bowel surgery. Since then, the SIBO research has advanced. Now we know SIBO has many causes, including:

  • Autoimmune reaction to food poisoning: The bacteria that cause food poisoning excrete a toxic called the cytolethal distending toxin (CdtB). To fight this toxin, our bodies begin producing an anti-CdbT antibody. However, some people’s bodies have an autoimmune cross-reaction. Their immune system essentially becomes confused and attacks itself. This happens when the body begins to produce, in addition to the anti-CdbtT antibody, an anti-vinculin antibody. Vinculin is a protein found in the digestive tract which helps convey electrical signals to the migrating motor complex. The migrating motor complex triggers the normal smooth muscle waves of the small intestine that clean out bacteria. Damage to that connection between the migrating motor complex and the cleansing waves can cause the body to produce less cleansing waves. When that happens, bacteria remain in the small intestine, leading to SIBO. This happens in approximately 10% of people who contract food poisoning.
  • Diabetes I and II: Both types of Diabetes can cause something called autonomic neuropathy. Autonomic neuropathy occurs when the autonomic nerves that manage many of the body’s day to day functions, including digestion, become damaged. When the nerves of the small intestine stop working properly the gut’s motility can decrease, resulting in SIBO.
  • Structural Abnormalities: Scar tissue from abdominal or intestinal surgeries, known as adhesions, can distort the normal function of the intestine. Adhesions can cause the intestines to slow the passage of food and bacteria because the normal shape and structure of the intestines has changed. Bacteria can become trapped due to the abnormal shape of the intestines, leading to overgrowth of bacteria in the small intestine and SIBO. This can be difficult to recover from even with antibiotics if the abnormal shape of the intestine remains unchanged.
  • Infections Disease (Lyme, Epstein Barr, etc.): Infectious diseases can deregulate the immune system. Particularly in chronic infections, such as Lyme Disease, this chronic deregulation of the immune system can affect the digestive system. The amount and composition of intestinal bacteria can become unbalanced and chronic inflammation can affect the gut. Furthermore, the neurological symptoms that may manifest in Lyme or other infections can damage or partially paralyze stomach or digestive tract. Both bacterial dysbiosis and neurological changes that impact the gut can cause bacteria to become trapped in the small intestine.
  • Hypothyroid: When the thyroid fails to produce enough of certain hormones (T3 and T4), this can change the gut’s motility. The thyroid controls time that it takes for food to enter your stomach for your stomach to empty into the digestive tract. This slowing of the digestive tract, or other changes to gut motility that occur when the thyroid fails to properly produce T3 and T4 hormones, may be what leds to overgrowth of bacteria in the small intestine in these cases.
  • Low Stomach Acid: Gastric acid helps to limit the bacteria present in the small intestine. As people age, or after an infection such as Helicobacter pylori, the body slows production of stomach acid. Overuse of proton pump inhibitors may also cause a reduction in stomach acid. When the stomach fails to secrete enough stomach acid, the bacteria in the small intestine can overpopulate. This overgrowth of bacteria in the small intestine develops into SIBO.

Often SIBO is mistaken or misdiagnosed as IBS.

IBS is a diagnosis of exclusion, meaning that it describes symptoms but not their cause. Often , a doctor will diagnose a patient with IBS without determining whether SIBO is the true cause of the patient’s digestive problems. This happened to me, and I went almost ten years without knowing that my IBS was actually SIBO. Had I known, I could have sought treatment much sooner and managed my symptoms better. So if you have IBS, please go get evaluated for SIBO by a doctor or other medical practitioner!

How is SIBO diagnosed?

Diagnosing SIBO can be controversial, because no set diagnostic criteria exist. However, several ways of evaluating SIBO (besides just evaluating symptoms) do exist.

Breath testing is probably the most common way that doctors diagnose SIBO. Breath tests measure the amount of gas produced by the bacteria present in the patient’s intestines. The test measures how much hydrogen, methane, and/or hydrogen sulfide a person produces over an approximately three hour testing period. These gases allow doctors to evaluate the presence and type of bacteria in the small intestine.

Before the breath test, the patient must fast for at least 12 hours. At the beginning of the test, the patient consumes a solution consisting of lactulose or glucose. Bacteria gobble up the lactulose or glucose, fermenting the carbohydrates, and producing gases: hydrogen, methane, and/or hydrogen sulfide. Humans don’t naturally make these gases on our own—they only occur when bacteria make the for us. The patient blows into a tube every 20 or so minutes and this measures the amount of these gases present in the patient’s breath. From this, a doctor can evaluate whether there is an overgrowth of bacteria in the small intestine.

But of course, we all have gut bacteria, right? Yes, but we shouldn’t really have much in our small intestine. The way that doctors can tell where in our intestines the bacteria creating these gases are is by looking at when during the test the gas levels sharply increase. The test creates a chart of the patient’s gas levels as they rise and fall over the three hour testing window. By looking at when the gas levels spike up, doctors can tell if the bacteria producing those gases are in the small intestine (where they shouldn’t be) or in the large intestine. As you can see below, food from the stomach enters the small intestine before the large intestine:

small intestine chart

Therefore, by seeing when the gas levels spike up, a doctor can tell if the bacteria producing those gases are in the small intestine (when the spike occurs early in the test) versus the large intestine (later in the test). A note of caution though: methane behaves differently from hydrogen sulfide. Methane may not exhibit the same spike that hydrogen does. A high level of methane throughout the test, even if it does not fluctuate, may indicate methane-based SIBO.

Breath testing is the most common way of diagnosing SIBO, but not the only one.

Duodenal aspiration is another method for diagnosing SIBO. Compared to breath testing, Duodenal aspiration costs more, carries more risk, and is much more invasive. Duodenal aspiration means undergoing an endoscopy of the small intestine—where you have a tube stuck down your esophagus into your small intestine so that the doctor can retrieve a sample of the fluid there. Doesn’t sound fun! Breathing in a tube for three hours sounds like a walk in the park in comparison…

The advantages of duodenal aspiration is that it’s a very reliable test. Doctors consider it the “gold standard” of testing for SIBO. But because of the costly and invasive nature of the test, it’s rarely performed.

Treating SIBO

Information on treating SIBO could take up way more time than you’ve already spent reading this entire post (if you’ve made it this far, congrats!). To make a very long story short, many ways exist to treat the overgrowth of bacteria in the small intestine. Very generally, treatment aims to kill the bacteria overpopulating the small intestine, and fix the motility or other issue that led to the bacteria overgrowth in the first place. After this stage, diet and prokinetic treatments seek to keep the SIBO from returning.

A few of the most common treatments are:

  • Pharmaceutical antibiotics: Antibiotics such as Rifaximin (generic name Xifaxan) and Neomycin kill the bacteria in the small intestine.
  • Herbal antibiotics: Herbal antibiotics like Garlic, Berberine, and Neem can be used to kill the bacteria.
  • Elemental Diet: This is where the patient consumes a solution of necessary nutrients, and nothing else, for several weeks. This “fast” starves the bacteria of their normal food (like carbohydrates) killing them off.
  • Prokinetics: Prokinetics seek to stimulate the gut’s motility which keeps bacteria from collecting in the small intestine. Prokinetics can be either pharmaceutical (such as Low-Dose Naltrexone or Low-Dose Erythromycin) or herbal (like Ginger or Iberogast).
  • Low FODMAP Diet: A low FODMAP diet reduces the amount and/or types of food consumed that bacteria ferment rapidly. Rapid fermentation of these foods feeds the overgrowth of bacteria in the small intestine and exacerbates SIBO symptoms. While this diet alone is unlikely to cure SIBO, because it doesn’t address the underlying motility symptoms, it can help manage symptoms. Typically a low FODMAP diet follows treatment with antibiotics for a limited period of time.

As you can see…there’s a lot to learn and absorb about SIBO! Hopefully this overview gives you a good place to start with your research.

Is there anything I didn’t cover that you’d like to see? Let me know!