If you’ve been struggling with bloating, abdominal discomfort, constipation, diarrhea, or increasing food sensitivities, you’ve probably come across the term SIBO.
While we often use the term SIBO broadly, there are actually different types of intestinal overgrowth. The differences come down largely to the microorganisms involved and the gases they produce: hydrogen, methane or hydrogen sulfide.
So, let’s start with the basics.
What is SIBO?
SIBO stands for Small Intestinal Bacterial Overgrowth.
Our digestive tract is home to trillions of microorganisms, the majority of which reside in our large intestine (colon) where they play important roles in digestion, immune function and overall health.
The small intestine, on the other hand, is primarily responsible for digesting our food and absorbing nutrients and it normally contains far fewer microorganisms than the colon.
With SIBO, excessive numbers of bacteria are present in the small intestine. Instead of waiting until food reaches the colon, these microorganisms begin fermenting carbohydrates much earlier in the digestive process. And fermentation produces gas.
This is why someone with SIBO may eat a meal and soon afterward experience bloating, distention, gas, pain or changes in their bowel movements.
Some people may also notice that certain carbohydrates, particularly more fermentable varieties, tend to make their symptoms worse.
Understanding the Three Gas Types
When microorganisms ferment carbohydrates in our digestive tract, they produce gases. Today, we recognize three primary gases associated with intestinal overgrowth: hydrogen, methane and hydrogen sulfide.
The type of gas being produced can give us important clues about someone’s symptoms.
Hydrogen (H2): SIBO
Hydrogen is produced by bacteria as they ferment carbohydrates and is the gas traditionally associated with SIBO.
Hydrogen-predominant SIBO is more commonly associated with:
- Bloating and abdominal distention
- Excess gas
- Abdominal discomfort or cramping
- Loose stools or diarrhea
- Urgency
Many people who have been diagnosed with diarrhea-predominant IBS (IBS-D) may discover that elevated hydrogen is contributing to their symptoms.
Methane (CH4): IMO
Methane isn’t actually produced by bacteria. It’s produced by microorganisms called archaea, or methanogens. Because methanogens can overgrow in both the small and large intestine, we no longer technically call this methane SIBO.
The more accurate term is Intestinal Methanogen Overgrowth, or IMO.
Methane has been associated with slower intestinal transit and is most commonly associated with:
- Constipation
- Sluggish digestion
- Abdominal fullness
- Significant bloating and distention
- Feeling uncomfortably full after eating
Many people who have been diagnosed with constipation-predominant IBS (IBS-C) may discover that elevated methane is contributing to their symptoms.
Hydrogen Sulfide (H2S): ISO
Hydrogen sulfide is the newest gas that we can measure through breath testing. An excess of hydrogen sulfide is referred to as Intestinal Sulfide Overproduction, or ISO.
Hydrogen sulfide is more commonly associated with:
- Diarrhea
- Urgency
- Bloating
- Abdominal pain
- Gas
Some people notice particularly foul or “rotten egg” smelling gas, although this certainly isn’t present in everyone.
And while hydrogen sulfide has been associated with diarrhea, practitioners in my SIBO study group have also observed that some people with elevated hydrogen sulfide experience constipation instead.
Mixed Gases
Some people produce more than one gas and have a mixed picture. Hydrogen and methane, for example, frequently occur together.
This can help explain why someone’s symptoms don’t fit neatly into one category or why someone may alternate between constipation and diarrhea.
What Are the Symptoms of SIBO?
SIBO can look different from one person to another, but common symptoms include:
- Bloating or visible abdominal distention
- Excessive gas
- Abdominal pain or cramping
- Constipation
- Diarrhea
- Alternating constipation and diarrhea
- Urgency
- Feeling overly full after eating
- Belching
- Reflux
- Increasing food intolerances or sensitivities
- Fatigue
- Nutrient deficiencies – such as low iron or vitamin B12
One clue I often look for is when symptoms occur.
With SIBO, bloating, gas, pain or changes in bowel movements commonly worsen after eating. However, you cannot diagnose SIBO based on symptoms alone.
The symptoms of SIBO overlap significantly with many other digestive conditions. This is why I look at symptoms, health history and breath testing together.
How is SIBO Diagnosed?
The most common way to evaluate intestinal overgrowth is through a breath test. It is non-invasive and can be done at home.
Before testing, you’ll follow specific preparation instructions, which typically include a limited diet the day before the test followed by an overnight fast.
On the morning of the test, you’ll collect a baseline breath sample and then drink a sugar solution, usually lactulose (my preference) or glucose.
You’ll then collect additional breath samples at regular intervals over the next two to three hours.
The laboratory measures the gases present in each breath sample.
Humans don’t produce hydrogen or methane on our own. These gases are produced by microorganisms as they ferment the sugar solution. The gases are absorbed into the bloodstream, travel to the lungs and are then exhaled in our breath.
By looking at which gases are produced, how much is produced and when those gases rise, we can gather information about where fermentation may be occurring in the digestive tract.
Lactulose vs. Glucose
You may see options for both lactulose and glucose breath testing. Both are accepted substrates for breath testing, but they behave differently in the digestive tract.
Glucose is absorbed relatively early in the small intestine. This can make it useful for detecting overgrowth in the upper portion of the small intestine, but it may potentially miss overgrowth farther down.
Lactulose, on the other hand, isn’t absorbed by us. It travels through the entire small intestine and eventually reaches the colon, where fermentation is expected to occur.
This is also why the timing of gas production matters when interpreting a lactulose breath test. A later rise in gas is expected as lactulose reaches the bacteria living normally in the colon. We are looking for evidence that fermentation is occurring earlier than expected.
What Does a Positive Breath Test Look Like?
For hydrogen, current criteria consider a rise of 20 parts per million (ppm) or more above baseline within the first 90 minutes of a glucose or lactulose breath test supportive of SIBO.
Methane is interpreted differently. A methane level of 10 ppm or greater at any point during the test is considered methane-positive and supports a diagnosis of IMO.
Hydrogen sulfide requires a breath test capable of directly measuring this third gas. It is newer than hydrogen and methane testing, and interpretation continues to evolve as we learn more about its role in digestive symptoms.
The results are usually displayed on a graph, which allows us to see not only whether a gas is elevated but how it behaves throughout the test.
The Limitations of Breath Testing
Breath testing has limitations, and this is one reason I don’t interpret a breath test in isolation.
Intestinal transit time can influence the results (and lactulose itself can speed up intestinal transit time). The difference between substrates (glucose and lactulose) can influence what we see. Improper preparation and breathing incorrectly can also affect the test.
Breath test results aren’t always black and white, which is why I interpret the results alongside a person’s symptoms and health history. Testing is just one piece of the puzzle.
Why SIBO Develops
SIBO is often not the entire story. Something allowed those microorganisms to accumulate where they shouldn’t.
Many factors can contribute, including:
- Impaired intestinal motility
- Prior food poisoning
- Certain medications
- Structural changes after surgery
- Connective tissue disorders
- Hypothyroidism
- Other digestive or medical conditions
Identifying and addressing these underlying factors is one of the best ways to reduce the risk of recurrence after treatment.
So rather than only asking: “How do I get rid of SIBO?” we also want to ask: “Why might you have developed SIBO in the first place?”
Understanding that piece can be incredibly important, especially for people who find themselves treating SIBO over and over again.
Treating SIBO
If testing confirms intestinal overgrowth (SIBO, IMO, ISO), there are several treatment options available. Depending on the individual, treatment may include prescription antibiotics, herbal antimicrobials, or an elemental diet.
Each approach has advantages and disadvantages, and the “best” treatment depends on your symptoms, gas type, medical history and personal preferences.
If you’d like to learn more, please see these blog posts:
