Small intestinal microvilli with bacteria present

4 Types of SIBO and Associated Symptoms

You may have heard of small intestinal bacterial overgrowth (SIBO), but did you know that there are different types of SIBO? Further, different symptoms correlate with each type of SIBO. 

So, if you have any type of irritable bowel syndrome like symptoms, it’s important to be familiar with SIBO as this may actually be the underlying cause in some cases. It’s also important to know which symptoms correlate with each specific type of SIBO. This can also help you determine if SIBO may be an issue for you. 


To understand the types of SIBO, we first need to take a small step back and briefly talk about the basics of SIBO.

In short, SIBO is the presence of our normal gut bacteria in our small intestine. Our gut bacteria should be living, for the most part, in our large intestine with very few numbers in the latter portion of our small intestine.

Additionally, when our normal gut bacteria begin to populate the small intestine, they become imbalanced in numbers as well. Certain bacteria become too large in numbers while others become too small. This disrupts the harmony that these bacteria should be living in.

So now, we have not only a displacement of bacteria as well as an imbalance of bacteria as well. 

One important point to note is that our gut bacteria are SUPPOSED to be living in our GI tract. It’s important not to demonize bacteria as they are actually very beneficial to us and serve many functions. One of their many functions is to help us digest food. 

This process is called fermentation and the byproduct of it is gas production. Now, this is a natural process, and one that most of us can handle without much ado. The problem comes in when these bacteria have started living in our small bowel as in the case of SIBO. 

If bacteria are populating the small bowel in large numbers, the fermentation process (and gas production that comes with it), can be very uncomfortable and lead to many other symptoms which we will address. 

Picture of both the large and small intestine with a happy smiley face on it.

So, how does this relate to the different types of SIBO? Well, as mentioned, bacterial fermentation produces gas. Each type of SIBO produces a different type of gas. 

A small bowel aspirate via an upper endoscopy has been the gold standard for testing for these gases. However, this is expensive and invasive and not often the most common choice. 

The more common way to test for SIBO is by taking a SIBO breath test which can measure and identify this gas. The test also measures the timing of the gas production.

The timing is important. If SIBO is not present, the bacteria will mostly be living in the large intestine. In this situation, the rise of gas on your breath with happen later than it would if SIBO is present. This is because in SIBO, the sugar solution will reach the bacteria and start to ferment sooner in your digestive tract since it doesn’t have to travel as far to reach the bacteria.

When it comes to breath testing, there are two different types of tests we can use – a glucose breath test and a lactulose breath test. For these tests, you will drink a sugar solution (either glucose based or lactulose based) and breathe into a sealed bag every 15-20 minutes for 90-120 minutes depending on the test. Side note: 120 minute tests are typically more accurate according to Dr. Allison Siebecker, world renowned SIBO expert. 

Your sealed breath bag will be sent to a lab for analysis. The gases, if any, will be identified, measured and timed. This will tell us which type of SIBO you have. This is important because each type of SIBO needs to be treated slightly differently for best efficacy. 

Got it? Good! Let’s dive into each type of SIBO as well as common symptoms of each.

Hydrogen SIBO

Hydrogen-dominant SIBO is one of the first known types of SIBO. The name “hydrogen SIBO” simply means that the organisms causing the overgrowth in this type of SIBO produce hydrogen gas. 

Remember we said that SIBO is a displacement of bacteria as well as an imbalance in numbers and type of bacteria. Certain bacteria produce certain gases more predominantly. 

The excess levels of hydrogen gas then appear on the breath test results, so we then know it’s hydrogen-dominant SIBO. There are some specifications around what qualifies as a positive result – a certain level of elevation within a certain time frame etc. A trained healthcare provider will be able to accurately read these tests. 

Symptoms of Hydrogen SIBO

Here are some common symptoms of hydrogen-dominant SIBO.  

  • Diarrhea or loose stool
  • Abdominal pain
  • Bloating 
  • Excessive flatulence
  • Sense of urgency with bowel movements 
  • Food sensitivities

Now, its important to remember that symptoms may not always present in a “typical” manner. Some may experience all of these symptoms, and some may experience only one or two. A few people may present with different symptoms entirely.

Methane SIBO (also known as IMO)

Technically speaking, methane-dominant SIBO is not an overgrowth of BACTERIA. Rather, it is an overgrowth of microorganisms called archaea. However, the concept is the same – archaea live in our large intestine and should not populate the small intestine in large numbers.

“Methanogen” is another name for these archae because they produce methane gas. This type of SIBO is now termed IMO and stands for intestinal methanogen overgrowth because of this. 

A SIBO breath test will pick up on elevated methane levels so a diagnosis can be made. 

Chemical structure of methane. Four hydrogen atoms attached to one carbon atom in the middle and pen next to it.

Symptoms of IMO

  • Constipation
  • Bloating
  • Excess flatulence
  • Distention
  • Abdominal pain
  • Food sensitivities
  • Belching

 Methane gas slows transit time in the digestive system often resulting in constipation. But here’s the kicker… constipation is actually one of the factors that can predispose someone to SIBO. 

If things aren’t moving through your gastrointestinal tract at the rate that they should, you’re at a higher risk of having bacteria or methanogens start to populate the small intestine. For this reason, constipation can also make overgrowth worse if SIBO is already present. 

You can see that we easily end up on the hamster wheel with this one. 

Hydrogen Sulfide SIBO

This is the newest type of SIBO and was discovered by Dr. Mark Pimentel with Pimentel Research Lab at Cedars Sinai Medical Center. Hydrogen sulfide SIBO produces, of course, hydrogen sulfide gas. Like the other types of SIBO, a SIBO breath test can pick up on this excess hydrogen sulfide gas. 

For many years, the breath tests we used to diagnose SIBO only had the capability to detect hydrogen and methane gases since hydrogen sulfide SIBO was recently discovered. So in reality, we were missing a fair amount of SIBO cases, possibly somewhere around one third. 

Up until recently, patients with hydrogen sulfide SIBO were taking breath tests and getting false negatives because the tests weren’t measuring this gas. The good news is that we are now able to capture hydrogen sulfide SIBO using the same type of breath testing as before. 

These new tests have the added capability of testing all three gases – hydrogen, methane and hydrogen sulfide. It is important to note that some clinics/providers may still be using two gas breath tests. This is certainly something to find out if you plan to get tested. 

Trio Smart offers a three gas breath test and you don’t need a provider order to order one. This may be an option for you if you do not otherwise have access to a three gas breath test. 

Symptoms of Hydrogen Sulfide SIBO

Here are some common symptoms of hydrogen sulfide SIBO. 

  • Diarrhea or loose stools
  • Urgency with bowel movements
  • Excessive flatulence
  • Food sensitivities 
  • Bloating
  • Abdominal pain or discomfort

As you can see, diarrhea is often associated with both hydrogen SIBO and hydrogen sulfide SIBO while constipation is often associated with methane SIBO/IMO. Other than that, there is clearly a lot of cross over with common symptoms of each type of SIBO. 

It’s important, however, to reiterate that these are common symptoms. Everyone will experience symptoms differently and it may not always manifest in the most “common” way. 

Mixed Type SIBO

In certain cases, more than one gas may be present due to the mixture of organisms causing the issue. Though it may not be considered a “type of SIBO” in and of itself, it is essentially a combination of the other types.

Brick wall with a sign for "restrooms" with an arrow on it.

Symptoms of Mixed Type SIBO

Symptoms of mixed type SIBO depend on which organisms are present of course. There may be a combination of any of the above listed symptoms. 

However, we often see alternating bowel habits oscillating between constipation and diarrhea with this type. This can make it difficult to know how to manage these symptoms because they are unpredictable and often changing.

Ultimately, testing for SIBO and treating it should resolve the symptoms, even if they are alternating. That said, hydrogen gas is needed to make both methane and hydrogen sulfide. So, in a mixed SIBO scenario, hydrogen may actually look low on testing even if hydrogen SIBO is present. 

This is because the hydrogen that is being created is then being used to make the other types of gases. This may require a short series of testing and treatments to get to the bottom of it and treat it appropriately but it can be done!

Knowing which type of SIBO is present is just the first step, but an important one. We want you feeling better! In order to do that, the SIBO needs to be treated. And in order to effectively treat it, we need to know which type you have. 

The treatment for each type of SIBO varies slightly so in most cases, we should not skip the testing step if at all possible. There are different treatment options – herbal antimicrobials, pharmaceutical antibiotic therapy and the elemental diet. Again, there are different treatment plan options within each of these categories and some of that is based on the type of SIBO you have.

If you would like to learn more about these treatments options, see my post Most Effective Treatments for SIBO According to SIBO Type

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