SIBO, IMO, or ISO? What Each Diagnosis Means for Your Gut Symptoms

I remember when I first heard that I had SIBO. It was Christmas time, and I was in our local toy store, shopping for presents for my son, nieces, and nephews. My phone rang, and I saw my doctor’s name on the screen. My heart skipped a beat. She let me know I had tested positive for SIBO, and it was methane dominant. That was about 14 years ago. I remember feeling both relieved and scared. I had been feeling so bad for so long. I was happy to have an answer, but nervous about what a SIBO diagnosis meant for me and my family.

Back then, SIBO—small intestinal bacterial overgrowth—was the only term being used. It was either methane dominant, hydrogen dominant, or mixed type. Today, we use three distinct names:

• SIBO for hydrogen overgrowth

• IMO (intestinal methanogen overgrowth) for methane

• ISO (intestinal sulfide overproduction) for hydrogen sulfide

Let’s break down each type so you can better understand what you might be experiencing.

What Is the Small Intestine’s Role?

The small intestine is a 19–21 foot-long tube that connects the stomach to the large intestine (colon). It’s where we absorb nutrients from food. In a healthy gut, most bacteria live in the large intestine. The small intestine contains very few bacteria—especially at the upper end. When an overgrowth occurs in the small intestine, it leads to SIBO, IMO, or ISO.

No matter which type you’re dealing with, these overgrowths disrupt digestion and nutrient absorption. That’s why symptoms can feel so widespread and oftentimes the symptoms are not even connected to the GI system at all.

Hydrogen Overgrowth (SIBO)

Hydrogen-producing bacteria release hydrogen gas in the small intestine, which is detected on a lactulose breath test. This type often presents with:

• Diarrhea

• Bloating

• Gas

• Weight loss

• Brain fog

• Fatigue

• Skin irritation

I’ve found that hydrogen overgrowth tends to be easier to treat. Both prescription antibiotics and herbal antimicrobials can be effective.

Treatment requires a step-by-step approach, including diet, motility support, and root-cause exploration - which is true for any type of SIBO that you may have.

Note: Now a days, lactulose breath testing is preferred over glucose testing, as it evaluates the entire length of the small intestine and reduces false negatives.

Methanogen Overgrowth (IMO)

IMO stands for intestinal methanogen overgrowth, caused by archaea—not bacteria. These ancient organisms are tenacious and can overgrow in both the small and large intestine.

Common symptoms include:

• Constipation

• Gas

• Brain fog

• Fatigue

• Skin issues

• Weight gain

• Bloating

Methane slows motility significantly (actually motility and SIBO might be a chicken and egg situation - what came first? Slow motility or SIBO?). According to Dr. Siebecker, up to two-thirds of methane-dominant cases can be chronic. Treatment requires more patience, as archaea feed on hydrogen—so when you reduce methane, hydrogen levels can spike temporarily. This flip-flopping is normal as your system rebalances, but this might also warrant more than one treatment cycle.

Hydrogen Sulfide Overproduction (ISO)

ISO is caused by excess sulfur-reducing bacteria, which release hydrogen sulfide gas. It’s measured with the Trio Smart Test, the only current test that measures all three gases.

Symptoms may include:

• Egg-smelling gas

• Diarrhea

• Bloating

• Sore joints

• A general “toxic” or inflamed feeling

Thankfully, this tends to be the easiest type to shift. The goal isn’t total eradication—these bacteria play a role in health—but rather balance. Treatment may include bismuth, herbal antimicrobials, and a short-term low-sulfur diet.

Why Knowing Your Gas Type Matters

Each gas type presents differently and requires a tailored treatment plan. The wrong protocol can lead to frustration and no symptom relief. I've seen many clients who had been treated without success—because the protocol didn't match their gas type.

 “Test, don’t guess.” —Andrea Nakayama

If you suspect you have SIBO, IMO, or ISO, the Trio Smart Test or a lactulose breath test are the best first steps.

My Healing Journey

Over the past 14 years, I’ve tried it all: restrictive diets, antibiotics, antimicrobials, Elemental Diets—you name it. I felt better in small increments, but full healing required a whole-body approach.

In my practice, every client is unique. SIBO is not a root cause—it’s a symptom of deeper imbalances, often linked to stress, food patterns, hormone shifts, gut motility, and emotional history and just life experiences in general. That’s where true healing begins.

Even now, I have periods of remission and moments when I need to support my gut more actively. And I am thriving. I have energy. I do what I love. And I help others do the same.

Key Takeaways

• SIBO, IMO, and ISO are different types of overgrowth in the small intestine.

• Each gas presents with different symptoms and requires unique support.

• Testing is essential—don’t guess your gas type.

• A holistic, whole-body approach is the most effective path to healing.

Ready for Next Steps?

Want more tips on gut health, SIBO recovery, or how Hashimoto’s connects to digestion? Explore my blog https://www.karenmitchellwellness.com/blog and on Instagram https://www.instagram.com/karenmitchellwellness/.

Looking for expert, personalized support from someone who’s lived it?

Schedule a Complimentary Conversation or join an upcoming Masterclass. I’d love to support you.

References:

• Small Intestinal Bacterial Overgrowth: Comprehensive Review

• Pros and Cons of Breath Testing for SIBO and IMO: Research Review

• Trio Smart Breath Test

• Dr. Allison Siebecker, SIBO Expert https://www.siboinfo.com/