SIBO: The 3 Kinds, and Why It Keeps Coming Back

When I share my story at summits and workshops, I list what my testing found, and then I add 1 more.

So that was me. One more thing on top of it, SIBO, small intestinal bacteria overgrowth. Yay! Okay, so this is when bacteria proliferate where they shouldn't be, further damaging the gut lining, creating bloating, fog, more inflammation.

Many Women live some version of it: the belly that is flat at breakfast and 5 months pregnant by dinner, the 'healthy' vegetables that make it worse, the bloating that no amount of eating clean seems to touch.

'SIBO and Your Thyroid' in the Gut chapter introduced SIBO, and 'Leaky Gut, IBS, IBD, and SIBO' set it beside the conditions it is most often confused with. This chapter goes further, because SIBO turns out to be at least 3 different overgrowths, each with its own gas, its own symptoms, and its own treatment.

Which kind do I have? Did my test even look for all 3?

What treats each 1? And why does it keep coming back?

6 Guards That Keep Your Small Intestine Quiet

Your small intestine is where most of your nourishment is absorbed, and it is meant to hold very few bacteria. In healthy people, a sample of fluid from the upper small intestine grows fewer than 1,000 bacteria per milliliter. Your large intestine, further along, holds most of the roughly 38 trillion bacteria in your body.

That quiet is actively kept. The American College of Gastroenterology's 2020 SIBO guideline lists the guards that hold the line:

• Stomach acid, which kills most of the bacteria that arrive on your food.
• Pancreatic enzymes, which digest food fully and leave less behind for bacteria to eat.
• Bile, which acts like a detergent on bacterial membranes.
• The Migrating Motor Complex, the cleaning wave that sweeps the small intestine between meals.
• The ileocecal valve, the 1-way door between your small and large intestine.
• Your gut's immune system, including secretory IgA.

SIBO takes hold where 1 or more of these guards has gone quiet. The guideline notes that in almost every case, SIBO is related to something else, usually a condition that lets contents sit still in the small intestine. That sentence is the Functional Medicine case for finding the root cause.

What SIBO Is, and Why the Name Is Changing

The ACG guideline defines SIBO as too many bacteria in the small intestine, causing gut symptoms. When a gastroenterologist samples fluid from the small intestine during an endoscopy, 1,000 or more bacteria per milliliter counts as SIBO.

The most common symptom is bloating. Pain, gas, distension, and diarrhea are close behind, along with constipation in some Women, and some people describe fatigue and poor concentration too. No single symptom proves SIBO, which is why the test matters so much.

The name itself is changing. Methane, 1 of the gases measured on a breath test, is made by microbes called archaea, which are a separate branch of life from bacteria. They can overgrow in the large intestine as well as the small.

So in 2020, the ACG proposed a new name for the methane pattern: intestinal methanogen overgrowth, or IMO.

Newer research has refined the picture further. The Gut chapter describes SIBO as bacteria from the large intestine moving up. Recent sequencing studies from Cedars-Sinai suggest the overgrowth is often dominated by just a few species, mainly strains of E. coli and Klebsiella.

The same group now describes 3 overgrowth patterns: SIBO, IMO, and intestinal sulfide overproduction, or ISO.

Most practitioners still call all 3 'SIBO', and so do I, naming the type whenever it changes the treatment.

The 3 Kinds, and the Gas Each 1 Makes

Your body cannot make hydrogen or methane. When those gases show up in your breath, microbes in your gut made them, and the gas tells you who is doing the fermenting.

Hydrogen-dominant SIBO. Bacteria ferment your food and release hydrogen. This is the classic SIBO pattern.

In a 2026 study of 3,004 people who took a 3-gas breath test at home, higher hydrogen went with more severe diarrhea. Bloating, gas, and loose stools are the typical picture.

Methane-dominant, or IMO. The methane makers, mainly 1 species called Methanobrevibacter smithii, feed on the hydrogen other microbes produce and turn it into methane. Methane slows the gut.

In the research behind the ACG guideline, a positive methane test was linked to constipation, and the higher the methane, the worse the constipation tended to be. Bloating with hard, slow stools is the typical picture.

Hydrogen sulfide, or ISO. Some bacteria use hydrogen to make hydrogen sulfide, the gas that smells like rotten eggs.

In the same 2026 study, higher hydrogen sulfide went with more severe diarrhea, more urgency, and more belly pain. Gas with a sulfur smell, urgent loose stools, and pain are the typical picture.

The 3 gases compete for the same hydrogen. When methane makers or sulfide makers use it up, the hydrogen line on a breath test can look low or flat even while a great deal of fermentation is happening. Some practitioners read a flat hydrogen line in a Woman with obvious symptoms as a clue to hydrogen sulfide, and researchers have described how these gases trade hydrogen back and forth.

It remains an interpretation, and a 3-gas test gives a direct answer.

Read this chart as text

Diagram titled '3 gases, 3 kinds of overgrowth.' Hydrogen, H2, SIBO. Pattern: bloating, gas, loose stools. Positive test: 20 ppm rise within 90 minutes. Methane, CH4, IMO. Pattern: bloating, constipation. Positive test: 10 ppm at any point. Hydrogen sulfide, H2S, ISO. Pattern: diarrhea, urgency, pain, rotten-egg gas. Positive test: 3 ppm on trio-smart. A bracket under hydrogen and methane says most breath tests measure these 2. A bracket under hydrogen sulfide says it needs a 3-gas test. The chart shows the idea, not measured data. The 2017 North American Consensus sets the thresholds for hydrogen and methane, and the hydrogen sulfide cutoff is the lab's own.

If Your Symptoms Began After Food Poisoning

Some Women can name the day their gut changed: a bout of food poisoning, a stomach bug on vacation, traveler's diarrhea that never quite resolved. That pattern is called post-infectious IBS.

In a 2017 analysis of 45 studies and more than 21,000 people, about 10 percent developed IBS within a year of a gut infection. Women had about 2 times the odds of men. After infections caused by parasites or protozoa, the rate was about 42 percent.

Dr. Mark Pimentel's group at Cedars-Sinai proposed a mechanism. Bacteria that cause food poisoning make a toxin called cytolethal distending toxin B, or CdtB. The antibodies your body makes against CdtB can also react with vinculin, a protein your own gut uses, and in animal research this cross-reaction produced IBS-like changes, including shifts in the bacteria of the small intestine.

Their blood test, now sold by Gemelli Biotech as ibs-smart, measures anti-CdtB and anti-vinculin antibodies. In their 2015 study of 2,375 people with diarrhea-type IBS, a high anti-CdtB result was quite specific, 91.6 percent, and it caught fewer than half the cases, 43.7 percent. An Australian research team found the antibodies did not separate IBS from health well in their community sample.

Read it as 1 more clue. A positive result can help explain why your SIBO keeps returning, and it points the plan toward protecting your gut's motility after treatment.

Why SIBO Comes Back: Finding the Root Cause

In Functional Medicine, the question underneath every symptom is 'why?' A root cause is the upstream driver, the reason the overgrowth could take hold, rather than the overgrowth itself. Antimicrobials clear the bacteria. The root cause decides whether they return.

And they often do. In a 2008 Italian study of 80 people whose SIBO had cleared with rifaximin, it returned in 12.6 percent by 3 months, 27.5 percent by 6 months, and 43.7 percent by 9 months. Older age, a past appendectomy, and long-term PPI use each raised the odds of a return.

The root causes worth looking for:

• Low stomach acid. Autoimmune gastritis, H. pylori, and long-term acid-suppressing medication all lower the acid that guards the door. The Stomach chapter covers how to test.
• A Migrating Motor Complex that has gone quiet. In a 2002 Cedars-Sinai study, people with IBS and SIBO had about a third as many cleaning waves during fasting as healthy controls. In a 1977 study of 18 people with bacterial overgrowth, 5 had a cleaning wave that was absent or badly disordered.
• Stress. In a 1986 study of 37 healthy volunteers, psychological stress during the day significantly reduced the cleaning waves. 'The Vagus Nerve and the Gut-Brain Axis' in the Stress chapter explains the wiring.
• A slow thyroid. In the 2007 study behind the Gut chapter's 'about half', 54 percent of people with a history of Hashimoto's-related hypothyroidism had SIBO, compared with 5 percent of controls.
• A weak ileocecal valve. In a small 2014 Johns Hopkins study, people with SIBO had lower pressure at the door between the small and large intestine, and less acidic stomachs.
• Past abdominal surgery. Surgery on the bowel, and the scar tissue called adhesions that can form afterward, can create places where contents sit still. The ACG suggests SIBO testing for Women with symptoms after abdominal surgery.
• Medications. Opioids, some antidiarrheals, and anticholinergic drugs slow the cleaning wave, and PPIs lower acid. GLP-1 medications slow the stomach and upper gut by design, and Dr. Mark Pimentel has raised the concern that this slowing may open the door to SIBO; 'GLP-1 Medications: What They Can Do, and What to Know First', in the Weight Gain chapter, covers the early studies. Every change stays with your prescriber.
• Diabetes, which can damage the nerves that run the gut's movement.
• A past gut infection, or celiac disease, both linked to SIBO.

Age and being a Woman also raise the odds, the ACG notes, possibly because food moves more slowly. That makes the root-cause work especially important in Midlife.

Keeping the Small Intestine Swept

Once the overgrowth has cleared, prevention comes back to the cleaning wave that 'Give Your Gut Time to Rest Between Meals' in the Gut chapter describes.

Space your meals. The wave runs only when your gut is empty, and food stops it. Leave 4 to 5 hours between meals, with nothing in between but water.

For a Woman prone to SIBO, grazing hands the bacteria a steady supply of food and takes away the wave that would have swept them along.

Let your Nervous System rest. Stress quiets the wave, so the breathing, sleep, and Connection practices in this book are part of SIBO prevention. Inhale deep, exhale long.

Ask about a prokinetic. A prokinetic is a medication or herb that encourages the gut to move. Many practitioners suggest 1 after SIBO treatment, especially when methane, a slow thyroid, or a post-infectious pattern is in the picture.

• Low-dose erythromycin. At low doses, this old antibiotic mimics motilin, the hormone that starts the cleaning wave. In 1 study, a low dose given by IV set off a cleaning wave in healthy volunteers within about 13 minutes. In a Cedars-Sinai chart review, 50 milligrams at bedtime lengthened the time before symptoms returned, a trend that did not reach statistical significance.
• Prucalopride. Sold as Motegrity, it is FDA-approved for chronic constipation, and some practitioners use it off-label after SIBO treatment.
• Ginger and artichoke. In a 2016 pilot study of 11 healthy volunteers, a ginger and artichoke extract helped the stomach empty faster after a meal.
• Iberogast. A liquid blend of 9 herbs studied in functional dyspepsia, not in SIBO. It contains greater celandine, linked to rare cases of liver injury.

Keep treating the root causes you found. Supporting stomach acid when testing shows it is low, caring for your thyroid, and steadying your blood sugar all protect the small intestine long after the antimicrobials are done.

1 of the Hashimoto 411 educators teaches 3 habits to keep long after treatment: a prokinetic, support for low stomach acid, and care for a healthy microbiome. In her experience, the prokinetic is the habit most worth protecting. As thyroid and gut health improve, many Women can lower their betaine HCl dose with their practitioner.

She suggests taking a prokinetic at night, on an empty stomach that will stay empty for hours, since the cleaning wave runs only in a fasting gut. Where blood sugar and adrenals allow, she favors long gaps between meals, a long overnight fast, and a kitchen closed 2 to 4 hours before bed.

She is just as clear that spacing bends to the body in front of you. A Woman with low blood sugar may need to eat every few hours, and a Woman with adrenal issues may need breakfast within 30 minutes of waking and a small protein and fat snack at bedtime. Space your meals as far as your blood sugar and your adrenals allow.

SIBO and the Rest of Your Body

SIBO can show up well beyond the gut, in places Women do not expect.

• Your thyroid. A slow thyroid slows the gut, and SIBO travels with Hashimoto's often. In the 2007 study, treating SIBO eased gut symptoms, and thyroid hormone levels did not change. Clearing SIBO helps your gut, and your thyroid still needs its own care. 'Your Thyroid and Your Gut' in the 'How It All Connects' chapter follows the loop.
• Your skin. Rosacea has 1 of the strongest links to SIBO in the research, as 'The Gut-Skin Axis' in 'Hair Loss, Skin, and Nails: What Your Body Is Telling You About Your Health' describes. If you have rosacea and bloating, a breath test is worth asking about.
• Your B12. The overgrowing bacteria use up B12 before you can absorb it, while making folate of their own. A low B12 with a high-normal or high folate can be 1 more clue. 'Vitamin B12, In Depth' in the Supplements chapter covers testing.
• Your iron. Severe SIBO can contribute to low iron, and the ACG notes these deficiencies are usually subtle. When ferritin will not climb despite good food, SIBO belongs on the list of reasons, alongside low stomach acid and celiac disease.

Histamine belongs on this list too. Some of the bacteria that overgrow in the gut make histamine, and an inflamed lining makes less of DAO, the enzyme that clears food histamine. If you flush, itch, or get headaches after fermented foods or wine, read 'Histamine Intolerance and MCAS: When Your Body Overreacts'.

My Own SIBO Story, Step by Step

Before I knew I had SIBO, healthy food made me miserable. Within 10 minutes of eating garlic, onions, celery, mushrooms, or certain fruits, I was so bloated, and it would not go down.

Those foods share a thread. Monash University's FODMAP research lists garlic and onions as rich in fructans, mushrooms and celery as rich in mannitol, and fruits such as apples, pears, and mango as high in excess fructose or sorbitol.

Because eating made me so uncomfortable, I began to eat less. That calorie restriction deepened the nutrient deficiencies I already had.

Thank goodness I did the trio-smart breath test, which measures all 3 gases. My methane came back at extreme levels, the pattern this chapter calls IMO.

On that first test, my methane measured about 75 ppm. The North American Consensus on breath testing treats 10 ppm or more at any point in the test as positive.

I followed an antimicrobial protocol from Dr. Allison Siebecker, a naturopathic doctor who has focused on SIBO since 2011. First came her methane protocol for 6 weeks, then a slower, longer round for 8 weeks, with a biofilm disruptor and low FODMAP eating alongside.

The tools I name, as education to bring to your practitioner:

• Allicin, the stabilized compound from garlic, such as Allimed, also sold as Allimax Pro. Allicin is the methane piece of the protocol.
• Neem or oil of oregano, each paired with allicin in Dr. Siebecker's combinations for methane.
• Berberine, paired with allicin or with Atrantil.
• Atrantil, described with its honest evidence note in the Stomach chapter.
• InterFase or InterFase Plus, Klaire Labs enzyme capsules used to break up biofilm. InterFase Plus adds disodium EDTA, contains egg, and carries a label warning against use in pregnancy, while nursing, with kidney or liver conditions, or for long-term daily use.
• Low FODMAP eating, during treatment and for a short stretch after.
• The elemental diet, a great treatment for some Women, as 'The Functional Medicine Toolkit' describes.

Dr. Siebecker's site lists berberine with allicin, berberine with Atrantil, neem with allicin, and oregano with allicin among her combinations for methane, usually in rounds of 4 to 6 weeks, and many Women need more than 1 round. Every product, amount, and length of treatment belongs with your practitioner, and none of these herbs has been tested on its own in a human SIBO trial.

And the order mattered as much as any herb. Thyroid first, then SIBO. When you have a list of things to treat, a skillful Functional Medicine practitioner helps you understand the proper order.

About 7 months later, after treatment, my methane read 23 ppm on FoodMarble, a handheld breath device I use at home. A home device and a lab breath test are different tools, so I read that number as a trend.

About 10 months after that, my methane climbed again, to 52 ppm on FoodMarble. I understand why: my thyroid was not yet optimized. A slow thyroid slows the gut, and a slow gut gives the methane makers time to settle back in.

As my thyroid has come closer to optimal, my antimicrobials have worked much better, and my results are holding. Thyroid first, then SIBO, is a lesson my own body keeps teaching me.

These are my own results, reached with my practitioners, and 1 Woman's numbers are not a promise for yours.