Why your IBS diagnosis may be hiding a treatable root cause — and what functional medicine testing can reveal.
Dr. Houston Anderson | Mesa, AZ

What If Your IBS Isn’t Really IBS?

If you’ve spent years dealing with bloating, unpredictable bowel movements, cramping, gas, urgency, constipation, or that constant feeling that your gut is running the show — and all you’ve been told is, “You have IBS” — you already know how frustrating that answer can be.

Maybe you were given an antispasmodic.
Maybe you were told to eat more fiber.
Maybe you were handed a low-FODMAP diet sheet.
Maybe stress was blamed.
Maybe everything “looked normal,” but you still didn’t feel normal.

Here’s the problem: IBS is often treated like a diagnosis, but in many cases, it’s really just a label.

IBS describes what your symptoms look like. It does not automatically tell you why those symptoms are happening.

And that distinction matters.

Because in many patients, what gets called IBS is actually SIBO — Small Intestinal Bacterial Overgrowth — a measurable, testable, and treatable condition that is commonly missed in conventional gut care.

The difference between SIBO vs IBS is not just academic. It changes the testing. It changes the treatment. And more importantly, it changes whether you actually get better — or just keep managing symptoms for years.

What the Research Shows

SIBO is found in a significant percentage of patients diagnosed with IBS.

A 2023 meta-analysis by Ndong et al. published in JGH Open found a 36.4% prevalence of SIBO among IBS patients using glucose breath testing.

A 2022 meta-analysis by Poon et al. in Scientific Reports reported prevalence rates as high as 49% when lactulose breath testing was used.

In plain English: roughly one in two people diagnosed with IBS may actually have an underlying bacterial overgrowth driving their symptoms.

That is not a small group.

In our Mesa, AZ functional medicine practice, we see this pattern all the time: patients who have carried an IBS diagnosis for years, only to discover that once the underlying SIBO is identified and treated, their symptoms finally begin to change.

Understanding the difference between SIBO and IBS is often the first step toward a completely different outcome.

What Is IBS? Understanding the Label

Irritable Bowel Syndrome, or IBS, is classified as a “functional” gastrointestinal disorder.

In conventional medicine, that means standard testing does not show obvious structural disease. Blood work may look normal. Colonoscopy may look normal. CT scans, endoscopy, and other standard evaluations may not show inflammation, tumors, ulcers, or visible pathology.

Everything looks “fine.”

But the patient is clearly not fine.

IBS is diagnosed using the Rome IV Criteria, the current international diagnostic standard.

That includes:

• Recurrent abdominal pain, on average, at least one day per week for the past three months
• Pain associated with two or more of the following:
  • Related to defecation
  • Associated with a change in stool frequency
  • Associated with a change in stool form or appearance

Once these criteria are met, and once major red-flag conditions like inflammatory bowel disease, celiac disease, and colorectal cancer have been ruled out, the IBS label is often applied.

From there, IBS is usually broken into subtypes:

IBS-D: Diarrhea-Predominant IBS

Frequent loose stools, urgency, post-meal rushing to the bathroom, and fear of being too far from a restroom.

IBS-C: Constipation-Predominant IBS

Hard stools, infrequent bowel movements, bloating, pressure, and that frustrating feeling of incomplete evacuation.

IBS-M: Mixed IBS

An unpredictable back-and-forth between constipation and diarrhea, sometimes changing within the same week.

Here’s the key point that often gets missed:

IBS tells you what is happening. It does not tell you why it is happening.

And when you do not know why something is happening, treatment usually becomes symptom management instead of root-cause resolution.

That is the heart of the SIBO vs IBS conversation.

What Is SIBO? The Root Cause IBS Often Misses

SIBO stands for Small Intestinal Bacterial Overgrowth.

It happens when excessive bacteria accumulate in the small intestine — an area of the digestive tract where bacterial levels are supposed to stay relatively low.

Your colon is designed to hold a large amount of bacteria. Your small intestine is not.

In a healthy gut, the upper small intestine usually contains very low but specific bacterial counts, often fewer than 1,000 microorganisms per milliliter. That low-bacteria environment is protected by several systems working together.

These include:

Stomach Acid

Stomach acid helps kill many bacteria before they can move deeper into the digestive tract.

Bile Acids

Bile acids do more than help digest fat. They also have natural antimicrobial properties that help regulate bacterial growth and act as the primary calming chemicals to your digestive tract.

The Ileocecal Valve

This valve separates the small intestine from the colon and helps prevent bacteria from moving backward from the colon into the small intestine.

The Migrating Motor Complex

The Migrating Motor Complex, or MMC, is one of the most important pieces of this entire conversation.

It acts like a cleaning wave through the small intestine. Every 90–120 minutes between meals, the MMC helps sweep leftover food particles and bacteria out of the small intestine and into the colon where they belong.

When any of these protective mechanisms break down — from food poisoning, medications like proton pump inhibitors, chronic stress, adhesions, low stomach acid, impaired motility, or other root causes — bacteria can accumulate where they do not belong.

Then those bacteria ferment the food you eat.

That fermentation produces gases.

And those gases create the classic SIBO symptoms:

• Bloating
• Distension
• Abdominal pain
• Gas
• Diarrhea
• Constipation
• Food reactions
• Brain fog
• Fatigue

Which, of course, looks almost exactly like IBS.

The Three Types of Overgrowth

SIBO is not one single condition that looks the same in every patient.

The type of overgrowth matters because it often determines the symptom pattern and the treatment strategy.

Hydrogen-Dominant SIBO

Hydrogen-dominant SIBO occurs when the overgrown bacteria produce hydrogen gas.

This is most commonly associated with:

• Diarrhea
• Bloating
• Abdominal pain
• Urgency
• IBS-D-type symptoms

This is one reason many patients with diarrhea-predominant IBS and even bile acid malabsorption may actually have hydrogen-dominant SIBO underneath the label.

Methane-Dominant Overgrowth / IMO

Methane-dominant overgrowth is now more accurately called IMO — Intestinal Methanogen Overgrowth.

The reason for the name change is important: the organisms that produce methane are not technically bacteria. They are Archaea.

The 2020 ACG clinical guidelines recognized IMO as a distinct entity, and this matters because methane behaves differently in the gut.

Methane is strongly associated with constipation because it slows intestinal transit time.

This is often what we see in stubborn IBS-C cases — the patient is bloated, constipated, uncomfortable, and has been told to take more fiber, but the real driver may be methane slowing the system down.

Hydrogen Sulfide-Dominant Overgrowth

Hydrogen sulfide-dominant overgrowth is an emerging area of research.

Hydrogen sulfide-producing organisms have been linked to:

• Diarrhea-predominant symptoms
• Visceral hypersensitivity
• Sulfur-smelling gas
• Abdominal pain
• Irritated bowel patterns

Testing for this subtype is still being refined, but clinical awareness is growing quickly.

The Key Difference

Unlike IBS, SIBO is not just a diagnosis of exclusion.

SIBO can be measured. It can be tested. And it can be specifically treated.

That is the major difference that changes the entire direction of care.

Why SIBO Gets Misdiagnosed as IBS

If SIBO is so common in IBS patients, why is it missed so often?

Because conventional gastroenterology often approaches IBS as a symptom-based condition after major pathology has been ruled out.

In other words, once the colonoscopy is normal, the blood work is normal, and there is no obvious inflammatory bowel disease, cancer, or celiac disease, the patient is often told they have IBS.

But that does not mean the root cause has been found.

It just means the standard workup did not find something obvious.

There are three big reasons SIBO gets mislabeled as IBS.

1. The Symptoms Are Nearly Identical

SIBO and IBS can both cause:

• Bloating
• Gas
• Abdominal pain
• Diarrhea
• Constipation
• Alternating bowel habits
• Food reactions
• Fatigue
• Brain fog

Without testing, there is no reliable way to separate the two based on symptoms alone.

2. SIBO Testing Is Not Always Part of the Standard IBS Workup

Most conventional IBS evaluations do not automatically include a SIBO breath test.

So the patient may go through blood work, colonoscopy, maybe imaging, and once those are normal, they receive the IBS diagnosis — without ever checking whether bacterial overgrowth is present.

3. The Rome IV Criteria Do Not Require Ruling Out SIBO

There is no required step in the Rome IV diagnostic process that says, “Test for bacterial overgrowth before diagnosing IBS.”

That means many patients are given an IBS label before SIBO has ever been considered.

The 2022 Poon et al. meta-analysis made this point clearly: a significant proportion of patients presenting with IBS may have an organic condition accounting for their symptoms, and failing to exclude those conditions can deny patients effective treatment.

That is exactly why this matters.

SIBO vs IBS: Side-by-Side Comparison

The Food Poisoning Connection: Post-Infectious IBS and SIBO

One of the most important pieces of the SIBO vs IBS puzzle is food poisoning.

And this is one of the most overlooked conversations in gut health.

Many patients can trace the beginning of their gut problems back to a specific event:

• Food poisoning
• Traveler’s diarrhea
• A stomach bug
• A bad meal while traveling
• An infection that “never fully went away”

This matters because post-infectious IBS is one of the best-documented pathways into chronic gut symptoms.

And the mechanism connecting food poisoning to SIBO is now much better understood.

Here’s the basic sequence.

Step 1: Food Poisoning Occurs

A patient is exposed to organisms like:

• Campylobacter
• Salmonella
• E. coli
• Shigella

These organisms can produce a toxin called cytolethal distending toxin, or CDT.

Step 2: The Immune System Reacts

The immune system produces anti-CDT antibodies to fight the toxin.

That part is normal.

Step 3: Molecular Mimicry Can Occur

In some people, the immune response cross-reacts with a protein in the body called vinculin.

This leads to the production of anti-vinculin antibodies.

Step 4: Vinculin Damage Affects Gut Motility

Vinculin helps support the structural connections involving the Interstitial Cells of Cajal, or ICC.

These are the electrical pacemaker cells that help drive the Migrating Motor Complex.

Step 5: The MMC Becomes Impaired

When this system is damaged, the MMC does not sweep the small intestine the way it should.

That means bacteria and food debris are not cleared properly.

Step 6: Bacteria Accumulate and SIBO Develops

Without a properly functioning MMC, bacteria can build up in the small intestine.

That is how a single episode of food poisoning can turn into years of bloating, diarrhea, constipation, food reactions, and an IBS diagnosis.

Research estimates that about one in nine people who experience food poisoning go on to develop IBS.

But the better clinical question is this:

How many of those people actually developed SIBO because food poisoning damaged their gut’s motility system?

That is the kind of question we ask in a root-cause model.

A Clinical Perspective from Dr. Anderson

This is why history matters.

When someone comes into our clinic with chronic gut symptoms, we are not just asking, “Do you have bloating?” or “How often do you go to the bathroom?”

We want to know when it started.

Did your gut symptoms begin after food poisoning?
After traveling?
After antibiotics?
After a stomach infection?
After a stressful season of life?
After starting acid blockers?

That timeline can change everything.

Because if there is a clear post-infectious pattern, we know what to look for. We know the MMC may be involved. We know SIBO may be part of the picture. And we know that simply calling it IBS may not be enough.

The Functional Medicine Difference

The difference between functional medicine and conventional medicine is not that we deny IBS exists.

The difference is that we do not stop at the label.

IBS should be a starting point for investigation — not the final answer.

If SIBO is driving your symptoms, we want to find it.
If methane is slowing your motility, we want to know that.
If food poisoning damaged your MMC, we want to identify that pattern.
If bile acid malabsorption, food sensitivities, dysbiosis, stress physiology, or gut-brain dysfunction are involved, we want to find those too.

Because patients do not need more labels.

They need answers.

Why Treatment Differs: IBS Management vs SIBO Root-Cause Treatment

Once you understand the difference between SIBO and IBS, the difference in treatment becomes obvious.

Conventional IBS Management

Conventional IBS treatment usually focuses on reducing symptoms.

That may include:

Low-FODMAP Diet

A low-FODMAP diet can reduce symptoms by limiting fermentable carbohydrates that bacteria feed on.

This can be helpful short-term.

But it does not necessarily treat the overgrowth itself.

Antispasmodics

These medications may reduce cramping or intestinal spasms, but they do not address the underlying reason the gut is irritated.

Loperamide or Laxatives

Loperamide may be used for diarrhea. Laxatives may be used for constipation.

Again, this may help manage symptoms, but it does not explain why the bowel pattern is abnormal in the first place.

SSRIs or Tricyclic Antidepressants

These may be used to modulate gut-brain signaling and reduce visceral hypersensitivity.

For some patients, this can help. But it still does not answer the deeper question: why is the gut hypersensitive or dysregulated?

Overall, the conventional IBS model often becomes long-term symptom management with no clear endpoint.

Functional Medicine SIBO Treatment

SIBO treatment is different because it is aimed at the underlying driver.

That may include:

Targeted Herbal Antimicrobial Protocols

The protocol depends on the type of overgrowth.

Hydrogen-dominant SIBO, methane-dominant IMO, and hydrogen sulfide patterns may require different strategies.

This matters because treating every SIBO case the same way is one of the reasons patients relapse or fail treatment.

Motility Support

This is one of the most overlooked pieces of SIBO care.

If you kill the overgrowth but do not restore the Migrating Motor Complex, bacteria can come right back.

Motility support helps keep the small intestine clean between meals and reduces the risk of recurrence.

Gut Barrier Repair

SIBO frequently overlaps with intestinal permeability, often called “leaky gut.”

When the gut barrier is irritated, immune activation and inflammation can increase, leading to more food reactions, more systemic symptoms, and more gut sensitivity.

Repairing the gut lining is often a key part of long-term recovery.

Microbiome Restoration

The goal is not simply to “kill bacteria.”

The goal is to restore a healthier microbial balance.

That means rebuilding beneficial organisms, improving diversity, and supporting the gut ecosystem after treatment.

Root Cause Correction

This is the biggest difference.

We want to know why the SIBO developed in the first place.

Possible root causes include:

• Food poisoning
• Impaired MMC function
• Low stomach acid
• Proton pump inhibitor use
• Structural adhesions
• Thyroid dysfunction
• Chronic stress
• Poor bile flow
• Digestive insufficiency
• Immune dysfunction

If the root cause is not addressed, recurrence is much more likely.

Dietary Support

Diet can be useful, but it should not become the entire treatment plan.

We use nutrition strategically to reduce symptoms, support healing, and avoid unnecessary long-term restriction.

The goal is not to keep patients afraid of food forever.

The goal is to get the gut working better.

The Bottom Line

If SIBO is the engine driving your IBS symptoms, then fiber, antispasmodics, and stress-management handouts are not going to fix the problem.

They may help temporarily.

But they will not remove the overgrowth, restore motility, or correct the reason the overgrowth developed in the first place.

That is the difference between managing a label and treating a cause.

When IBS Really Is IBS Without SIBO

Intellectual honesty matters.

Not every case of IBS is SIBO.

Some patients test negative for SIBO and still have very real, life-disrupting gut symptoms.

In those cases, we keep looking.

Because even when SIBO is not present, there is still a reason the gut is not functioning properly.

Other root causes may include:

Gut-Brain Axis Dysfunction

The gut and brain are constantly communicating through the nervous system, immune system, hormones, and microbial signals.

When that communication becomes dysregulated, patients can develop visceral hypersensitivity, altered motility, pain, urgency, nausea, bloating, and bowel changes without bacterial overgrowth being the main driver.

Bile Acid Malabsorption

Bile acid malabsorption is commonly overlooked, especially in IBS-D patients.

When excess bile acids reach the colon, they can trigger diarrhea, urgency, and irritation.

Many patients labeled with IBS-D may actually need evaluation for bile acid-related diarrhea.

Food Sensitivities and Histamine Intolerance

Immune reactions to foods can cause bloating, pain, loose stools, constipation, skin symptoms, headaches, flushing, fatigue, and other systemic complaints.

Histamine intolerance can create IBS-like symptoms without SIBO being the primary cause.

Stress and HPA Axis Dysregulation

Chronic stress directly affects the gut.

It can alter motility, increase intestinal permeability, change the microbiome, and amplify pain perception.

The gut-stress connection is not “all in your head.”

It is physiology.

Large Intestine Dysbiosis

Sometimes the imbalance is not in the small intestine.

Sometimes the colon microbiome is disrupted, inflammatory, or lacking diversity.

This can produce many of the same symptoms as SIBO but requires a different treatment approach.

Even when SIBO is not present, functional medicine still looks deeper than the IBS label.

There is almost always a “why.”

Finding it changes everything.

Frequently Asked Questions: SIBO vs IBS

What is the difference between SIBO and IBS?

IBS is a symptom-based diagnosis defined by the Rome IV criteria. It describes recurrent abdominal pain with changes in bowel habits.

SIBO is a measurable condition where excessive bacteria accumulate in the small intestine.

The symptoms overlap significantly, but SIBO can be tested through breath testing and treated with targeted protocols. IBS is often managed based on symptoms alone.

Research shows that 36–49% of IBS patients may have underlying SIBO driving their symptoms.

Can SIBO be misdiagnosed as IBS?

Yes. This is extremely common.

SIBO and IBS share many of the same symptoms, including bloating, gas, abdominal pain, diarrhea, constipation, urgency, and food reactions.

Because standard gastroenterology workups do not always include SIBO breath testing, many patients carry an IBS diagnosis for years without knowing bacterial overgrowth may be part of the problem.

If you have been told you have IBS and have not improved, SIBO is worth investigating.

How do you test for SIBO?

The most common non-invasive test is a hydrogen and methane breath test.

After drinking lactulose or glucose, you provide breath samples over a 2–3 hour period.

Elevated hydrogen suggests bacterial overgrowth. Elevated methane suggests Intestinal Methanogen Overgrowth, or IMO.

At our Mesa, AZ clinic, we often combine breath testing with stool analysis and other functional labs to understand the full picture.

Can food poisoning cause SIBO?

Yes.

Food poisoning is one of the most well-documented triggers for post-infectious IBS and SIBO.

Certain bacteria produce a toxin called CDT. In some people, the immune response to that toxin cross-reacts with vinculin, a protein involved in gut motility.

When this process damages the Migrating Motor Complex, the small intestine does not clear bacteria properly.

That can allow SIBO to develop.

What is the Migrating Motor Complex and why does it matter?

The Migrating Motor Complex, or MMC, is a wave-like cleaning mechanism that moves through the small intestine every 90–120 minutes between meals.

It helps clear leftover food and bacteria out of the small intestine.

When the MMC is impaired, bacteria can accumulate and contribute to SIBO.

This is why motility support is so important in preventing SIBO recurrence.

Should I try a low-FODMAP diet for IBS?

A low-FODMAP diet can reduce symptoms temporarily by limiting fermentable foods that bacteria feed on.

But it does not fix the underlying cause.

If SIBO is driving your symptoms, the overgrowth can remain even if symptoms improve while eating a restricted diet.

Long-term FODMAP restriction may also reduce beneficial bacterial diversity.

In functional medicine, diet is used as part of a broader strategy — not as the entire treatment plan.

Where can I get tested for SIBO in Mesa, AZ?

Dr. Houston Anderson offers comprehensive SIBO breath testing, stool analysis, and functional medicine evaluation at his Mesa, AZ practice.

The clinic serves patients throughout Mesa, Gilbert, Chandler, Tempe, Scottsdale, and the greater Phoenix metro area.

Testing helps identify whether SIBO is present, what type of overgrowth may be involved, and what root causes need to be addressed for long-term improvement.

Ready to Move Beyond the IBS Label?

If you have been living with an IBS diagnosis that has not led to real improvement — if you are still bloated, still uncomfortable, still reacting to foods, still planning your life around your gut — it may be time to ask a better question.

Not just:

“How do I manage my IBS?”

But:

“Why do I have these symptoms in the first place?”

That one shift changes everything.

Dr. Houston Anderson and the team at our Mesa, AZ clinic specialize in identifying root causes behind IBS symptoms, including SIBO, gut dysbiosis, food sensitivities, bile acid issues, gut-brain axis dysfunction, and post-infectious gut dysfunction.

We serve patients throughout Mesa, Gilbert, Chandler, Tempe, Scottsdale, and the greater East Valley.

Schedule a new patient visit to find out whether SIBO may be the missing diagnosis behind your IBS.

Want to learn more about how gut dysfunction affects your brain, mood, and cognitive function? Read our in-depth guide:

The Gut-Brain Axis: How Your Gut Affects Brain Fog and Mood

Written by Dr. Houston Anderson — Functional Medicine Doctor, Chiropractor, and Sports Rehab Specialist | Mesa & Scottsdale, AZ | drhoustonanderson.com