Is Your IBS Actually SIBO? How Functional Medicine Finds Out

This article is part of Dr. Harrison’s gut health series. For how root-cause digestive care works at our Westminster clinic, start with our guide to gut health programs in Denver.

By Dr. Glenn Harrison, DC, DACBN, CFMP

Doctor of Chiropractic | Diplomate, American Clinical Board of Nutrition (DACBN) | Certified Functional Medicine Practitioner (CFMP)

| Westminster, Colorado

 

The Short Answer IBS is a diagnosis based on symptoms. SIBO is a testable root cause of those symptoms. Research consistently shows that bacterial overgrowth is present in 60–80% of IBS patients — meaning most people walking around with an IBS label may actually have an undiagnosed, treatable condition driving their gut dysfunction. Functional medicine for IBS doesn’t stop at the diagnosis. It asks why — and then runs the testing to find out.

 

You’ve been told you have IBS. You’ve tried the low-FODMAP diet. You’ve taken the antispasmodics. Maybe you’ve even had a colonoscopy that came back completely normal. And yet here you are: still bloated after every meal, still running to the bathroom at the worst possible moments, still exhausted from a digestive system that refuses to cooperate.

Here is what most patients in this position have never been told: IBS is not actually a diagnosis. It is a description. It stands for Irritable Bowel Syndrome — a collection of symptoms for which conventional medicine has found no structural explanation. The problem with accepting that label at face value is that it forecloses the investigation into why the gut is behaving this way.

Functional medicine for IBS takes a fundamentally different approach. Rather than managing a symptom cluster, it investigates the specific mechanisms driving gut dysfunction — the most common and most consistently overlooked of which is SIBO: Small Intestinal Bacterial Overgrowth.

What IBS Actually Is — And What It Isn’t

IBS is classified as a “functional” gastrointestinal disorder. In medical terminology, “functional” means that standard diagnostic tests — blood work, colonoscopy, imaging — reveal no obvious structural abnormality. The bowel appears structurally intact. The symptoms, however, are very real.

To receive an IBS diagnosis, a patient must meet the Rome IV criteria: recurrent abdominal pain at least one day per week for the past three months, associated with two or more of the following:

• Pain related to defecation
• Change in stool frequency
• Change in stool form or appearance

 

IBS is then typically subcategorized by predominant bowel pattern:

• IBS-C (Constipation-Predominant): Hard, infrequent stools; bloating; abdominal discomfort relieved by bowel movements that are difficult to complete
• IBS-D (Diarrhea-Predominant): Frequent, loose stools; urgency; post-meal rushing; anxiety around access to bathrooms
• IBS-M (Mixed): Alternating constipation and diarrhea, often unpredictably

 

What the IBS diagnosis does not tell you is why any of this is happening. And that is precisely where the conventional model runs out of answers — and where functional medicine for IBS begins.

Critical Point: Accepting an IBS diagnosis without further investigation is like accepting “cough” as a final diagnosis without looking for pneumonia, asthma, or reflux. IBS describes what is happening. It does not explain why.

 

What SIBO Is — And Why It Explains So Many IBS Cases

The small intestine is meant to be a relatively bacteria-sparse environment. Its primary job is digestion and nutrient absorption, and it functions best with limited microbial competition. The large intestine, by contrast, houses the vast majority of the gut’s 38 trillion bacteria.

SIBO occurs when bacteria migrate upward and colonize the small intestine in excessive numbers. These bacteria — which do not belong there in those quantities — begin fermenting food as it passes through, producing gases (primarily hydrogen and methane) that cause:

• Bloating and visible abdominal distension, often worse after eating and later in the day
• Excess gas and flatulence, sometimes foul-smelling
• Abdominal pain and cramping
• Altered motility — either diarrhea, constipation, or both, depending on the gas type
• Nutrient malabsorption — particularly B12, iron, fat-soluble vitamins (A, D, E, K), and fat
• Fatigue and brain fog, often driven by malabsorption and systemic bacterial byproducts

 

The connection to IBS is not theoretical. Multiple studies have found bacterial overgrowth present in 60–80% of IBS patients when breath testing is performed. A landmark study published in the American Journal of Gastroenterology found that eradicating SIBO with antibiotics produced IBS symptom normalization in a substantial percentage of patients — far exceeding placebo outcomes.

In other words: for a majority of people diagnosed with IBS, SIBO may be the actual diagnosis. And SIBO is treatable when it is properly identified.

Hydrogen vs. Methane SIBO: Why the Type Matters for Treatment

Not all SIBO is the same, and the specific gas pattern identified on breath testing has direct implications for both symptom presentation and treatment approach. This is one of the reasons why functional medicine for IBS is far more precise than a standard gastroenterology workup.

Hydrogen-Dominant SIBO → IBS-D Pattern

Hydrogen gas is produced primarily by bacteria fermenting carbohydrates in the small intestine. Elevated hydrogen on breath testing is most commonly associated with diarrhea-predominant symptoms. Hydrogen accelerates small intestinal transit time — moving contents through the gut more quickly than normal — producing urgency, loose stools, and post-meal rushing that many patients experience as their “normal.”

Patients with hydrogen-dominant SIBO frequently report that symptoms are worst within 30–90 minutes of eating, particularly after high-carbohydrate or high-sugar meals, which provide the most fermentable substrate for overgrown bacteria.

Methane-Dominant SIBO (IMO) → IBS-C Pattern

Methane is produced not by bacteria but by archaea — a distinct class of microorganisms called methanogens. Methane gas has a direct inhibitory effect on intestinal motility, slowing the movement of contents through the gut and producing the constipation, bloating, and fullness characteristic of IBS-C.

This condition is now more precisely classified as Intestinal Methanogen Overgrowth (IMO) to distinguish it from bacterial SIBO, as the organisms and optimal treatment protocols differ. Methane-dominant patients often report feeling “full all day,” with bloating that builds throughout the day regardless of food intake, and bowel movements that are infrequent, incomplete, or require significant straining.

The clinical significance of distinguishing hydrogen from methane SIBO is that treatment protocols differ — both the antimicrobial agents used and the dietary approaches employed during eradication. Testing both gases is essential for accurate functional medicine for IBS treatment planning.

Hydrogen Sulfide SIBO → The Overlooked Third Gas

A third gas pattern — hydrogen sulfide — has gained increasing clinical attention in recent years. Hydrogen sulfide-producing bacteria cause a distinct presentation characterized by diarrhea, foul-smelling gas, and abdominal pain that does not fit neatly into the hydrogen or methane pattern. Newer breath testing technology now measures hydrogen sulfide alongside the traditional markers, providing a more complete picture of the bacterial environment in the small intestine.

Post-Infectious IBS: When a Gut Infection Changes Everything

One of the most important and consistently underrecognized drivers of IBS — and one that functional medicine for IBS investigates directly — is post-infectious IBS (PI-IBS).

PI-IBS develops following an acute gastrointestinal infection: food poisoning, traveler’s diarrhea, a stomach virus, or a course of antibiotics that disrupted the gut microbiome. The mechanism is now well-understood. During the acute infection, bacterial toxins — particularly CdtB (cytolethal distending toxin B) and vinculin antibodies — can damage the interstitial cells of Cajal (ICCs), which regulate the migrating motor complex (MMC).

The Migrating Motor Complex (MMC): The MMC is the gut’s “housekeeping wave” — a series of electrical impulses that sweep bacteria and undigested material from the small intestine into the large intestine between meals. When MMC function is impaired, bacteria are not cleared from the small intestine efficiently, creating the conditions for SIBO to develop and recur.

 

This is why so many patients can trace their IBS back to a specific event: “I got food poisoning in Mexico and my gut has never been the same.” Or: “I took a course of antibiotics for a sinus infection and that’s when the bloating started.” These are not coincidences — they are clinically recognized pathways to IBS and SIBO.

Functional medicine evaluation for IBS includes testing for anti-CdtB and anti-vinculin antibodies (via the IBS-Smart test), which can confirm post-infectious damage to the MMC. When this mechanism is identified, treatment must include prokinetic support — agents that stimulate the MMC and prevent SIBO recurrence after eradication — rather than simply treating the overgrowth and hoping it does not return.

This is one of the primary reasons SIBO recurrence is so common after antibiotic treatment alone: if the underlying MMC dysfunction is not addressed, bacterial overgrowth will re-establish within months of clearance.

IBS, SIBO, and the Bloating No One Can Explain

Bloating is the symptom that brings more IBS patients to functional medicine than perhaps any other. It is also one of the most dismissed symptoms in conventional gastroenterology, frequently attributed to “sensitivity,” “anxiety,” or “normal variation.”

For patients with SIBO, bloating is physiological and measurable. Gas produced by bacterial fermentation in the small intestine accumulates faster than it can be expelled, creating genuine distension — not perceived distension. Many patients describe being able to fit comfortably into their clothes in the morning and look visibly pregnant by evening. This progression across the day is a hallmark presentation of SIBO-driven bloating and is directly correlated with bacterial fermentation accumulating after meals throughout the day.

Food sensitivities compound this significantly. In patients with concurrent intestinal permeability (leaky gut), partially digested food proteins cross the gut barrier and trigger delayed immune responses — IgG and IgA mediated reactions that cause ongoing gut inflammation, worsening both bloating and IBS symptoms. Standard allergy testing does not identify these delayed sensitivities, which is why functional medicine for IBS includes food sensitivity panels as part of the evaluation.

→ [INTERNAL LINK: Gut Health Condition Page] — Learn more about how Dr. Harrison’s functional medicine approach addresses gut health, leaky gut, and intestinal permeability at his Westminster, Colorado practice.

The Thyroid-IBS Connection: Why These Two Conditions Travel Together

For patients who have both IBS symptoms and thyroid dysfunction — particularly Hashimoto’s thyroiditis or hypothyroidism — the overlap is not a coincidence. It reflects a bidirectional physiological relationship that functional medicine is uniquely equipped to address.

Thyroid hormone directly regulates gut motility. When thyroid hormone levels are suboptimal — even within conventional “normal” reference ranges — the entire digestive tract slows. This reduced motility impairs the migrating motor complex, creating the bacterial stasis that drives SIBO and IBS-C. It is one of the most common systemic causes of constipation-predominant IBS, and it is routinely missed because standard IBS workups do not include comprehensive thyroid testing.

In the other direction, gut dysbiosis and intestinal permeability are among the most potent drivers of autoimmune thyroid disease. The gut houses 70% of the immune system, and a compromised gut environment is a key factor in the loss of immune tolerance that underlies Hashimoto’s.

For patients navigating both gut and thyroid symptoms, Dr. Harrison’s Westminster practice evaluates and treats these systems together — because addressing one without the other consistently produces incomplete results.

Thyroid Condition Page — Read more about Dr. Harrison’s functional medicine approach to Hashimoto’s and thyroid disease.

IBS vs. SIBO: Side-by-Side Comparison

For patients trying to understand how these two diagnoses relate, the following comparison clarifies the key clinical distinctions:

 

	IBS (Irritable Bowel Syndrome)	SIBO (Small Intestinal Bacterial Overgrowth)
What it is	A symptom-based functional diagnosis — no identifiable structural cause	A specific, testable condition — excess bacteria in the small intestine
How it’s diagnosed	Rome IV criteria (symptom questionnaire); colonoscopy to rule out structural disease	Hydrogen/methane/hydrogen sulfide breath test; IBS-Smart antibody test
Primary symptoms	Abdominal pain, bloating, altered bowel habits	Bloating, gas, abdominal pain, altered motility, fatigue, nutrient malabsorption
IBS-C connection	Constipation-predominant subtype; cause often unknown	Methane/IMO slows motility and causes constipation — a root cause of IBS-C
IBS-D connection	Diarrhea-predominant subtype; cause often unknown	Hydrogen gas accelerates motility and causes diarrhea — a root cause of IBS-D
Post-infectious link	Recognized but rarely investigated	MMC damage from infection is a primary SIBO mechanism; testable via IBS-Smart
Conventional treatment	Low-FODMAP diet, antispasmodics, antidepressants, fiber	Rifaximin (antibiotic); neomycin for methane; herbal antimicrobials
Functional medicine treatment	Root cause investigation — test for SIBO, dysbiosis, food sensitivities, leaky gut, thyroid	Targeted eradication + prokinetics + gut repair + addressing recurrence drivers
Recurrence rate	High — without root cause resolution, symptoms return	High if MMC dysfunction, diet, or systemic factors not addressed after eradication

 

How Functional Medicine Tests for IBS and SIBO

One of the most meaningful differences patients notice when they move from conventional gastroenterology to functional medicine for IBS is the depth of the diagnostic workup. A standard IBS evaluation rules out serious disease. A functional medicine evaluation investigates what is actually causing the problem.

Dr. Harrison’s diagnostic approach for IBS and suspected SIBO typically includes:

1. SIBO Breath Test

The foundational test for suspected SIBO. Patients consume a lactulose or glucose substrate and breathe into collection tubes at timed intervals over two to three hours. The test measures hydrogen, methane, and (with newer technology) hydrogen sulfide gases. Elevated early readings indicate small intestinal fermentation rather than colonic. Results guide the specific antimicrobial protocol used for eradication.

2. GI-MAP Comprehensive Stool Analysis

A DNA-based stool test that identifies the presence and relative abundance of commensal bacteria, opportunistic pathogens, parasites, yeast, and viruses in the large intestine. Provides inflammatory markers (calprotectin, zonulin), immune markers (secretory IgA), and indicators of digestive enzyme sufficiency. This test reveals dysbiosis patterns in the colon that often coexist with SIBO and perpetuate IBS symptoms even after SIBO is addressed.

3. IBS-Smart Antibody Test

A blood test that measures anti-CdtB and anti-vinculin antibodies, which indicate prior intestinal infection and resultant MMC damage. A positive result confirms post-infectious IBS pathophysiology and signals that prokinetic therapy will be essential to prevent SIBO recurrence. This test is particularly valuable for patients who can identify a specific infection or antibiotic course as the trigger for their IBS symptoms.

4. Intestinal Permeability Panel

Assesses the integrity of the gut lining through markers including zonulin, occludin, and claudin antibodies. Intestinal permeability (leaky gut) perpetuates systemic immune activation, food sensitivities, and the inflammatory environment that sustains IBS and dysbiosis. Identifying and repairing leaky gut is often the key to resolving food sensitivities that persist even after SIBO eradication.

5. Food Sensitivity Testing (IgG and IgA)

Delayed-type food sensitivities — distinct from IgE-mediated allergies — are among the most common perpetuating factors in IBS. These reactions occur hours to days after food consumption, making them nearly impossible to identify through symptom tracking alone. Testing identifies specific trigger foods that can be temporarily eliminated while the gut heals, reducing the inflammatory load and allowing the gut lining to recover.

6. Comprehensive Thyroid Panel and Adrenal Assessment

Given the established relationship between thyroid function, gut motility, and SIBO risk, Dr. Harrison evaluates thyroid status — including Free T3, Free T4, Reverse T3, and antibodies — in IBS patients, particularly those with IBS-C. Adrenal cortisol testing is included because chronic stress dysregulates the MMC, impairs digestive enzyme output, and promotes the inflammatory gut environment that perpetuates both IBS and SIBO.

Functional Medicine Treatment for IBS and SIBO: The Protocol

Once testing identifies the specific mechanisms driving a patient’s gut dysfunction, Dr. Harrison builds an individualized protocol. While no two protocols are identical, the general framework addresses eradication, restoration, and prevention of recurrence in a structured sequence.

Phase 1: Eradication

For confirmed SIBO, antimicrobial treatment is initiated — either pharmaceutical (rifaximin for hydrogen-dominant; rifaximin plus neomycin or metronidazole for methane-dominant) or herbal antimicrobials (berberine, oregano oil, allicin, neem), which research suggests are comparably effective to rifaximin for hydrogen-dominant SIBO with a favorable side effect profile. The specific protocol depends on gas type, severity, and patient history with prior antibiotic use.

During eradication, dietary modification reduces fermentable substrate available to overgrown bacteria. A low-FODMAP, low-fermentation diet — sometimes called the SIBO-specific diet — is typically employed to reduce symptoms during treatment rather than as a long-term dietary strategy.

Phase 2: Prokinetic Support and MMC Restoration

This phase is the most commonly skipped in conventional SIBO treatment and the primary reason for the high SIBO recurrence rate. Prokinetic agents — substances that stimulate the migrating motor complex — are introduced after eradication to prevent bacterial re-migration into the small intestine.

Prokinetic options include low-dose naltrexone (LDN), iberogast (a botanical formula with strong MMC evidence), ginger, and 5-HTP. The appropriate prokinetic depends on the patient’s gas type, symptom profile, and any concurrent medications. Prokinetics are typically continued for three to six months after eradication to allow the MMC to re-establish consistent function.

Phase 3: Gut Repair

After eradication, the gut lining and microbiome require active restoration. This phase uses targeted nutrients — L-glutamine, zinc carnosine, collagen peptides, deglycyrrhizinated licorice (DGL), slippery elm, and aloe vera — to repair intestinal permeability. Probiotic and prebiotic reintroduction is timed carefully: certain probiotic strains can perpetuate bloating during active SIBO and are better introduced after successful eradication.

Phase 4: Root Cause Resolution

The final phase addresses whatever systemic factors created the conditions for SIBO in the first place. For patients with thyroid dysfunction driving motility issues, thyroid optimization is a central part of preventing recurrence. For patients with stress-driven MMC impairment, stress physiology interventions — HPA axis support, sleep optimization, vagal tone practices — are essential. For patients with ongoing food sensitivities and leaky gut, continued elimination and gut healing support is maintained until retesting confirms resolution.

Why Does My IBS Keep Coming Back? The Answer Functional Medicine Finds

If there is one question that defines the frustration of IBS patients who have already tried conventional treatment, it is this one. And the answer, in nearly every case, is that the root cause was never properly identified.

The most common reasons IBS and SIBO recur after treatment include:

• Unaddressed MMC dysfunction: Bacterial overgrowth is eradicated but the housekeeping mechanism that prevents re-colonization was never restored. Recurrence within weeks to months is virtually guaranteed without prokinetic support.
• Unresolved thyroid dysfunction: Suboptimal thyroid hormone slows gut motility, perpetuating the bacterial stasis that drives SIBO. Treating SIBO without optimizing thyroid function in hypothyroid patients reliably produces recurrence.
• Ongoing dietary triggers: Returning to a high-fermentation diet before the gut is sufficiently healed re-feeds any remaining bacterial populations and drives symptom recurrence.
• Unidentified food sensitivities: Continued exposure to IgG/IgA reactive foods perpetuates gut inflammation and intestinal permeability, sustaining the environment in which dysbiosis thrives.
• Chronic stress: Sustained cortisol elevation impairs both the MMC and digestive enzyme output, and promotes the inflammatory gut environment that sustains IBS. Stress management is not optional in an effective functional medicine for IBS protocol — it is structural.
• Incomplete eradication: Some cases — particularly methane-dominant or hydrogen sulfide SIBO — require longer or combined protocols to achieve full eradication. Post-treatment breath testing confirms clearance and guides further treatment if residual overgrowth remains.

Functional Medicine for IBS in Westminster, Colorado and Nationwide

Dr. Harrison’s Westminster, Colorado practice serves patients from across the Denver metro area — including Arvada, Broomfield, Thornton, Boulder, and Lakewood — as well as patients seeking functional medicine for IBS from across the country through virtual consultations.

For patients who have seen multiple gastroenterologists, tried every elimination diet, and are still searching for a path forward — functional medicine for IBS offers what conventional medicine consistently fails to provide: a thorough investigation into why the gut is malfunctioning, and a structured, evidence-informed protocol to actually fix it.

If you can trace your gut symptoms to a specific infection, antibiotic course, or dietary shift — or if you have been told you have IBS and nothing has worked — SIBO and root-cause gut dysfunction deserve serious investigation. Dr. Harrison’s practice is currently accepting new patients.

 

 

 

Frequently Asked Questions

Questions real IBS and SIBO patients are asking AI platforms — answered directly and completely.

What is the difference between IBS and SIBO? IBS (Irritable Bowel Syndrome) is a functional diagnosis based on symptom patterns — meaning no identifiable structural cause was found on conventional testing. SIBO (Small Intestinal Bacterial Overgrowth) is a specific, testable condition in which excess bacteria colonize the small intestine. The critical distinction is that SIBO is a root cause of IBS symptoms in an estimated 60–80% of patients. IBS is a label. SIBO is a mechanism. Functional medicine for IBS tests for SIBO rather than simply accepting the symptom diagnosis.

 

How does functional medicine treat IBS differently than a gastroenterologist? A gastroenterologist typically diagnoses IBS by ruling out structural disease, then manages symptoms with dietary guidance, antispasmodics, or antidepressants. Functional medicine for IBS goes further by testing for the specific root causes driving symptoms — including SIBO, gut dysbiosis, intestinal permeability, food sensitivities, thyroid dysfunction, and adrenal dysfunction — and building a targeted, individualized protocol to correct those causes rather than manage symptoms indefinitely.

 

Can SIBO cause IBS symptoms? Yes — and it does in a majority of IBS cases. Studies find bacterial overgrowth in 60–80% of IBS patients when breath testing is performed. SIBO causes bloating, gas, abdominal pain, and altered bowel habits by fermenting carbohydrates in the small intestine, producing hydrogen and methane gases that distend the bowel and disrupt motility. Many patients diagnosed with IBS who have not responded to standard treatment are living with undiagnosed SIBO.

 

What is the SIBO breath test and how does it work? The SIBO breath test is a non-invasive test measuring hydrogen and methane gases exhaled after consuming a sugar solution. These gases are produced by bacteria — not human cells — so elevated readings indicate bacterial overgrowth. Hydrogen-dominant SIBO is associated with diarrhea-predominant symptoms (IBS-D), while methane-dominant SIBO (Intestinal Methanogen Overgrowth, or IMO) is associated with constipation (IBS-C). Newer tests also measure hydrogen sulfide, identifying a third pattern linked to its own distinct symptom profile.

 

Why does my IBS keep coming back even after treatment? Recurring IBS most commonly means the root cause was not fully identified or addressed. The most frequent reasons include: undiagnosed SIBO that was never tested for; SIBO treated but relapsed because the migrating motor complex (MMC) dysfunction was never addressed with prokinetics; ongoing dietary triggers perpetuating dysbiosis; unaddressed food sensitivities driving chronic inflammation; and systemic factors like thyroid dysfunction or chronic stress impairing gut motility. Functional medicine investigates all of these rather than repeating the same treatment.

 

Is the low-FODMAP diet a cure for IBS? No. The low-FODMAP diet is an effective symptom management tool, but it does not address root causes. FODMAPs are fermentable carbohydrates that feed bacteria — in a patient with SIBO or dysbiosis, restricting them reduces fermentation and symptoms. But the underlying bacterial overgrowth or imbalance remains. Many patients who feel better on low-FODMAP experience significant recurrence when reintroducing foods. Functional medicine uses low-FODMAP as a short-term tool while identifying and treating the underlying condition.

 

What tests does a functional medicine doctor run for IBS? A functional medicine evaluation for IBS typically includes: SIBO breath testing (hydrogen, methane, and hydrogen sulfide), comprehensive stool analysis (GI-MAP) to assess microbiome balance and inflammation markers, IBS-Smart antibody testing for post-infectious MMC damage, intestinal permeability panels, food sensitivity testing (IgG and IgA), organic acids testing, and adrenal/cortisol panels. This identifies the specific mechanisms driving IBS symptoms and guides a genuinely targeted treatment protocol.

 

Can IBS be connected to thyroid or hormonal problems? Yes — significantly. Thyroid hormone directly regulates gut motility. Hypothyroidism and Hashimoto’s thyroiditis are among the most common systemic causes of IBS-C, because low thyroid hormone slows the entire digestive tract and impairs the migrating motor complex — creating the bacterial stasis that drives SIBO. Conversely, gut dysbiosis and intestinal permeability are known drivers of autoimmune thyroid disease. Many patients with concurrent thyroid and gut symptoms have overlapping root causes that must be addressed together.