IBS Root Causes: What Your Diagnosis Leaves Out

Quick Summary:

IBS is diagnosed by symptom pattern using the Rome IV criteria after other diseases have been ruled out. The label describes what you feel and leaves the cause unnamed, which is why two people with the same diagnosis can need completely different treatment.
Hidden drivers are common. Pooled data show about 38 percent of people with IBS test positive for SIBO, and about 28 percent of people with IBS-D have bile acid malabsorption [3][4]. Neither one shows up on a colonoscopy.
IBS often starts with an infection. About 1 in 10 people develop IBS after a bout of infectious gastroenteritis, roughly four times the risk of people who never got sick [5]. That history belongs in every IBS workup.
Standard treatment is usually chosen by stool type: fiber and laxatives for constipation, antidiarrheals for diarrhea, antispasmodics for pain. A root cause approach tests for what is driving the pattern, then treats in order, starting with diet and lifestyle.

Six years of urgent mornings. That's how one patient described her IBS the first time we met.

She was 38, a trial attorney who knew every courthouse bathroom in Maricopa County and skipped breakfast on hearing days.

Two colonoscopies had come back clean. Her gastroenterologist diagnosed IBS with diarrhea, recommended more fiber, and suggested she work on her stress.

When I took her history, one detail stood out. Her symptoms started three weeks after a case of food poisoning on vacation. Nobody had asked about it.

A breath test showed hydrogen-dominant SIBO, and over the next four months her urgency and bloating improved substantially once we treated the overgrowth and rebuilt her gut motility.

I hear some version of her story every week. IBS is one of the most common diagnoses in gastroenterology, and for many people it's where the investigation stops.

This guide explains what an IBS diagnosis tells you, what it leaves out, and what a root cause approach looks for next.

What IBS Actually Means

Irritable bowel syndrome (IBS) is classified as a disorder of gut-brain interaction. That's the current medical term for conditions where the gut looks normal on a scope but doesn't work normally.

Doctors diagnose IBS with the Rome IV criteria. You need recurrent abdominal pain at least one day per week over the last three months, and the pain has to be linked to bowel movements or to a change in how often you go or what your stool looks like. Symptoms need to have started at least six months before diagnosis [1].

IBS is common. In a Rome Foundation survey of more than 54,000 adults across 26 countries, about 4 percent met the Rome IV criteria [2]. Many people with IBS never see a doctor for it at all.

Here's what most patients never hear: IBS is a pattern, and the diagnosis is made by recognizing that pattern once red flags are ruled out.

Two people can meet identical criteria for completely different reasons. One has bacteria overgrowing the small intestine. Another has bile acids spilling into the colon. A third has pelvic floor muscles that won't relax during a bowel movement. The same label can sit on top of very different problems, and each one needs its own treatment.

The Four Types of IBS

Rome IV sorts IBS into subtypes based on what your stool looks like on the days your bowels are abnormal [1]. The subtype matters because it points toward different root causes.

IBS-D (diarrhea-predominant)

More than a quarter of abnormal bowel movements are loose or watery, and fewer than a quarter are hard. In my practice, IBS-D most often traces back to hydrogen-dominant SIBO, bile acid malabsorption, a past gut infection, or an undiagnosed food intolerance.

IBS-C (constipation-predominant)

More than a quarter of abnormal bowel movements are hard or lumpy, and fewer than a quarter are loose. IBS-C points me toward slow motility, methane-producing microbes (now called intestinal methanogen overgrowth, or IMO), pelvic floor problems, low thyroid function, and medications that slow the gut [11].

IBS-M (mixed)

Both hard and loose stools each show up in more than a quarter of abnormal bowel movements. Mixed patterns are common with SIBO, where different microbes and different meals push the gut in opposite directions.

IBS-U (unclassified)

Symptoms meet the IBS criteria but don't fit cleanly into the other three groups.

Subtypes can shift over time, especially after antibiotics, a new medication, pregnancy, or perimenopause. If yours has changed, the drivers may have changed too.

Symptoms, Overlaps, and Warning Signs

The core IBS symptoms are abdominal pain tied to bowel movements, bloating, urgency, a feeling of incomplete emptying, and mucus in the stool.

Many of my patients also deal with reflux, early fullness after meals, fatigue, brain fog, poor sleep, and anxiety. Those overlaps are clues. Symptoms that span several organ systems usually share an upstream cause.

Some symptoms need a conventional workup before anything else. See a gastroenterologist promptly if you have blood in your stool, unintentional weight loss, iron deficiency anemia, diarrhea that wakes you from sleep, fever, new symptoms after age 50, or a family history of colon cancer, inflammatory bowel disease, or celiac disease [8].

These alarm features point away from IBS and toward conditions a scope is designed to find.

What Your Normal Colonoscopy Did and Didn't Rule Out

A clean colonoscopy is genuinely good news.

It rules out colon cancer, polyps, and visible inflammatory bowel disease, and with biopsies it can catch microscopic colitis. Those are serious conditions, and conventional gastroenterology does an excellent job finding them.

A colonoscopy can't see the small intestine, where SIBO lives.

It can't measure how your body handles bile acids, how fast food moves through you, how much digestive enzyme your pancreas makes, or which foods ferment in your gut.

Celiac disease needs a blood test and biopsies from the small intestine on an upper endoscopy. So your colonoscopy answered the cancer question. The IBS question usually needs different tools.

Seven Root-Cause Drivers Behind an IBS Diagnosis

1. SIBO and IMO

Small intestinal bacterial overgrowth (SIBO) means too many bacteria are living in the small intestine, where they ferment food early and produce gas. A 2018 meta-analysis of 50 studies found SIBO in about 38 percent of people with IBS, with nearly five times the odds seen in healthy controls [3]. Methane-dominant overgrowth is linked to constipation [11]. SIBO is common enough and complex enough that it gets its own guide and its own podcast episode.

2. Post-Infectious IBS

A single bad bout of food poisoning can reset how your gut behaves for years. A meta-analysis of 45 studies found that about 10 percent of people developed IBS after infectious enteritis, with roughly four times the risk of people who never had the infection [5].

One proposed mechanism involves a bacterial toxin called CdtB, which can trigger antibodies that damage the nerves controlling gut motility. A blood test for anti-CdtB and anti-vinculin antibodies was developed to help identify this pattern in IBS-D [6]. The research is still developing, and I use it as one piece of the picture.

3. Bile Acid Malabsorption

Your liver makes bile acids to digest fat, and your small intestine normally reabsorbs most of them. When that recycling fails, bile acids reach the colon and cause watery diarrhea and urgency. A meta-analysis found bile acid malabsorption in about 28 percent of people diagnosed with IBS-D [4]. It's especially common after gallbladder removal, and it responds well to targeted treatment once someone thinks to test for it.

4. Celiac Disease and Gluten

Celiac disease and IBS share many symptoms. A meta-analysis of 36 studies found people with IBS symptoms had about 4.5 times the odds of biopsy-proven celiac disease compared with healthy controls, though the North American studies in that analysis didn't show the same increase [7]. Testing is simple, and I test anyway. I was that patient. My own celiac diagnosis came after three years of being told my labs were fine.

5. Dysbiosis and Food Fermentation

An imbalanced gut microbiome changes how you ferment carbohydrates, how much gas you make, and how sensitive your gut lining becomes. FODMAPs are a group of fermentable carbohydrates found in foods like onions, garlic, wheat, apples, and beans, and they drive symptoms in many people with IBS. Stool testing can show which organisms are overgrown, which beneficial species are missing, and whether there are signs of inflammation or poor digestion.

6. Motility and Pelvic Floor Problems

Between meals, your small intestine runs a cleaning wave called the migrating motor complex (MMC). Constant snacking, infections, low thyroid function, and some medications can weaken it, which sets the stage for SIBO. In IBS-C, the problem is sometimes at the exit. Dyssynergic defecation means the pelvic floor muscles tighten when they should relax, and no amount of fiber fixes a muscle coordination problem. Pelvic floor physical therapy does.

7. The Gut-Brain Connection

Your gut has its own nervous system, and it talks to your brain constantly through the vagus nerve. In IBS, that conversation often becomes oversensitive, so normal gas and stretching register as pain. Stress changes gut motility and pain signaling in measurable ways. That's physiology, and it's why gut-directed psychotherapy has real evidence behind it for IBS [8].

The Hormone Connection

Many women notice their IBS gets worse right before or during their period, and many notice new or changing symptoms in perimenopause. Estrogen and progesterone affect gut motility and pain sensitivity. Low thyroid function slows the entire digestive tract. In men, gut inflammation and poor microbial diversity track with lower testosterone. When hormones and digestion shift together, I evaluate both, because the gut and hormones influence each other in both directions.

Why the Standard Treatment Falls Short

The conventional IBS toolkit is real and many of its tools work. The 2021 American College of Gastroenterology guideline supports soluble fiber, peppermint oil, antispasmodics, a limited low-FODMAP diet, prescription secretagogues for constipation, rifaximin for IBS without constipation, low-dose tricyclic antidepressants, and gut-directed hypnotherapy [8]. The same guideline recommends a positive diagnostic strategy over an endless cycle of exclusion testing [8].

The problem is how these tools usually get chosen. Treatment is picked by stool type and symptom, one medication at a time, with the driver left unidentified. Constipation gets a laxative. Diarrhea gets loperamide. Pain gets an antispasmodic. If the first choice doesn't help, you try the next.

Look at rifaximin. In two large trials, 40.7 percent of patients reported adequate relief of global IBS symptoms, compared with 31.7 percent on placebo [9]. That's a genuine benefit. It also tells you rifaximin works best when you know who has bacterial overgrowth, and that relapse is likely when the reason the bacteria overgrew in the first place goes unaddressed.

The low-FODMAP diet ranked first among dietary approaches for IBS in a 2022 network meta-analysis [10]. It was designed as a short elimination phase followed by careful reintroduction. In my practice I meet patients who have stayed on a restricted version for years, eating a shrinking list of foods and starving the gut bacteria that need fiber to thrive.

Here's my position: an IBS diagnosis should be where the investigation begins. For most patients I see, it was where the investigation ended.

How I Approach IBS in My Practice

My approach follows the naturopathic therapeutic order: diet and lifestyle first, then targeted supplements, then pharmaceuticals when they're the right tool. As a naturopathic physician with prescribing authority in Arizona, I use all three.

Step 1: A history that goes back to the beginning

With new patients, we're having a 60-90 minute discussion. I want to know exactly when your symptoms started and what happened right before: an infection, a course of antibiotics, gallbladder surgery, a new medication, a move, a loss. The trigger is often the most important clue in the case.

Step 2: Testing for drivers

Depending on your pattern, that can mean SIBO breath testing, comprehensive stool analysis, celiac blood work, bile acid testing when diarrhea dominates, fecal calprotectin to screen for inflammation, pancreatic elastase, thyroid labs, and iron studies. If this pattern sounds familiar, it's worth getting properly tested before trying another medication.

Step 3: Diet and daily habits

When FODMAPs are a factor, I use a structured elimination with a planned reintroduction so the diet ends up as broad as your gut allows. I often recommend spacing meals 3-4 hours apart so the migrating motor complex can do its job. Chewing thoroughly, eating sitting down, and protecting sleep matter more than most people expect.

Step 4: Targeted treatment

When testing confirms SIBO, treatment may involve herbal antimicrobials or rifaximin, followed by support for motility to lower the risk of relapse. For pain and spasm, research supports enteric-coated peppermint oil, and it's worth discussing with your practitioner [12]. In my practice I often recommend partially hydrolyzed guar gum for patients who can't tolerate other fibers. Bile acid binders are an option when bile acid malabsorption is confirmed.

Step 5: The nervous system and hormones

I address vagal tone, stress physiology, and sleep, and I refer to gut-directed hypnotherapy or pelvic floor physical therapy when the pattern calls for it. When hormones are part of the picture, I evaluate them alongside the gut.

Many of my IBS patients already have a gastroenterologist, and some have a therapist, a gynecologist, and an endocrinologist too. My job is to act as the clinical quarterback who puts those findings together into one plan.

IBS, SIBO, and Functional GI Hub Directory

The Bottom Line on IBS

IBS is a real condition with real suffering behind it, and the diagnosis describes a pattern of symptoms. The cause behind the pattern is often findable: bacterial overgrowth, a past infection, bile acids, celiac disease, an imbalanced microbiome, a motility or pelvic floor problem, or a nervous system stuck on high alert.

A normal colonoscopy is the right first step, and it rules out the conditions that matter most to rule out. What comes next is testing for the drivers and treating them in order, least invasive first. If you've been told you have IBS and handed a fiber supplement, you have more options than you've been shown.

Is IBS a real diagnosis or just a label?

Quick Answer: It's a real diagnosis that describes a symptom pattern without naming its cause.

Full Answer: IBS is diagnosed with the Rome IV criteria, which require recurrent abdominal pain linked to bowel movements over at least three months [1]. The diagnosis confirms that your symptoms fit a recognized pattern and that red flags have been ruled out. It doesn't identify why the pattern exists. SIBO, bile acid malabsorption, a past gut infection, and celiac disease can all produce symptoms that meet IBS criteria [3][4][5][7].

Can IBS go away completely?

Quick Answer: For many patients, yes, when the underlying driver is found and treated.

Full Answer: IBS managed only by symptom tends to come and go for years. In my practice, patients whose IBS traces back to a specific driver, such as SIBO or bile acid malabsorption, often see lasting improvement once that driver is treated and the reasons it developed are addressed. Some patients have more than one driver, and progress comes in stages.

What is the difference between IBS and SIBO?

Quick Answer: IBS is a symptom-based diagnosis, and SIBO is one of its most common causes.

Full Answer: SIBO means too many bacteria are living in the small intestine. It is diagnosed with a breath test, while IBS is diagnosed by symptoms. A meta-analysis found about 38 percent of people with IBS test positive for SIBO [3]. If you've been diagnosed with IBS, especially with bloating, a SIBO breath test is worth discussing with your practitioner.

Why did my IBS start after food poisoning?

Quick Answer: Gut infections are one of the best-documented triggers for IBS.

Full Answer: About 10% of people develop IBS after infectious gastroenteritis, roughly four times the risk of people without an infection [5]. Infections can damage the nerves that control gut motility, change the microbiome, and leave the gut lining more sensitive. Post-infectious IBS often overlaps with SIBO, which is why the timing of your first symptoms matters.

What tests should I ask about if I've been diagnosed with IBS?

Quick Answer: It depends on your subtype, but several tests are worth discussing.

Full Answer: Celiac blood work is reasonable for most people with IBS [7]. For diarrhea-predominant IBS, ask about bile acid testing and fecal calprotectin. For bloating or mixed symptoms, ask about a SIBO breath test. Comprehensive stool testing, thyroid labs, and iron studies can round out the picture. Your practitioner can help decide which tests fit your history.

Does the low-FODMAP diet work for IBS?

Quick Answer: Yes, for many people, when it's done as a short elimination followed by reintroduction.

Full Answer: A 2022 network meta-analysis ranked the low-FODMAP diet first among dietary approaches for IBS symptoms [10]. The diet was designed to identify your specific triggers, then reintroduce everything you tolerate. Staying highly restricted long term can reduce the fiber your beneficial gut bacteria depend on. Working with a practitioner makes the reintroduction phase far more useful.

Is IBS caused by stress?

Quick Answer: Stress can worsen IBS, but it's rarely the whole story.

Full Answer: The gut and brain communicate constantly, and stress changes gut motility and pain signaling. That's why gut-directed psychotherapy helps many people with IBS [8]. Stress alone rarely explains years of symptoms, though. If you've been told your IBS is only stress, it's worth testing for SIBO, bile acid malabsorption, and celiac disease before accepting that answer.

Your IBS Has a Cause. Let's Find It.

Dr. Wool's Gut Repair Plan starts with extended visits, specialty testing conventional workups rarely include, and a treatment plan built around root cause rather than symptom management.

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Disclaimer:

The information provided on this page is for educational purposes only and is not intended as medical advice, diagnosis, or treatment. Dr. Dan Wool nor his affiliates do not make claims about the effectiveness of supplements, peptides, hormones or other therapies outside of the contexts supported by cited clinical evidence and regulatory approval. Always consult a qualified healthcare provider before starting, changing, or stopping any medical or wellness program.

References

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2. Sperber AD, Bangdiwala SI, Drossman DA, et al. Worldwide prevalence and burden of functional gastrointestinal disorders, results of Rome Foundation Global Study. Gastroenterology. 2021;160(1):99-114.e3. https://doi.org/10.1053/j.gastro.2020.04.014

3. Chen B, Kim JJ, Zhang Y, Du L, Dai N. Prevalence and predictors of small intestinal bacterial overgrowth in irritable bowel syndrome: a systematic review and meta-analysis. J Gastroenterol. 2018;53(7):807-818. https://pubmed.ncbi.nlm.nih.gov/29761234/

4. Slattery SA, Niaz O, Aziz Q, Ford AC, Farmer AD. Systematic review with meta-analysis: the prevalence of bile acid malabsorption in the irritable bowel syndrome with diarrhoea. Aliment Pharmacol Ther. 2015;42(1):3-11. https://pubmed.ncbi.nlm.nih.gov/25913530/

5. Klem F, Wadhwa A, Prokop LJ, et al. Prevalence, risk factors, and outcomes of irritable bowel syndrome after infectious enteritis: a systematic review and meta-analysis. Gastroenterology. 2017;152(5):1042-1054.e1. https://pubmed.ncbi.nlm.nih.gov/28069350/

6. Pimentel M, Morales W, Rezaie A, et al. Development and validation of a biomarker for diarrhea-predominant irritable bowel syndrome in human subjects. PLoS One. 2015;10(5):e0126438. https://doi.org/10.1371/journal.pone.0126438

7. Irvine AJ, Chey WD, Ford AC. Screening for celiac disease in irritable bowel syndrome: an updated systematic review and meta-analysis. Am J Gastroenterol. 2017;112(1):65-76. https://pubmed.ncbi.nlm.nih.gov/27753436/

8. Lacy BE, Pimentel M, Brenner DM, et al. ACG clinical guideline: management of irritable bowel syndrome. Am J Gastroenterol. 2021;116(1):17-44. https://doi.org/10.14309/ajg.0000000000001036

9. Pimentel M, Lembo A, Chey WD, et al. Rifaximin therapy for patients with irritable bowel syndrome without constipation. N Engl J Med. 2011;364(1):22-32. https://doi.org/10.1056/NEJMoa1004409

10. Black CJ, Staudacher HM, Ford AC. Efficacy of a low FODMAP diet in irritable bowel syndrome: systematic review and network meta-analysis. Gut. 2022;71(6):1117-1126. https://doi.org/10.1136/gutjnl-2021-325214

11. Pimentel M, Saad RJ, Long MD, Rao SSC. ACG clinical guideline: small intestinal bacterial overgrowth. Am J Gastroenterol. 2020;115(2):165-178. https://doi.org/10.14309/ajg.0000000000000501

12. Ingrosso MR, Ianiro G, Nee J, et al. Systematic review and meta-analysis: efficacy of peppermint oil in irritable bowel syndrome. Aliment Pharmacol Ther. 2022;56(6):932-941. https://doi.org/10.1111/apt.17179