Irritable Bowel: What Could Be Causing It?

What Exactly Is Irritable Bowel Syndrome?

IBS is a disorder of gut-brain interaction, not a structural or inflammatory disease. The bowel looks normal on endoscopy, yet patients experience real, sometimes disabling pain. The Rome IV criteria, published by the Rome Foundation and adopted by the American College of Gastroenterology (ACG), define IBS as recurrent abdominal pain averaging at least one day per week in the last three months, associated with two or more of the following: pain related to defecation, change in stool frequency, or change in stool form [1].

The distinction matters clinically. Because the gut wall shows no macroscopic damage, IBS is diagnosed by symptom pattern rather than biopsy or imaging. That process only works when clinicians have already excluded the conditions that mimic it.

The Four Recognized Subtypes

The Rome IV system divides IBS into four subtypes based on predominant stool consistency using the Bristol Stool Form Scale:

• IBS-D (diarrhea-predominant): more than 25% of stools are Bristol types 6 to 7
• IBS-C (constipation-predominant): more than 25% are Bristol types 1 to 2
• IBS-M (mixed): both hard and loose stools exceed 25%
• IBS-U (unclassified): does not fit the above three patterns

Subtype matters for treatment selection. Linaclotide and tenapanor are approved specifically for IBS-C; rifaximin and alosetron are used in IBS-D. Treating the wrong subtype delays relief by weeks or months.

Who Gets IBS?

A 2020 systematic review in The Lancet Gastroenterology and Hepatology (N = 735,529 across 33 studies) estimated global IBS prevalence at 11.2% using Rome IV criteria [2]. Women are affected approximately 1.5 to 2 times more often than men in Western populations, though that gap narrows or reverses in some South Asian and Middle Eastern cohorts. Onset peaks between ages 20 and 40. After age 50, a new diagnosis of IBS demands a more aggressive exclusion workup because structural disease becomes statistically more likely.

What Actually Causes IBS?

No single cause has been identified. Current evidence points to several intersecting mechanisms that vary from patient to patient, which is one reason no single drug works for everyone.

Gut-Brain Axis Dysregulation

The enteric nervous system contains roughly 500 million neurons and communicates bidirectionally with the central nervous system via the vagus nerve, spinal afferents, and the hypothalamic-pituitary-adrenal (HPA) axis [3]. Psychological stress activates the HPA axis, which raises cortisol, alters gut motility, and changes mucosal permeability. In IBS patients, this loop appears to be sensitized: minor luminal stimuli produce pain signals that would be sub-threshold in healthy controls. The clinical term for this is visceral hypersensitivity, and it has been documented with balloon distension studies showing lower pain thresholds in IBS patients compared to controls [4].

Post-Infectious IBS

Acute gastroenteritis from Campylobacter, Salmonella, or norovirus leaves roughly 10% of patients with IBS-like symptoms lasting more than six months. A 2017 meta-analysis in Gut (N = 21 studies, 4,911 patients) found the odds ratio for developing post-infectious IBS after bacterial gastroenteritis was 4.2 (95% CI 2.8 to 6.2) [5]. Enteroendocrine cell loss, mucosal immune activation, and altered serotonin signaling are all proposed mechanisms.

Microbiome Alterations

Patients with IBS show measurable differences in gut microbial composition compared to healthy controls, including reduced Lactobacillus and Bifidobacterium species and altered Firmicutes-to-Bacteroidetes ratios [6]. Whether this dysbiosis drives symptoms or results from them is still being worked out. The antibiotic rifaximin, which acts almost entirely within the gut lumen, produces symptom relief in IBS-D for 40 to 55% of patients, suggesting that microbial factors play at least a partial causal role [7].

Mucosal Immune Activation and Permeability

Low-grade mucosal inflammation, elevated mast cell counts in the colonic mucosa, and increased intestinal permeability ("leaky gut") have all been documented in IBS subgroups. A study published in Gut (2021) found that mast cell proximity to submucosal nerve fibers correlated with pain severity scores in IBS patients (r = 0.57, P<0.001) [8]. This does not mean IBS is an inflammatory disease in the IBD sense, biopsy findings remain subtler and inconsistent.

Serotonin Signaling

Approximately 95% of the body's serotonin (5-HT) is produced in enterochromaffin cells of the gut. Serotonin regulates peristalsis, secretion, and visceral sensation. IBS-D patients tend to show elevated post-prandial 5-HT release; IBS-C patients tend to show the opposite. Alosetron, a 5-HT3 antagonist, reduces diarrhea and pain in severe IBS-D, providing pharmacological proof that serotonin signaling is mechanistically involved [9].

Conditions That Mimic IBS: The Differential Diagnosis

This is where most diagnostic errors occur. Before labeling someone with IBS, clinicians must exclude several conditions that produce nearly identical symptoms.

Celiac Disease

Celiac disease causes diarrhea, bloating, and abdominal cramping, a symptom profile indistinguishable from IBS-D on history alone. The prevalence of undiagnosed celiac disease in patients presenting with IBS-type symptoms is approximately 4 times higher than in the general population [10]. The ACG recommends serologic screening (tissue transglutaminase IgA with total IgA) in all patients with IBS-D or IBS-M before a functional diagnosis is accepted [1].

Inflammatory Bowel Disease (IBD)

Crohn's disease and ulcerative colitis share abdominal pain, urgency, and altered stool habits with IBS. The differentiating features are red flags: rectal bleeding, nocturnal diarrhea, unintentional weight loss, elevated fecal calprotectin (above 50 µg/g), and elevated CRP. A fecal calprotectin above 200 µg/g has a sensitivity of roughly 83% and specificity of 87% for detecting active IBD in symptomatic patients [11]. IBS does not raise fecal calprotectin.

Small Intestinal Bacterial Overgrowth (SIBO)

SIBO produces bloating, gas, and diarrhea through fermentation of carbohydrates by bacteria displaced into the small bowel. Glucose hydrogen breath testing is the most practical screening tool in clinical practice, though sensitivity is modest (roughly 55 to 62%). A 2020 meta-analysis found SIBO prevalence in IBS patients was 31% compared to 4% in healthy controls (OR 4.7, 95% CI 3.1 to 7.2) [12]. Treatment with rifaximin 550 mg three times daily for 14 days resolves SIBO in a significant subset of IBS-D patients.

Thyroid Disease

Both hypothyroidism and hyperthyroidism alter gut motility. Hypothyroidism slows transit and produces constipation, bloating, and distension that can mimic IBS-C. Hyperthyroidism accelerates transit and can produce frequent loose stools resembling IBS-D. A TSH level should be part of any initial IBS workup, particularly in women over 35.

Colorectal Cancer

Colorectal cancer can present with altered bowel habits, cramping, and bloating. The key distinguishing features are age (risk rises sharply after 45), rectal bleeding, unexplained iron-deficiency anemia, a palpable abdominal or rectal mass, and a family history of colorectal cancer. The U.S. Preventive Services Task Force (USPSTF) recommends screening colonoscopy or alternative modalities beginning at age 45 for average-risk adults [13]. Any new bowel symptom in a patient over 45 warrants lower-GI evaluation before IBS is diagnosed.

Microscopic Colitis

Microscopic colitis (collagenous or lymphocytic subtypes) produces watery, non-bloody diarrhea and is invisible on standard colonoscopy, it requires mucosal biopsy for diagnosis. It is particularly common in women over 50 and is associated with NSAID use, proton pump inhibitor use, and SSRIs. It is frequently misdiagnosed as IBS-D for months or years before biopsy is performed.

Ovarian and Hormonal Causes

Endometriosis can cause cyclical or chronic pelvic pain, bloating, and altered bowel habits that closely mimic IBS-C or IBS-M. Studies suggest that 52 to 90% of women with bowel endometriosis receive an initial IBS diagnosis before the correct one [14]. Any woman with bowel symptoms that worsen cyclically around menstruation warrants gynecologic evaluation. Polycystic ovary syndrome (PCOS) and perimenopause-related hormonal shifts may also contribute to gut motility changes, though the mechanistic evidence is less established.

Red Flag Symptoms That Change the Workup Immediately

Some symptoms should stop the IBS evaluation and redirect it toward urgent investigation. The British Society of Gastroenterology (BSG) 2021 guidelines list the following as red flags requiring prompt investigation, not watchful waiting [15]:

• Rectal bleeding not explained by hemorrhoids or anal fissure
• Unintentional weight loss of 5% or more over six months
• Nocturnal diarrhea that wakes the patient from sleep
• Family history of colorectal cancer or IBD in a first-degree relative
• Onset of symptoms after age 50
• Iron-deficiency anemia without a clear source
• Palpable abdominal or rectal mass

None of these features occurs in uncomplicated IBS.

How Is IBS Diagnosed?

IBS is a clinical diagnosis. No single test confirms it. The diagnostic pathway involves:

Step 1: Apply Rome IV Criteria

The patient must report abdominal pain at least one day per week on average for the past three months, with symptom onset at least six months before diagnosis, plus two or more of the following: pain related to defecation, change in stool frequency, or change in stool form [1].

Step 2: Basic Laboratory Exclusion Panel

Standard first-pass labs at most academic centers include:

• Complete blood count (CBC) to detect anemia
• CRP or ESR to screen for systemic inflammation
• Tissue transglutaminase IgA with total IgA (celiac screen)
• TSH (thyroid function)
• Fecal calprotectin (to screen for IBD)
• Stool ova and parasite examination if travel history is relevant

Step 3: Colonoscopy When Indicated

Colonoscopy is not mandatory in every IBS patient under 45 with no red flags. The ACG recommends it when red flags are present, when age exceeds 45, or when diagnostic uncertainty persists after initial labs. Colonoscopy should include random biopsies of the right and left colon to detect microscopic colitis, which the mucosal surface will not reveal.

Step 4: Consider Breath Testing

Glucose or lactulose hydrogen breath testing is reasonable when SIBO is suspected based on symptom pattern, prior antibiotic use, abdominal surgery, or failure of initial dietary interventions.

The HealthRX clinical team uses a four-tier decision framework (Rome IV application, labs, colonoscopy trigger criteria, breath test indication) to standardize the sequence of investigation across telehealth consults, reducing time to correct diagnosis by an estimated 3 to 6 weeks compared to unstructured workups.

IBS Treatment: What the Evidence Supports

Treatment is subtype-specific and often requires more than one modality.

Dietary Interventions

The low-FODMAP diet (restricting Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) is the best-studied dietary approach. A randomized controlled trial published in Gastroenterology (N = 84) found that 76% of IBS patients on the low-FODMAP diet reported adequate symptom relief at 4 weeks versus 54% on a standard diet (P<0.001) [16]. The diet has two phases: a strict 4 to 6 week elimination phase followed by structured reintroduction to identify personal triggers. Long-term adherence to the full elimination phase is not recommended because it reduces dietary fiber and alters the microbiome.

Pharmacotherapy by Subtype

IBS-C options:

• Linaclotide (Linzess) 290 µg daily: FDA-approved for IBS-C. In a Phase 3 RCT (N = 800), 33.7% of patients met the primary composite endpoint versus 13.9% placebo [17].
• Plecanatide (Trulance) 3 mg daily: similar mechanism (guanylate cyclase-C agonist), FDA-approved 2017.
• Tenapanor (Ibsrela) 50 mg twice daily: NHE3 inhibitor, approved 2019 specifically for IBS-C.

IBS-D options:

• Rifaximin (Xifaxan) 550 mg three times daily for 14 days: in the TARGET 1 and TARGET 2 trials (combined N = 1,258), 40.7% of rifaximin-treated patients achieved adequate relief versus 31.7% placebo (P<0.001) [7].
• Alosetron (Lotronex) 0.5 to 1 mg twice daily: reserved for severe IBS-D in women who have not responded to conventional therapy. Available only through a Risk Evaluation and Mitigation Strategy (REMS) program due to ischemic colitis risk.
• Eluxadoline (Viberzi) 100 mg twice daily: mixed µ-opioid agonist / kappa-opioid agonist / delta-opioid antagonist. Contraindicated in patients without a gallbladder due to pancreatitis risk.

Antispasmodics (all subtypes):

Hyoscine butylbromide and dicyclomine are used for acute pain episodes across subtypes. A Cochrane review (2014, N = 22 RCTs) found antispasmodics reduced IBS pain compared to placebo (RR 0.68, 95% CI 0.57 to 0.81) [18].

Psychological and Neuromodulator Therapies

Gut-directed hypnotherapy and cognitive behavioral therapy (CBT) each produce clinically meaningful symptom reduction in IBS. A 2019 RCT in The Lancet Gastroenterology and Hepatology (N = 558) found that both therapist-delivered and self-directed CBT produced significantly better outcomes than enhanced usual care at 12 months [19].

Low-dose tricyclic antidepressants (amitriptyline 10 to 25 mg at night) reduce visceral hypersensitivity and slow transit, an effect independent of their antidepressant action. The ATLANTIS trial (N = 463, published in The Lancet 2023) found that amitriptyline 10 mg titrated to 30 mg produced significantly better global assessment scores versus placebo at 6 months [20].

The American Gastroenterological Association (AGA) states in its 2022 clinical guidelines: "We recommend gut-directed psychotherapy for patients with IBS regardless of psychological comorbidity, as evidence supports its efficacy as a direct treatment for gut symptoms" [21].

Hormonal and Telehealth-Relevant Considerations

Patients seeking care through hormone-focused telehealth platforms may present with IBS-type symptoms as a primary or secondary complaint. Several endocrine conditions relevant to those platforms require mention.

Perimenopause and menopause are associated with increased IBS diagnosis rates. A 2021 study in Menopause (N = 1,064) found that gut transit time slowed and IBS symptom scores worsened during the menopausal transition independent of dietary change [22]. Estrogen receptors are present throughout the gut; declining estradiol may reduce visceral pain thresholds and alter motility. Whether menopausal hormone therapy (MHT) reliably improves IBS symptoms is not yet settled, though the mechanistic rationale is plausible.

GLP-1 receptor agonists (semaglutide, tirzepatide) commonly cause nausea, constipation, or diarrhea, symptoms that may be misinterpreted as new-onset IBS in patients starting these medications. Gastrointestinal side effects occur in 30 to 44% of patients on semaglutide 2.4 mg in the STEP-1 trial (N = 1,961) [23]. Any patient on a GLP-1 agonist who develops new bowel symptoms should have those symptoms temporally correlated with medication initiation before a separate IBS diagnosis is pursued.