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PubMed This is a summary of 24 peer-reviewed journal articles Updated
Gastroenterology

Biological Mimics: When It Might Not Be IBS

At a Glance

If your diarrhea-predominant IBS (IBS-D) isn't improving with standard care, you may have a biological mimic. Common mimics include Bile Acid Malabsorption (BAM), SIBO, CSID, and microscopic colitis. These conditions cause identical symptoms but require specific tests and targeted treatments.

While IBS is a real and biological condition, it is also a “syndrome”—a collection of symptoms that can sometimes be caused by other, more specific medical issues. If you have been diagnosed with IBS-D (diarrhea-predominant) but your symptoms are not improving with standard treatments, it is possible that an “IBS mimic” is the true cause of your distress [1][2].

Why the Gut Overreacts

In a healthy body, the gut and brain work together to move food along smoothly. In IBS, this system is disrupted by gut-brain axis dysregulation [3]. The nerves in your gut become over-sensitive (visceral hypersensitivity), and the balance of your gut bacteria may be off (microbiome dysbiosis) [4][3]. However, several other conditions can cause these exact same sensations and symptoms by different biological means.

Major IBS-D Mimics

If your symptoms are persistent, your care team may investigate these conditions, which require very different treatments than typical IBS:

Bile Acid Malabsorption (BAM)

Bile acids are produced by your liver to help digest fats and are normally recycled by your intestines. In BAM, these acids are not recycled properly and leak into the colon [5]. Once there, they act like a laxative, pulling water into the stool and speeding up bowel movements [6].

  • Prevalence: Research suggests that up to one-third of people diagnosed with IBS-D actually have BAM [1][7].
  • Testing: Doctors use the SeHCAT scan (the gold standard for measuring bile acid retention) or a blood test called serum C4, which measures how much bile your liver is making [8][9].

Small Intestinal Bacterial Overgrowth (SIBO)

In SIBO, bacteria that normally live in the large intestine migrate up into the small intestine [10]. When you eat, these bacteria ferment your food prematurely, producing gases and metabolites that lead to intense bloating and diarrhea [11]. It is often identified through hydrogen/methane breath tests [12].

Congenital Sucrase-Isomaltase Deficiency (CSID)

CSID is a genetic condition where your body lacks the enzymes needed to break down sucrose (table sugar) and certain starches [13]. When these sugars reach the colon undigested, they ferment, causing osmotic diarrhea (watery diarrhea caused by unabsorbed sugars pulling excess water into the bowel) and gas that perfectly mimics IBS-D [14][15].

  • Who it affects: While often diagnosed in children, many adults have milder versions (heterozygous variants) that can cause lifelong “IBS” symptoms [16][17].
  • Testing: This can be checked via a sucrose breath test or by taking a small tissue sample during an endoscopy to measure enzyme levels [18][19].

Microscopic Colitis

This is a type of inflammation in the colon that can only be seen under a microscope—your colon will look completely normal during a standard colonoscopy [20]. It causes chronic, watery diarrhea and is more common in women over age 50 [21]. Diagnosis requires taking biopsies (small tissue samples) during a colonoscopy [22].

Finding the right diagnosis is vital because treatments for these mimics—such as bile acid sequestrants for BAM or specific enzyme replacements for CSID—can often provide the relief that standard IBS management cannot [23][24].

Common questions in this guide

What if my IBS-D symptoms don't improve with standard treatments?
If standard treatments aren't working, you may have an IBS mimic. Conditions like Bile Acid Malabsorption (BAM), SIBO, or CSID can cause identical symptoms but require different medical tests and treatments to resolve.
How do doctors test for Bile Acid Malabsorption (BAM)?
Doctors typically diagnose BAM using a specialized SeHCAT scan, which measures how much bile your body retains. They can also use a blood test called serum C4 to see how much bile your liver is producing.
Could a sugar intolerance be causing my IBS symptoms?
Yes, a condition called Congenital Sucrase-Isomaltase Deficiency (CSID) makes it difficult to break down sugars and starches. When these undigested sugars reach your colon, they can cause gas and watery diarrhea that perfectly mimics IBS-D.
Can a normal colonoscopy rule out microscopic colitis?
No, a colon can look completely normal during a standard colonoscopy even if you have microscopic colitis. Your doctor must take small tissue biopsies during the procedure and examine them under a microscope to make this diagnosis.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Given that my symptoms haven't improved with standard IBS treatments, could we test for Bile Acid Malabsorption (BAM) using a SeHCAT scan or a serum C4 blood test?
  2. 2.Do you recommend a breath test for Small Intestinal Bacterial Overgrowth (SIBO) based on my bloating and diarrhea?
  3. 3.Should I be screened for Congenital Sucrase-Isomaltase Deficiency (CSID), and would a sucrose challenge test or a disaccharidase assay be appropriate?
  4. 4.Since I have persistent watery diarrhea, were biopsies taken during my colonoscopy to check for Microscopic Colitis, even if the tissue looked normal?
  5. 5.If my symptoms are related to sugar or starch malabsorption, should I try a trial of enzyme replacement therapy?

Questions For You

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References

References (24)
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    Bile acid malabsorption investigated by selenium-75-homocholic acid taurine (75SeHCAT) scans, a retrospective single-centre experience.

    Vulsteke F, De Gersem R, Arts J, Vanuytsel T

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    Prevalence of biliary acid malabsorption in patients with chronic diarrhoea of functional characteristics: a prospective study.

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This page is for informational purposes only and does not replace professional medical advice. Always consult your gastroenterologist or healthcare provider if your IBS treatments are not working.

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