Bile Acid Malabsorption: A Common but Missed Cause of Chronic Diarrhoea

For many people with Crohn’s disease, or those who have had part of their small bowel removed, chronic watery diarrhoea can persist even when scans and blood tests look reassuring. It is frustrating to be told inflammation is under control while symptoms continue regardless. In a proportion of these cases, the cause is not ongoing inflammation at all, but bile acid malabsorption (BAM), a condition that is often overlooked but relatively straightforward to test for and manage.

What is bile acid malabsorption?

Bile acids are made in the liver from cholesterol and released into the small bowel after meals to help digest and absorb fat. Normally, around ninety-five per cent are reabsorbed in the terminal ileum, the final section of the small bowel, and recycled back to the liver via the enterohepatic circulation.

BAM occurs when this reabsorption step fails. Excess bile acids pass into the colon, where they stimulate the lining to secrete water and salt and speed up muscular contractions that move stool along. The result is frequent, urgent, watery diarrhoea, often worse after eating and sometimes with a pale, oily quality to stool if fat absorption is also affected.

Why it matters for gut inflammation

BAM is not itself inflammatory, but it is closely tied to IBD because the terminal ileum, the site responsible for bile acid reabsorption, is also the area most commonly affected by Crohn’s disease. Inflammation there, or surgical removal of that section, can disrupt reabsorption even after the underlying disease has settled. Diarrhoea can therefore continue for mechanical and chemical reasons, separate from active immune activity. Recognising this distinction matters because it changes what treatment is likely to help.

Key mechanisms

Terminal ileal disease or resection

When the terminal ileum is inflamed or surgically removed, the transporter proteins that pull bile acids back into the bloodstream are reduced in number or function. The shorter or more damaged this segment, the more bile acids escape into the colon, which is why BAM is particularly common after ileal resection for Crohn’s disease.

Bile acids and colonic secretion

Excess bile acids act directly on colon lining cells, triggering release of fluid and electrolytes into the bowel. This secretory effect is separate from inflammation and explains why the diarrhoea can be sudden, watery, and unpredictable.

Effects on gut motility

Bile acids also stimulate colonic muscle contractions, speeding up transit. Faster transit gives the colon less chance to absorb water from stool, compounding the secretory diarrhoea described above.

Overlap with inflammation and other conditions

Because BAM causes symptoms similar to active IBD, such as urgency and frequent loose stools, it can be mistaken for a flare. This can lead to unnecessary escalation of anti-inflammatory treatment when the real issue is bile acid handling. It may also coexist with small intestinal bacterial overgrowth or fat malabsorption, which need separate consideration.

Diagnosis and confirming the cause

The most specific test is a SeHCAT scan, which measures how much of a labelled bile acid analogue remains in the body after seven days, giving a direct measure of retention and loss. Where unavailable, a practical response to bile acid binding medication, taken as a trial, can support the diagnosis. Blood markers such as fibroblast growth factor 19 (FGF19), a hormone regulating bile acid production, are used mainly in research rather than routine practice.

Persistent diarrhoea does not automatically mean inflammation has returned, and calm inflammatory markers do not rule out a treatable cause. BAM is one of several possibilities a gastroenterologist may investigate alongside disease activity, and should always be assessed by a clinician rather than assumed from symptoms alone.

Practical takeaways

• Keep a diary of stool frequency, urgency, and relationship to meals, as this pattern can help distinguish BAM from a flare.
• Ask your IBD team about SeHCAT testing if diarrhoea persists despite low or normal inflammation markers, particularly after ileal resection.
• Avoid assuming ongoing diarrhoea always means active disease, as this can lead to unnecessary changes in inflammatory treatment.
• If a bile acid binder is prescribed, take it consistently and around meal times, as irregular use reduces effectiveness.
• Discuss fat intake with a dietitian if stools become pale or oily, which may indicate combined fat and bile acid malabsorption.
• Review any new or worsening bowel symptoms with your clinical team rather than managing them through diet alone.

Conclusion

Chronic diarrhoea in IBD is not always a sign of active inflammation, and bile acid malabsorption is a recognised, treatable explanation that is frequently missed. Understanding the link between terminal ileal disease, bile acid recycling, and colonic function helps explain why symptoms can persist even when disease markers look stable. Testing and targeted treatment can meaningfully improve day-to-day symptom control. Long-term stability comes from identifying the correct cause of symptoms rather than assuming all diarrhoea reflects disease activity, best achieved in partnership with a gastroenterology team.

References

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This article is intended for informational and educational purposes only. It does not constitute medical advice and should not be used as a substitute for professional medical guidance, diagnosis, or treatment.

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