What is bile acid malabsorption?

Bile Acid Malabsorption: Definition, Diagnosis, and Management

Bile acid malabsorption (BAM) is a condition characterized by impaired absorption of bile acids in the terminal ileum, resulting in excess bile acids entering the colon and causing chronic watery diarrhea. BAM can significantly impact quality of life and requires specific diagnosis and management approaches.

Pathophysiology

• BAM occurs when the normal enterohepatic circulation of bile acids is disrupted, leading to excess bile acids in the colon that cause secretory diarrhea 1, 2
• Bile acids are normally produced in the liver, secreted into the intestines during digestion, and approximately 95% are reabsorbed in the terminal ileum to complete the enterohepatic cycle 3, 4
• When this reabsorption is impaired, the excess bile acids in the colon stimulate electrolyte and water secretion, resulting in diarrhea 5

Classification of BAM

BAM is typically classified into three types:

1. Type 1 (Secondary BAM): Due to ileal disease or resection

   • Most commonly seen in Crohn's disease with ileal involvement or after ileal resection 6
   • Occurs in more than 80% of patients following ileal resection 6

2. Type 2 (Primary BAM/Idiopathic):

   • No structural abnormality of the ileum
   • May be related to defective production of Fibroblast Growth Factor 19 (FGF19), which normally inhibits hepatic bile acid synthesis 1
   • Results in overproduction rather than malabsorption of bile acids 1, 2

3. Type 3 (Secondary to other conditions):

   • Associated with conditions such as post-cholecystectomy, small intestinal bacterial overgrowth, celiac disease, or radiation enteropathy 6

Clinical Presentation

• Chronic watery diarrhea, often worse after meals (particularly high-fat meals) 6
• Abdominal pain and bloating 5
• In severe cases, steatorrhea (fat malabsorption) may occur 6
• Symptoms may mimic irritable bowel syndrome with diarrhea (IBS-D) 6

Diagnosis

Several diagnostic approaches are available:

• 75Se-HCAT (SeHCAT) scan: Gold standard where available

  • Measures retention of a synthetic bile acid analog after 7 days
  • Values less than 15% suggest BAM 6
  • Should be considered when response to empiric therapy fails or diagnosis is unclear 6

• Serum 7α-hydroxy-4-cholesten-3-one (C4):

  • Elevated levels indicate increased bile acid synthesis 1
  • Alternative diagnostic method where SeHCAT is unavailable 5

• Empiric therapeutic trial:

  • Treatment with bile acid sequestrants is appropriate, particularly if fecal calprotectin is not significantly raised 6
  • Clinical response to bile acid sequestrants supports the diagnosis 5

• Other tests:

  • Measurement of serum FGF19 levels 1
  • 48-hour fecal bile acid test (less commonly used) 5

Management

First-line treatment is with bile acid sequestrants:

• Cholestyramine: Effective but may be unpalatable 6

  • Binds bile acids in the intestine forming an insoluble complex excreted in feces 4
  • Prevents reabsorption of bile acids from the enterohepatic circulation 4

• Alternative sequestrants:

  • Colestipol or colesevelam can be used for patients who don't tolerate cholestyramine 6
  • These agents are more expensive but may be better tolerated 6

• Dietary modifications:

  • Low-fat diet may help reduce symptoms 7
  • In patients with hyperoxaluria (common with BAM), a diet low in fat and oxalate and high in calcium may be beneficial 6

• Loperamide:

  • Can be used as an adjunctive treatment 6

Special Considerations

• Post-ileal resection: BAM occurs in >80% of patients following ileal resection for Crohn's disease 6

  • A therapeutic trial of bile acid sequestrants is strongly recommended in these patients 6

• Differential diagnosis:

  • Small intestinal bacterial overgrowth often coexists with BAM and may require separate treatment 6
  • In patients with IBS-D symptoms with atypical features (nocturnal diarrhea, prior cholecystectomy), BAM should be considered 6

• Long-term outcomes:

  • Some patients with idiopathic BAM may experience spontaneous improvement over time 8
  • Regular follow-up is recommended as some patients may develop other gastrointestinal conditions 8

Pitfalls and Caveats

• BAM is often underdiagnosed, particularly where SeHCAT testing is not widely available 2
• An abnormal SeHCAT scan after ileal resection or with ileal inflammation does not necessarily prove that symptoms are due to BAM 6
• In severe BAM, steatorrhea may paradoxically worsen with cholestyramine treatment 6
• Bile acid sequestrants need to be stopped during the SeHCAT test 6
• BAM should be considered in patients with chronic diarrhea before labeling them as having functional bowel disorders 5