Bile acid diarrhea (BAD) is a chronic digestive condition characterized by persistent, watery stools resulting from excessive bile acids entering the large intestine. While bile acids normally aid in fat breakdown, their presence in the colon acts as a powerful irritant when not properly recycled. BAD is often overlooked and misdiagnosed as diarrhea-predominant Irritable Bowel Syndrome (IBS-D). Up to one-third of individuals diagnosed with IBS-D may actually have BAD.
Bile Acids: Normal Function and Pathophysiology
Bile acids are compounds synthesized in the liver from cholesterol and are released into the small intestine after a meal. Their primary function is to facilitate the digestion and absorption of dietary fats and fat-soluble vitamins. This process is tightly regulated by the enterohepatic circulation.
Approximately 95% of bile acids are reabsorbed from the intestinal contents back into the bloodstream in the terminal ileum. They then travel back to the liver via the portal vein, where they are reused, completing the circulation cycle multiple times daily. Only a small fraction of bile acids passes into the large intestine for excretion.
BAD occurs when reabsorption is inefficient, causing excess bile acids to spill over into the colon. Once there, these unabsorbed bile acids interact with the lining, stimulating the secretion of water and electrolytes. This influx of fluid, combined with increased colonic muscle contractions, results in chronic watery diarrhea. This diarrhea is classified as secretory.
Types and Underlying Causes of Bile Acid Diarrhea
Type 1 BAD (Secondary Bile Acid Malabsorption)
Type 1 BAD is caused by a disease or surgical procedure that structurally damages the terminal ileum. Common examples include Crohn’s disease, which causes inflammation, or surgical removal of the ileum (ileal resection). Radiation enteritis, a complication of radiation therapy, can also impair the ileum’s ability to reabsorb bile acids.
Type 2 BAD (Primary Bile Acid Diarrhea)
Type 2 BAD is considered idiopathic, meaning it has no identifiable underlying cause. The reabsorption mechanism in the ileum appears structurally normal, but the liver may be overproducing bile acids. This overproduction overwhelms the ileum’s capacity to recycle them, leading to overflow into the colon. Researchers suggest a regulatory issue involving the hormone Fibroblast Growth Factor 19 (FGF19) may be implicated.
Type 3 BAD
Type 3 BAD is associated with various other gastrointestinal conditions that indirectly affect bile acid absorption or regulation. This category includes post-cholecystectomy diarrhea, which occurs after gallbladder removal and can overwhelm the system with a constant drip of bile. Other related conditions include celiac disease, chronic pancreatitis, small intestinal bacterial overgrowth (SIBO), and certain medications like metformin.
Identifying Symptoms and Diagnostic Procedures
The symptoms of bile acid diarrhea are disruptive and significantly affect quality of life. The primary symptom is chronic, watery diarrhea, often occurring multiple times per day. This diarrhea is frequently accompanied by extreme urgency, and sometimes fecal incontinence occurs due to rapid transit and colon irritation.
Individuals often experience significant abdominal cramping, bloating, and discomfort. Stools may sometimes appear pale or greasy, which can be a sign of fat malabsorption in severe cases. Since BAD mimics common disorders like IBS-D, a precise diagnosis requires specific testing.
The 75-selenium homotaurocholic acid test (SeHCAT) is widely regarded as the gold standard for diagnosis globally. This nuclear medicine test involves swallowing a capsule containing a synthetic bile acid tagged with radioactive selenium. Clinicians measure the amount of tracer retained after seven days to quantify bile acid malabsorption. A low retention rate confirms inefficient reabsorption.
When SeHCAT is unavailable, a 48-hour fecal bile acid measurement is often used. This involves collecting a stool sample over two days to directly measure the total amount of bile acids excreted. An elevated level confirms an excess of bile acids is reaching the colon. A simpler approach is an empirical diagnostic trial: if chronic diarrhea rapidly improves after starting a bile acid sequestrant, a presumptive diagnosis of BAD is often made.
Effective Treatment and Dietary Management
The primary treatment for bile acid diarrhea involves medications called Bile Acid Sequestrants (BAS). These medications work by binding to the excess bile acids in the intestinal tract, preventing them from irritating the colon lining and reducing water secretion and diarrhea.
BAS medications include:
- Cholestyramine
- Colestipol
- Colesevelam
Cholestyramine and Colestipol are typically administered as a powder mixed with liquid, while Colesevelam is available in tablet form. Side effects can include bloating, gas, and constipation, often managed by adjusting the dosage. Patients must take BAS at least one hour before or four hours after other medications, as they can interfere with the absorption of other drugs.
Dietary modifications are an important complement to medication for managing symptoms. Following a low-fat diet is often recommended because fat consumption triggers the release of bile acids from the gallbladder. Reducing the fat content of meals decreases the total amount of bile acids released.
There is a risk of malabsorption of fat-soluble vitamins (A, D, E, and K) due to the condition and the action of BAS. Physicians may recommend supplementing these vitamins to prevent deficiencies, especially during long-term use. Incorporating soluble fiber into the diet can also be helpful, as it may bind to bile acids and slow down intestinal transit.