Bile Acid Diarrhea (BAD) is a chronic digestive condition. It occurs when excessive amounts of bile acids escape the small intestine and enter the colon, where they act as irritants. This condition is frequently overlooked or misdiagnosed as Irritable Bowel Syndrome with Diarrhea (IBS-D) due to the overlap in symptoms, despite having a distinct underlying cause. Understanding the mechanism by which bile acids irritate the colon is the first step in recognizing this specific type of chronic diarrhea.
The Role of Bile Acids in Healthy Digestion
Bile acids are compounds synthesized in the liver from cholesterol and are essential for the digestion and absorption of fats and fat-soluble vitamins. They are then stored and concentrated in the gallbladder. When a person eats a meal, the gallbladder releases bile into the small intestine to help emulsify dietary fats.
The body has an efficient recycling system for these compounds, known as the enterohepatic circulation. After performing their digestive work in the small intestine, approximately 95% of bile acids are actively reabsorbed in the terminal ileum, which is the last section of the small intestine. They are then transported back to the liver.
Only a small fraction of bile acids normally reaches the large intestine, where they are typically processed by gut bacteria and excreted. In Bile Acid Diarrhea, this recycling process is disrupted, causing a much larger quantity of bile acids to flow into the colon. These unabsorbed bile acids stimulate the colon to secrete water and electrolytes, which accelerates colonic transit and results in diarrhea.
Identifying the Specific Symptoms of Bile Acid Diarrhea
The characteristics of the stools and the pattern of bowel movements can often distinguish Bile Acid Diarrhea from other forms of chronic diarrhea. The excess bile acids reaching the colon have a strong detergent and osmotic effect, which directly causes the distinct symptoms.
The diarrhea associated with this condition is often described as watery, explosive, and sometimes voluminous, with high frequency. Patients commonly report five to ten or more bowel movements per day. A defining feature is the sudden, severe urgency that often requires immediate access to a restroom.
The unabsorbed bile acids passing through the colon may also give the stool a distinct color. Stools can appear yellow, orange, or sometimes green because the bile salts are still present and have not been fully metabolized. Symptoms are often worse shortly after eating, as food triggers the release of bile from the gallbladder into the small intestine.
Primary and Secondary Causes
Clinicians often classify the causes into three main types based on the underlying mechanism.
Type 1 (Secondary Malabsorption)
Type 1 Bile Acid Diarrhea is classified as secondary malabsorption and occurs when there is a defect or damage to the terminal ileum. This damage prevents the small intestine from effectively reabsorbing the bile acids. Common causes include inflammatory bowel conditions like Crohn’s disease or surgical removal of part of the ileum, known as ileal resection.
Type 2 (Primary or Idiopathic)
Type 2 is referred to as primary or idiopathic Bile Acid Diarrhea, meaning the cause is unknown or structural damage is absent. This type is often linked to an overproduction of bile acids by the liver or a defect in the regulatory feedback loop that controls bile acid synthesis. This overproduction overwhelms the ileum’s normal reabsorption capacity, even though the ileum itself is healthy.
Type 3 (Secondary to Other Conditions)
Type 3 is secondary to other gastrointestinal conditions or surgical alterations that indirectly disrupt bile flow. These can include conditions like celiac disease, chronic pancreatitis, or, notably, gallbladder removal, known as cholecystectomy. Following gallbladder removal, bile acids flow continuously into the small intestine rather than being stored and released only upon eating, which can overwhelm the recycling system.
Confirming a Diagnosis and Treatment Pathways
Diagnosing Bile Acid Diarrhea can be challenging because its symptoms mimic other common gastrointestinal disorders. The gold standard diagnostic test is the 75-Selenium Homotaurocholic Acid Test, or SeHCAT scan, which tracks a synthetic bile acid over seven days to measure reabsorption efficiency. However, this test is not widely available in all countries, including the United States.
In settings where the SeHCAT scan is unavailable, physicians often use a therapeutic trial as an alternative diagnostic method. This involves prescribing a Bile Acid Sequestrant (BAS) medication to see if the patient’s symptoms improve significantly. A positive response provides strong clinical evidence for the diagnosis.
The primary treatment involves the use of Bile Acid Sequestrants, such as cholestyramine, colestipol, or colesevelam. These medications are designed to bind to the excess bile acids in the intestine, forming a large, insoluble complex. This binding prevents the bile acids from irritating the colon wall and is effective in reducing the frequency and urgency of diarrhea.
Patients may also find support in certain dietary adjustments, particularly managing fat intake. Since bile acid release is stimulated by the presence of fat in the diet, following a low-fat diet can reduce the overall demand for bile acid production and subsequent release.