Bile acid malabsorption (BAM) is a chronic digestive disorder where the body fails to properly reabsorb bile acids in the small intestine, leading to their excess presence in the colon. This condition, also commonly referred to as bile acid diarrhea, results from a failure in the efficient recycling system for bile acids. Though it is a frequent cause of chronic diarrhea, BAM is often underdiagnosed because its symptoms can mimic other common gastrointestinal disorders.
The Normal Role of Bile Acids in Digestion
Bile acids are compounds synthesized in the liver from cholesterol and released into the small intestine to aid in digestion. Their primary function is to act as detergents, emulsifying dietary fats and fat-soluble vitamins (A, D, E, and K) for absorption. They are stored in the gallbladder between meals and secreted into the duodenum, the first part of the small intestine, upon eating.
This process is called enterohepatic circulation, a recycling system designed to conserve the body’s bile acid supply. After completing digestion, approximately 95% of bile acids are actively reabsorbed in the terminal ileum, the last section of the small intestine. They then travel back to the liver via the portal vein to be reused.
Categorizing the Underlying Causes
The breakdown in this recycling system is categorized into three distinct types of bile acid malabsorption.
Type 1 BAM, known as secondary or ileal dysfunction, arises from structural damage to the terminal ileum. This damage often occurs due to inflammatory conditions like Crohn’s disease, surgical removal of the ileum (resection), or radiation injury. In these cases, the physical site responsible for reabsorption is compromised, preventing bile acids from being taken back into the bloodstream.
Type 2 BAM is classified as primary or idiopathic, meaning it occurs without any identifiable intestinal disease or damage. This form is linked to a regulatory defect controlling bile acid synthesis in the liver. It may involve the hormone Fibroblast Growth Factor 19 (FGF19), which normally signals the liver to slow production. Low levels of FGF19 fail to send this “stop” signal, causing the liver to overproduce bile acids that overwhelm the intestine’s reabsorption capacity.
Type 3 BAM is secondary to various other gastrointestinal disorders that affect the digestive tract. Conditions such as celiac disease, chronic pancreatitis, and small intestinal bacterial overgrowth (SIBO) can indirectly impair bile acid reabsorption. Additionally, gallbladder removal (cholecystectomy) can lead to Type 3 BAM because the continuous, unregulated flow of bile can overwhelm the reabsorption capacity.
Identifying the Symptoms
When bile acids are not adequately reabsorbed, the excess concentration spills into the large intestine, causing symptoms. The primary manifestation is chronic watery diarrhea, often called bile salt diarrhea. The unabsorbed bile acids irritate the lining of the colon, triggering the secretion of water and electrolytes into the bowel.
This influx of fluid leads to frequent, urgent, and sometimes explosive bowel movements. Other associated symptoms include abdominal cramping, bloating, and excessive flatulence. Severe malabsorption can also lead to steatorrhea, which is the presence of pale, greasy, and foul-smelling stools due to fat malabsorption.
Diagnostic Procedures and Management
Diagnosing BAM involves tests that confirm the presence of excess bile acids in the colon. The gold standard diagnostic method in many regions is the Selenium Homocholic Acid Taurine (SeHCAT) test. This nuclear medicine test involves swallowing a capsule containing a synthetic, mildly radioactive bile acid analogue, and measuring its retention after seven days. A low retention rate (often less than 10% to 15%) confirms that the bile acids are not being recycled properly.
Other methods, such as measuring serum markers like Fibroblast Growth Factor 19 (FGF19) or 7-alpha-hydroxy-4-cholesten-3-one (C4), are sometimes used when the SeHCAT test is unavailable.
The most common and effective management strategy involves the use of medications called Bile Acid Sequestrants (BAS), such as cholestyramine, colesevelam, or colestipol. These medications work by binding to the excess bile acids in the intestine, forming a complex that is too large to irritate the colon. This complex is then safely excreted in the stool. Dietary adjustments, such as adopting a low-fat diet, are often recommended because they reduce the overall amount of bile acid the body needs to produce.