Bile acid malabsorption (BAM) refers to the presence of increased amounts of bile acids in the colon, resulting in symptoms. As bile acids are important in the absorption of fats, intolerance to fatty foods or even normal amounts of dietary fat are often observed. Bile acids are produced in the liver and play a major role in the absorption of dietary fats from the small intestine. About 95% of bile acids are actively reabsorbed in the lowest part of the small intestine and returned to the liver. About 5% of bile acids normally reach the colon to be excreted in the stool.
Bile acid malabsorption leads to increased amounts of bile acids in the colon, causing higher water content, raised gut permeability, accelerated transport in the colon and increased mucus secretion, amongst other effects.
These actions give rise to the typical symptoms of BAM and fat intolerance.
There are currently three types of BAM. Type 1 occurs after surgical removal of the lowest part of the small intestine, the ileum (e.g. obesity surgery or Crohn’s disease). Type 2 may well have underlying genetic causes, including abnormal regulation of a hormone factor called ﬁbroblast growth factor 19 (FGF19), which reduces liver bile acid production via farnesoid X receptor (FXR). Type 3 BAM can occur secondary to a wide range of causes, such as upper gastrointestinal surgery (e.g. after gallbladder surgery), chronic pancreatitis, celiac’s disease, small intestinal bacterial overgrowth and radiation treatment for cancer.
Frequency in population and natural history
BAM is implicated in about 25% of cases with unexplained watery diarrhea.
In patients with the diagnosis of IBS with diarrhea, BAM has been shown to be the underlying cause in between 17% and 35%. It is estimated approximately 1% of the population may have BAM.
Classic symptoms of bile acid malabsorption are watery diarrhea and often also bloating, the sudden need to rush to the toilet and cramps.
Due the sudden diarrhea, fecal incontinence (inability to hold back stool) sometimes occurs.
Testing and diagnosis
The clinically most appropriate test for BAM is the selenium homocholic acid taurine (SeHCAT) radionucleide test.
This is a nuclear medicine test with low radiation exposure, which measures the amount of radiolabeled substance remaining in the body after 7 days. A retention value below 10–15% is usually considered diagnostic of BAM. This test is only available in specialist centers, which is why the response to a trial of a bile acid-binding drug, such as cholestyramine, is often used for diagnostic purposes instead. Disadvantages of this approach are the poor taste and, sometimes, tolerability of these agents, and a miss rate of about 30%. The measurement in the blood of the bile marker, C4, and of the hormone FGF19 are being investigated and may in the near future be helpful alternatives. Stool measurement of bile acids is too cumbersome and no longer performed
In most countries the bile acid-binding drug, cholestyramine, is used for treating BAM. Success rates of 70% are achieved. As fat-soluble drugs or vitamins are also absorbed by this drug, regular monitoring by a doctor is advised. In some countries other, better tolerated bile acid-binding drugs are available, such as colesevelam or colestipol.
Other effective treatments include aluminium hydroxide (e.g. antacids) and a fat-reduced diet. The new FXR agonists are yielding early promising results