Identifying the distinguishing features of bile acid diarrhea

New research has provided much-needed insight into the nature of a particularly distressing form of chronic diarrhea, setting the foundation for improved ways to diagnose and treat it. Irritable bowel syndrome (IBS)—a debilitating group of symptoms involving recurring abdominal pain and changes in bowel movements—affects millions of people in the United States and can severely impact quality of life. Women are twice as likely to develop IBS than men. A common symptom is chronic diarrhea, called “IBS with diarrhea” or IBS-D. About a third of people with IBS-D have a more severe form of diarrhea, called bile acid diarrhea, caused by too much bile acid in the large intestine (colon). Bile acids are produced by the liver and released into the small intestine to aid in fat metabolism, then later reabsorbed in the colon for reuse. Little is known about the underlying causes of bile acid diarrhea, and many of the tests for it can be unreliable or inaccessible for many people. Also, new targeted therapies are needed because many people have difficulty adhering to current treatments, which are limited to unpleasant-tasting medicines that bind bile acids in the gut and prevent them from affecting the functions of the colon.

Researchers recently set out to identify defining characteristics of bile acid diarrhea that could lead to new ways to diagnose and treat the disorder by undertaking an in-depth analysis of chronic diarrhea in a group of 205 adults diagnosed with IBS-D, more than three-quarters of whom were women. To understand what differentiates bile acid diarrhea from other types of diarrhea, they compared people with the disorder to those who have IBS-D without bile acid diarrhea. The people with bile acid diarrhea experienced faster movement of stool through their large intestines, consistent with their more severe symptoms. Bile acid diarrhea was also associated with changes in the gut microbiome; in particular, the microbiomes of people with bile acid diarrhea were less diverse and had different compositions of bacteria than those from people without the disorder. The researchers also found several genes that were more active in people with bile acid diarrhea, including those involved in regulating inflammation and the permeability of the intestinal wall, pointing to a possible role for intestinal damage as a contributor to the symptoms of this disorder.

This study suggests that, at least in people diagnosed with IBS-D, there are fundamental, biological differences that set bile acid diarrhea apart from other types of diarrhea. The findings offer new potential markers— clinical, bacterial, and genetic—that can be used to help detect this disorder in people with chronic diarrhea. They also provide hints of the underlying causes of bile acid diarrhea, offering possible new pathways to facilitate diagnosis and treatment.