It may come as a surprise, but bile acids play many important roles when it comes to the microbiome, gastrointestinal health, metabolism, and a number of other physiological functions. This issue of the BIG Microbiome newsletter is dedicated to highlighting a few of these critical roles, including:
- Helping to keep gut bacteria in check and preventing small intestinal bacterial overgrowth
- Helping to maintain the integrity and health of the intestinal mucosal barrier
- Playing a key role in gut motility via the stimulation of serotonin synthesis in the colon by certain bacteria and their metabolic products
Keeping Gut Bacteria in Check
Bile acids are released into the duodenum to help emulsify fats and facilitate the action of pancreatic lipase. Most bile acids (roughly 95%) are then reabsorbed further along the small intestine (in the ileum), and then transported via the portal vein back to the liver.
But the importance of bile acids in the small intestine does not end there – there is much more to the story. Bile acids also have a major effect on gut bacteria. Their detergent-like action that is so helpful for emulsifying fats can also disrupt bacterial membranes. Many types of bacteria are sensitive bile acids, and this is one reason why the concentration of bacteria is relatively low throughout most of the small intestine (in healthy people).
When bile levels are low (e.g. from an obstruction in the bile duct), bacteria can overgrow in the small intestine, which in turn can result in increased levels of circulating endotoxin (also called lipopolysaccharide or LPS), which is a component of the cell wall of gram-negative bacteria. Elevated levels of endotoxin can cause increased systemic inflammation.
Experimental administration of bile acids has been shown to reduce small intestinal bacterial overgrowth (SIBO) in a rat model, but it is not yet clear if this approach will be beneficial for people with SIBO. However, the research does suggest that ensuring healthy liver function and consuming adequate fats (which stimulate bile release) may be important factors in maintaining normal levels of bacteria in the small intestine.
A second way in which bile acids may help to keep bacterial populations in check in the small intestine is via bile acid receptor signaling. There are receptors for bile acids on intestinal epithelial cells, and these receptors participate in signaling pathways that influence metabolism as well as some aspects of the immune response. One type of immune response that is activated by bile acids via receptor signaling is the production of antimicrobial peptides, which can also contribute to controlling bacterial populations.
A recent study showed that bile acids are also key in helping to protect against Clostridium difficile infection – by a complex mechanism that involves a non-pathogenic species of Clostridium (Clostridium scindens).
Together, the detergent activity of bile acids, the production of antimicrobial peptides by bile acid signaling, and the control of pathogens all function in helping to maintain healthy bacterial populations in the small intestine.
- The Interaction Between Bacteria & Bile
- Oral Bile Acids Reduce Bacterial Overgrowth, Bacterial Translocation and Endotoxemia in Cirrhotic Rats
- Regulation of Antibacterial Defense in the Small Intestine by the Nuclear Bile Acid Receptor
- Precision Microbiome Reconstitution Restores Bile Acid Mediated Resistance to CLostridium difficile