Motility and Health of the Intestinal Lining
As we saw above, bile acids can have a large impact on gut microbiota. But gut bacteria also have an impact on bile acids; the interaction is two-way. Different types of gut bacteria produce enzymes that can modify bile acids in different ways, producing what are called secondary bile acids. Some of these actions take place in the small intestine, while others take place in the colon.
A recent study showed that in mice, some species of Clostridia (a group of bacteria that was highlighted in the last issue of this newsletter) increased serotonin synthesis in enterochromaffin cells of the colon, resulting in faster intestinal transit rates. The researchers found that this effect was due to a secondary bile acid (deoxycholic acid, or DCA) produced by these Clostridia. This suggests that normal gut motility depends upon sufficient bile and sufficient amounts of Clostridia species to enable the conversion to DCA, and that too little – or too much – of either bile or Clostridia might contribute to constipation or diarrhea.
Another recent study showed that colonic bacteria can also promote motility via the production of short-chain fatty acids (SCFAs, which are produced by gut bacteria from fermentable carbohydrates). In this study, SCFAs were shown to also promote the production of serotonin in enterochromaffin cells.
Bile acids also play a role on fostering a healthy intestinal mucosal lining. As discussed above, one way that contribute to a healthy intestinal lining is by helping to control the growth and composition of the gut microbiota, but they can also promote intestinal health in other ways, via bile acid receptor signaling. Bile acid signaling via these receptors regulates the expression of genes involved in the integrity of the intestinal lining. Interestingly, in this study, they showed that licorice root extract containing glycyrrhizic acid could activate one of the bile acid receptors and promote the health of the stomach lining (licorice root extracts have been used to promote healing from peptic ulcers).
A Final Note on Bile Acids
The biological roles of bile acids and their derivatives is very complex and there is still much to be learned as their roles in health and dysfunction are clarified. It should be noted that bile acids and their derivatives can also have negative effects, such as promoting damage to intestinal cells at high concentrations (or in cases where the mucosal lining is already compromised by other factors). Some of the secondary bile acids produced by gut bacteria have also been linked to colon cancer. However, we still have a lot to learn about the conditions under which bile acids are beneficial vs. potentially harmful. As with most things in biology, too little or too much may increase the risk of harmful effects.