Bile acids are cholesterol-derived amphipathic molecules synthesized in the liver, secreted into the duodenum, and extensively modified by gut bacteria. The host-microbe co-metabolism of bile acids creates a signaling network with profound effects on lipid metabolism, glucose homeostasis, immune regulation, and cancer risk. Alterations in the bile acid pool are documented in cardiovascular disease, inflammatory bowel disease, multiple sclerosis, colorectal cancer, and metabolic disease.
Primary Bile Acids (Host-Produced)
Synthesized in hepatocytes from cholesterol via CYP7A1 (classic pathway) or CYP27A1 (alternative pathway):
- Cholic acid (CA): conjugated with glycine or taurine to form glycocholic/taurocholic acid.
- Chenodeoxycholic acid (CDCA): the most potent endogenous FXR agonist; conjugated similarly.
Conjugated primary bile acids are secreted into bile, stored in the gallbladder, and released into the duodenum postprandially to emulsify dietary fats. ~95% are reabsorbed in the terminal ileum via ASBT (apical sodium-dependent bile acid transporter) and recycled to the liver -- the enterohepatic circulation.
Microbial Bile Acid Transformation
The ~5% of bile acids escaping ileal absorption enter the colon, where bacteria perform transformations creating secondary bile acids:
Deconjugation (BSH Activity)
- Bile salt hydrolase (BSH) removes the glycine/taurine conjugate. Found in collinsella, Lactobacillus, Clostridium, bifidobacterium, Listeria, and Bacteroides [ryan 2017 bile acids gut microbiome cardiometabolic interactions].
- Collinsella is notable for its BSH activity and is enriched in atherosclerotic patients.
- Deconjugation is required before further microbial modifications can occur.
7-alpha-Dehydroxylation
- Performed primarily by Clostridium scindens and related Clostridium cluster XIVa species.
- Converts CA to deoxycholic acid (DCA) and CDCA to lithocholic acid (LCA).
- DCA and LCA are the dominant secondary bile acids in the human colon.
Other Transformations
- Epimerization: CDCA to ursodeoxycholic acid (UDCA) by certain Clostridium species.
- Oxidation/reduction at various hydroxyl positions.
- Esterification by gut bacteria, affecting bile acid solubility and activity.
Receptor Signaling
FXR (Farnesoid X Receptor)
- Nuclear receptor activated most potently by CDCA, then CA, DCA.
- Metabolic effects: Decreases triglycerides, decreases insulin resistance, increases reverse cholesterol transport, decreases blood pressure via iNOS expression [ryan 2017 bile acids gut microbiome cardiometabolic interactions].
- Gut barrier: Maintains epithelial integrity; FXR-deficient mice have increased intestinal permeability.
- Immune modulation: Suppresses NF-kB-driven inflammation in intestinal epithelium.
- Feedback regulation: Induces FGF15/19 (mouse/human), which suppresses CYP7A1, reducing bile acid synthesis.
TGR5 (GPBAR1)
- Membrane receptor activated most potently by LCA, then DCA, CDCA, CA.
- Anti-atherosclerotic: Inhibits NF-kB in macrophages, reduces monocyte infiltration, decreases foam cell formation [ryan 2017 bile acids gut microbiome cardiometabolic interactions].
- Metabolic: Increases GLP-1 secretion from enteroendocrine L-cells, improving glucose homeostasis.
- Anti-inflammatory: Reduces pro-inflammatory cytokine production.
Disease Connections
Colorectal Cancer
- DCA and LCA at high colonic concentrations promote CRC through DNA damage, ROS generation, NF-kB activation, and Wnt/beta-catenin pathway stimulation [ajouz 2014 secondary bile acids colon cancer].
- High-fat diets increase DCA production; this is a key mechanistic link between Western diet and CRC risk.
- UDCA (ursodeoxycholic acid) is chemoprotective and may counteract DCA/LCA toxicity.
Cardiovascular Disease
- Ratio of primary to secondary bile acids linked to CVD severity [hoffelner 2025 emerging therapy targets microbiome cvd].
- FXR agonists (OCA, CDCA) reduce blood pressure in hypertensive models.
- BSH-expressing Lactobacillus reuteri NCIMB 30242 reduced LDL-C, non-HDL-C, and apoB100 in clinical trials [ryan 2017 bile acids gut microbiome cardiometabolic interactions].
- Altered serum bile acid composition linked to atrial fibrillation in heart failure patients.
IBD and Multiple Sclerosis
- Bile acid pool disrupted in inflammatory bowel disease due to dysbiosis and ileal inflammation impairing reabsorption.
- In multiple sclerosis, bile acid supplementation is an emerging gut-oriented intervention; bile acid-FXR signaling may suppress Th17-driven neuroinflammation [martinelli 2022 gut oriented interventions ms].
Graves' Disease and Thyroid
- Altered bile acid metabolism documented in autoimmune thyroid disease, potentially via FXR-mediated immune modulation.
Metal Connections
- Bile acids as metal chelators: Bile acids can bind divalent cations (Ca2+, Fe2+, Zn2+, Cu2+) in the intestinal lumen, affecting both metal bioavailability and bile acid solubility.
- Metal-induced dysbiosis disrupts bile acid transformation: Loss of BSH-expressing and 7-alpha-dehydroxylating bacteria alters the primary/secondary bile acid ratio.
- FXR and metal homeostasis: FXR regulates intestinal barrier integrity; its disruption by altered bile acid signaling compounds metal-induced barrier failure.
- Coprostanol pathway: Microbial conversion of cholesterol to coprostanol (by Eubacterium coprostanoligenes) is sex-dependent and may be disrupted by metals, connecting to the cholesterol-CVD axis [ryan 2017 bile acids gut microbiome cardiometabolic interactions].
Connections
- cardiovascular disease -- bile acid-FXR/TGR5 axis regulates cholesterol, inflammation, and vascular function
- colorectal cancer -- DCA and LCA are tumor promoters at high concentrations
- collinsella -- BSH-expressing pathobiont enriched in atherosclerosis
- short chain fatty acids -- co-produced by colonic anaerobes; bile acid-SCFA balance reflects community health
- inflammation -- FXR activation suppresses NF-kB; dysregulated bile acids promote inflammation
- dysbiosis -- microbial bile acid transformation is highly sensitive to community shifts
- gut metal microbiome -- bile acids modulate metal absorption and are themselves altered by metal-induced dysbiosis