Bile Acid Metabolism

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.

Deconjugation (BSH Activity)#

Bile salt hydrolase (BSH) removes the glycine/taurine conjugate. Found in Collinsella, Lactobacillus, Clostridium, Bifidobacterium, Listeria, and Bacteroides.^{[1]Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactionsPaul M. Ryan, Catherine Stanton, Noel M. Caplice · 2017Open reference 1 ↓}

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.

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.^{[1]Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactionsPaul M. Ryan, Catherine Stanton, Noel M. Caplice · 2017Open reference 1 ↓}

Gut barrier: Maintains epithelial integrity; FXR-deficient mice have increased intestinal permeability. Immune modulation: Suppresses NF-kB-driven Metal-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.^{[1]Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactionsPaul M. Ryan, Catherine Stanton, Noel M. Caplice · 2017Open reference 1 ↓} Metabolic: Increases GLP-1 secretion from enteroendocrine L-cells, improving glucose homeostasis.

Anti-inflammatory: Reduces pro-inflammatory cytokine production.

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.^{[2]Secondary bile acids: an underrecognized cause of colon cancerHana Ajouz, Deborah Mukherji, Ali Shamseddine · 2014Open reference 2 ↓}

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.^{[3]Emerging therapy targets to modulate microbiome-mediated effects evident in cardiovascular diseaseDorothea Katharina Hoffelner, Tim Hendrikx · 2025Open reference 3 ↓} 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.^{[1]Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactionsPaul M. Ryan, Catherine Stanton, Noel M. Caplice · 2017Open reference 1 ↓}

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 (IBD) 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.^{[4]Gut-oriented interventions in patients with multiple sclerosis: fact or fiction?V. Martinelli, M. Albanese, M. Altieri et al. · 2022Open reference 4 ↓}

Metal Connections#

Bile acids as metal chelators: Bile acids can bind divalent cations (calcium(II) (Ca2+), iron(II) (Fe2+), zinc(II) (Zn2+), copper(II) (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.^{[1]Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactionsPaul M. Ryan, Catherine Stanton, Noel M. Caplice · 2017Open reference 1 ↓}