Bile acids: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Bile acids (stool): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Bile acids are derivatives of cholesterol that the liver forms and releases into the gut with the bile. There they surround fat droplets; gut bacteria modify them before part of them appears in stool.

The pathway step by step

1. Cholesterol → 7α-Hydroxycholesterol CYP7A1 · NADPH, Oxygen The enzyme CYP7A1 attaches an OH group at position 7 of the ring. This is the slowest step of the whole pathway; here the liver regulates the amount formed. Source 1
2. 7α-Hydroxycholesterol → Cholic acid, CDCA CYP8B1, CYP27A1 · NADPH Several further steps yield the two primary bile acids: cholic acid and chenodeoxycholic acid. The enzyme CYP8B1 decides which of the two is formed. Source 1
3. Cholic acid, CDCA → Conjugated bile acid BAAT · Glycine, Taurine The enzyme BAAT attaches glycine or taurine. The bile acid thus stays dissolved even in acidic gut contents and is not simply absorbed through the gut wall. Source 1, 2
4. Conjugated bile acid → Bile BSEP · ATP The BSEP pump moves bile acids out of the liver cell into the bile ducts, using ATP. From there they reach the small intestine via the gallbladder. Source 3
5. Bile acids in the gut → Reuptake in the ileum ASBT · Sodium At the end of the small intestine, the ASBT transporter retrieves most of the bile acids. They return to the liver via the portal vein; this circuit is called enterohepatic. Source 3
6. Reuptake in the ileum → FXR and FGF19 FXR In the gut cell, bile acids bind to the switch FXR. The cell then forms FGF19, which travels through the blood to the liver and dampens the formation of CYP7A1 there. Source 5, 3
7. Bile acids in the gut → Free bile acids Bile salt hydrolase Many gut bacteria carry the enzyme bile salt hydrolase (BSH). It splits off glycine or taurine, leaving the free bile acid. Source 4
8. Free bile acids → Deoxycholic acid 7α-dehydroxylation A few bacterial species in the colon then remove the OH group at position 7. This turns cholic acid into deoxycholic acid. Source 4
9. Free bile acids → Lithocholic acid 7α-dehydroxylation By the same route, chenodeoxycholic acid becomes lithocholic acid. It dissolves poorly in water and therefore largely stays in the gut. Source 4
10. Deoxycholic acid → In the stool Whatever does not return to the blood leaves the body with the stool. This is why stool mainly contains the secondary bile acids remodelled by bacteria. Source 4, 2
11. Lithocholic acid → In the stool Whatever does not return to the blood leaves the body with the stool. This is why stool mainly contains the secondary bile acids remodelled by bacteria. Source 4, 2

Cofactors in this pathway

• Glycine — Attached to the bile acid by BAAT, which keeps it dissolved in the gut content Source 1, 2In the ORY catalogue as a laboratory value: Glycin
• Taurine — The second conjugation partner of BAAT, alongside glycine Source 1, 2In the ORY catalogue as a laboratory value: Taurin
• NADPH — Electron donor for CYP7A1, the first step from cholesterol Source 1
• Sodium — The transporter ASBT retrieves bile acids together with sodium ions Source 3
• ATP — The BSEP pump uses ATP to move bile acids into the bile ducts Source 3

What acts on this pathway

• Colestyramine — Colestyramine binds bile acids in the gut into an insoluble complex. Bound bile acids no longer reach the ASBT transporter and leave with the stool. The product information describes this mechanism. Source 6

Sources

1. Chiang JY. Bile acids: regulation of synthesis. J Lipid Res 2009 · PubMed 19346330
2. Di Ciaula A, Garruti G et al. Bile Acid Physiology. Ann Hepatol 2017 · PubMed 29080336
3. Dawson PA, Karpen SJ. Intestinal transport and metabolism of bile acids. J Lipid Res 2015 · PubMed 25210150
4. Ridlon JM, Kang DJ et al. Bile salt biotransformations by human intestinal bacteria. J Lipid Res 2006 · PubMed 16299351
5. Chiang JYL, Ferrell JM. Discovery of farnesoid X receptor and its role in bile acid metabolism. Mol Cell Endocrinol 2022 · PubMed 35283218
6. US prescribing information Colestyramine (DailyMed), section Clinical Pharmacology · Prescribing information

Related pathways

• Cholesterol and lipoproteins — cholesterol
• Taurine — bile
• Cortisol — cholesterol
• DHEA — cholesterol
• Estradiol — cholesterol

As of 2026-09-16. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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