Bile acid synthesis !

The liver turns cholesterol into bile acids, which help digest fat and are the body's main way out for cholesterol.

Updated 2026-11-01

Key facts

The rate-limiting step is cholesterol 7α-hydroxylase (CYP7A1), slowed by bile acids returning to the liver.
Primary (liver): cholic and chenodeoxycholic. Secondary (colonic bacteria): deoxycholic and lithocholic.
They are conjugated with glycine or taurine (bile salts), stored in the gallbladder and reabsorbed in the terminal ileum: enterohepatic circulation.
Only a small fraction (about 5%) is lost in faeces and replaced by new synthesis.

Classic route: CYP7A1 adds a 7α-OH to cholesterol; the nucleus is then modified (CYP8B1 adds cholic acid's 12α-OH).
The side chain is oxidised and shortened (CYP27A1 and peroxisomal β-oxidation): from 27 to 24 carbons.
Alternative route: starts with mitochondrial CYP27A1 and makes mainly chenodeoxycholic acid.
The liver conjugates them with glycine or taurine, so they are ionised in the gut as bile salts.
In the gut they form micelles with fats, cholesterol and fat-soluble vitamins.
In the colon bacteria deconjugate them and remove the 7α-OH: secondary bile acids form.
In the terminal ileum (ASBT transporter) most are reabsorbed and return via the portal vein; FXR and FGF19 then slow CYP7A1.

If bile holds more cholesterol than bile salts and phospholipids, cholesterol stones form.
Bile acid sequestrants lower LDL by forcing the liver to use up cholesterol.
CYP27A1 deficiency causes cerebrotendinous xanthomatosis: tendon xanthomas, cataracts and neurological damage.
Cholestasis retains bile salts: itching and jaundice.

Think of the answer, then tap the question to see it.

Which enzyme limits bile acid synthesis?

Cholesterol 7α-hydroxylase (CYP7A1).

Where are bile salts reabsorbed?

In the terminal ileum, through the ASBT transporter.

What separates a primary from a secondary bile acid?

The liver makes primary ones from cholesterol; secondary ones form when colonic bacteria modify a primary one.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Synthesis of bile acids and bile salts (R-HSA-192105) (Reactome, 2026); Cerebrotendinous xanthomatosis: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.