Bile in the Small Intestine: Fat Digestion, Absorption & Enterohepatic Circulation

Bile is essential for dietary fat digestion and absorption in the small intestine.

Composition

• Bile salts (acids): Cholic, chenodeoxycholic (primary); deoxycholic, lithocholic (secondary, bacterial). Conjugated with taurine/glycine.
• Phospholipids: Lecithin (phosphatidylcholine).
• Cholesterol, bilirubin, water, electrolytes.

Mechanism: Emulsification

1. Bile salts adsorb at oil-water interface → reduce surface tension.
2. Large fat globules → micelles (3–10 µm) → vastly ↑ surface area for pancreatic lipase.
3. Lipase hydrolyzes triglycerides → 2-monoglyceride + 2 free fatty acids.
4. Products + bile salts → mixed micelles (3–7 nm) → solubilize lipids for absorption.

Absorption

• Mixed micelles deliver lipids to brush border → diffuse into enterocytes.
• Inside enterocyte: re-esterified to triglycerides → packaged into chylomicrons (with apoB-48) → exocytosed to lacteals → lymphatic system → thoracic duct → blood.
• Fat-soluble vitamins (A, D, E, K) co-absorbed via same pathway.

Enterohepatic Circulation

• 95% of bile salts reabsorbed in terminal ileum (ASBT transporter) → portal blood → hepatocytes (NTCP) → resecreted.
• Pool cycles 6–10×/day; synthesis replaces ~5% loss.

Clinical Correlates

• Bile acid malabsorption (ileal resection/disease): Diarrhea, steatorrhea, cholesterol gallstones.
• Obstructive jaundice: No bile → steatorrhea, vitamin K deficiency (coagulopathy).
• Cholestyramine: Binds bile salts → lowers cholesterol, treats bile acid diarrhea.

Bile transforms insoluble fats into absorbable forms — critical for nutrition.