Bile's Role in Fat Digestion: Understanding Organic Compound Breakdown

Bile is a complex aqueous secretion essential for lipid digestion and absorption of fat-soluble vitamins.

Composition & Function

• Bile salts (conjugated bile acids): Amphipathic molecules — hydrophilic face outward, hydrophobic inward. Critical micellar concentration (CMC) ~2–5 mM. Above CMC, form micelles that solubilize lipids.
• Phospholipids (lecithin): Stabilize mixed micelles with bile salts.
• Cholesterol: Solubilized in micelles; excreted in feces (major elimination route).
• Bilirubin: Waste pigment; gives bile color.

Digestion Sequence

1. Gastric lipase initiates triglyceride hydrolysis (10–30%).
2. Duodenal fat → CCK/secretin → pancreatic lipase + colipase + bile.
3. Bile salts emulsify fat → ↑ surface area for lipase.
4. Lipase hydrolyzes TG → 2-monoglyceride + 2 free fatty acids.
5. Products + bile salts + phospholipids → mixed micelles.
6. Micelles ferry lipids to brush border for absorption.

Key Point

Bile salts are not enzymes—they don’t hydrolyze bonds. They enable enzymatic digestion by creating the interface. Without bile, fat absorption drops from >95% to <40%.