Lipid Digestion and Emulsification
📌 Dietary lipids are primarily triacylglycerols (TAGs), composed of a glycerol backbone attached to three fatty acids, representing 90% of dietary lipids.
💧 Digestion begins minimally in the stomach via lingual lipases, but the majority occurs in the small intestine.
🔬 Because lipids are water-insoluble, they must be emulsified by bile salts (derivatives of cholesterol from the liver) to increase the surface area for water-soluble enzymes like pancreatic lipase to act.
🧬 Pancreatic lipase, aided by colipase which anchors it to the interface, hydrolyzes TAGs into glycerol and three free fatty acids.

Hormonal Regulation and Absorption
☀️ The presence of lipids in the small intestine stimulates the release of cholecystokinin (CCK), which triggers the gallbladder to release bile and increases pancreatic juice secretion (containing lipases).
🧪 The acidity from the stomach stimulates secretin release, which signals bile and pancreatic ducts to secrete bicarbonate ($\text{HCO}_3^-$) to neutralize the acidic chyme, optimizing the pH for pancreatic enzymes.
💧 Digestion products (free fatty acids, cholesterol) mix with bile salts to form micelles, which transport these hydrophobic molecules across the aqueous intestinal lumen to the brush border of the enterocytes (intestinal cells).

Lipid Reassembly and Transport
🔄 Inside the enterocyte's smooth endoplasmic reticulum, the absorbed components are reassembled into TAGs, cholesterol esters, and phospholipids.
🚀 These reassembled lipids are packaged with apolipoprotein B into large lipoprotein particles called chylomicrons for systemic transport.
🌊 Chylomicrons are exocytosed into the lacteals (lymphatic system) instead of the bloodstream, eventually entering the systemic circulation near the left subclavian vein.

Lipid Utilization and Remnants
⚡ In the bloodstream, chylomicrons are broken down by lipoprotein lipase mainly in the capillary beds of adipose tissue and skeletal/cardiac muscle.
💪 Free fatty acids taken up by muscle cells are used for energy, while adipocytes store them.
💧 Free glycerol released during TAG breakdown is phosphorylated in the liver to glycerol 3-phosphate, which can enter glycolysis/gluconeogenesis or be used to synthesize new TAGs.
➡️ Liver cells eventually hydrolyze and recycle the remaining chylomicron remnants.

Key Points & Insights
➡️ Cystic Fibrosis is mentioned as the most common lethal genetic disease in Caucasians, caused by a defective chloride channel leading to blocked ducts and fat malabsorption due to inadequate fat digestion/emulsification.
➡️ Bile salts are crucial for lipid digestion because they emulsify water-insoluble fats, allowing water-soluble digestive enzymes to access the lipid interface.
➡️ CCK controls the release of digestive enzymes (lipases) and bile necessary for fat breakdown and absorption.
➡️ Lipids are initially transported from the small intestine via the lymphatic system (lacteals) inside chylomicrons, bypassing the portal vein circulation initially.

📸 Video summarized with SummaryTube.com on Oct 10, 2025, 14:17 UTC