Overview
This lecture explains the digestion, absorption, and transport of dietary fats (lipolysis), focusing on the breakdown process and how fats are distributed throughout the body.
Composition and Ingestion of Fats
- Dietary fats mainly include triglycerides, cholesterol, and phospholipids.
- Triglycerides consist of a glycerol backbone plus three fatty acids.
- Fats are not digested in the mouth or stomach but start breaking down in the duodenum.
Emulsification and Enzyme Activation
- In the watery lumen of the intestines, fats aggregate into large globules.
- Bile, stored in the gallbladder and released upon stimulation by CCK and secretin, emulsifies these globules into smaller droplets.
- Bile salts act like detergent, allowing enzymes to access fats.
Enzymatic Digestion in the Duodenum
- The pancreas releases lipases, colipase, and bicarbonate into the duodenum upon stimulation by CCK and secretin.
- Bicarbonate neutralizes stomach acid to create a suitable pH for lipase activity.
- Lipases, assisted by colipase, break triglycerides into two free fatty acids and one 2-monoglyceride.
Micelle Formation and Absorption
- Fatty acids, monoglycerides, and cholesterol are surrounded by bile salts to form micelles.
- Micelles facilitate transport of lipids across the intestinal epithelial cells (enterocytes).
- Inside enterocytes, bile salts are left behind, and fats are absorbed.
Intracellular Processing and Chylomicron Formation
- Absorbed fatty acids and monoglycerides are reassembled into triglycerides in the smooth endoplasmic reticulum.
- Triglycerides and cholesterol are packaged with phospholipids and ApoB-48 protein to form chylomicrons.
- Chylomicrons enter the lymphatic system, not the blood portal system.
Transport and Utilization of Fats
- Chylomicrons travel through lymph to the bloodstream, delivering triglycerides to tissues.
- Lipoprotein lipase breaks down chylomicron triglycerides for uptake by muscle (energy use) and adipose tissue (storage).
- Chylomicron remnants, rich in cholesterol, return to the liver for processing.
Lipoproteins in Fasting and Cholesterol Transport
- In fasting, the liver produces VLDL (very low-density lipoprotein), containing ApoB-100, to transport endogenous fats.
- VLDL becomes IDL and then LDL as it loses triglycerides and delivers cholesterol to tissues.
- High LDL promotes cholesterol buildup in blood vessels, increasing the risk of atherosclerosis.
Key Terms & Definitions
- Triglyceride — A fat molecule with three fatty acids and a glycerol backbone.
- Lipolysis — Breakdown of fats into fatty acids and glycerol.
- Emulsification — Process where bile salts break fat globules into smaller droplets.
- Bile salts — Detergent-like molecules in bile that emulsify fats.
- Lipase — Enzyme that digests triglycerides.
- Colipase — Protein aiding lipase function.
- Micelle — Small, bile salt-coated aggregates carrying digested fats to intestinal cells.
- Chylomicron — Lipoprotein formed in intestinal cells to transport fats via lymph.
- ApoB-48 — Protein component of chylomicrons.
- VLDL/LDL — Lipoproteins made by the liver to transport fats and cholesterol.
- Atherosclerosis — Artery disease caused by cholesterol deposition.
Action Items / Next Steps
- Review the process of fat digestion and absorption.
- Study the roles of bile, pancreatic enzymes, and various lipoproteins.
- Memorize key terms and their functions.
- Compare fat digestion with carbohydrate and protein digestion.