Digestion and Transport of Dietary Fat

Overview

• Focus: pathway of dietary fat from cheeseburger through digestion, absorption, packaging, and entry into circulation.
• Main structures: mouth, esophagus, stomach, small intestine, colon, intestinal epithelial cells, lymphatic system.
• Main molecules: triacylglycerides (TAG), bile, lipases, fatty acids, glycerol, lipoproteins, chylomicrons.

Path of Food and Site of Fat Absorption

• Food path: mouth → esophagus → stomach → small intestine → colon → excretion.
• Small intestine has many loops and high surface area, ideal for nutrient absorption including fats.
• Colon receives material that cannot be absorbed in small intestine.

Structure and Properties of Dietary Fat (Triacylglycerides)

• Dietary fat considered as triacylglycerides (TAG).
• TAG structure: glycerol backbone (three carbons) plus three acyl groups.
• Each acyl group: carbon double bond oxygen, followed by many carbons and hydrogens.
• Many C–H bonds make TAG very hydrophobic and water-insoluble.
• In aqueous intestinal environment, TAG molecules cluster into fat droplets.

Key Fat-Related Terms Table

Small Intestine Lining and Enzymes

• Small intestine lined by specialized epithelial cells that absorb nutrients.
• Lumen: inner space of small intestine where food passes.
• Lipases: class of enzymes that break down fat molecules.
• Some lipases secreted by pancreas into intestine.
• Other lipases located on border (surface) of intestinal epithelial cells.
• Lipases require an aqueous (water-based) environment to function.

Problem of Fat Digestion in Aqueous Environment

• Body fluids and intestinal lumen are water-based and hydrophilic.
• TAG and other fats are very hydrophobic and do not dissolve in water.
• Hydrophobic TAGs cluster together into large fat droplets.
• Large droplets reduce access area for lipase enzymes to act.

Role of Bile in Fat Digestion

• Bile is secreted by the liver into small intestine when food arrives.
• Bile acts like a detergent for dietary fats.
• Bile molecules have both hydrophobic and hydrophilic functional groups.
• This dual nature allows bile to emulsify fat droplets into smaller pieces.
• Emulsification increases surface area of fat available to lipases.

Bile and Emulsification Table

Chemical Breakdown of Triacylglycerides by Lipases

• Lipases cleave TAG at ester linkages between glycerol and acyl groups.
• Reaction: water added (hydrolysis) across each ester bond.
• Products: one glycerol backbone plus three fatty acids.
• Glycerol now has hydroxyl group(s) from water.
• Fatty acids formed when acyl groups gain hydroxyl group from water.
• Fatty acids are carboxylic acids specifically derived from TAG.

Absorption into Intestinal Epithelial Cells

• After breakdown, glycerol and fatty acids are small enough to diffuse into epithelial cells.
• Entry into the cell counts as absorption from intestinal lumen.
• Absorption is not the final step, because fats must still be transported to tissues.

Re-esterification and Re-formation of Triacylglycerides

• Inside epithelial cells, absorbed fatty acids and glycerol are reassembled into TAG.
• Ester linkages between glycerol and fatty acids are re-formed.
• Purpose: pack fatty acids into compact TAG units for efficient transport.

Packaging into Lipoproteins and Chylomicrons

• Re-formed TAGs must be moved through aqueous body fluids.
• Cells package TAG and other hydrophobic molecules (e.g., cholesterol) into lipoproteins.
• Lipoprotein structure: hydrophobic core with TAG and other lipids.
• Protein components with polar (hydrophilic) heads on surface contact aqueous environment.
• This allows hydrophobic contents to be transported in blood or lymph.
• Specific lipoprotein made in intestinal cells from absorbed fats is called chylomicron.

Lipoprotein and Chylomicron Structure Table

Exit from Epithelial Cells and Entry into Lacteals

• After packaging, chylomicrons leave the intestinal epithelial cell.
• Surrounding each epithelial cell: blood capillaries and lymphatic capillaries (lacteals).
• Regular capillaries have tiny fenestrations (small gaps) for molecular passage.
• These gaps are sufficient for proteins and carbohydrate absorption.
• Chylomicrons are too large to pass through regular capillary fenestrations.
• Lacteals are specialized lymphatic capillaries with larger pores.
• Large pores allow bulky chylomicrons to enter the lymphatic system.

Capillaries vs Lacteals Table

Lymphatic Transport and Entry into Blood

• Lymphatic vessels are distributed throughout the body.
• Lymphatics drain regions in legs, thorax, arms, and head.
• Lymph from many regions coalesces near neck and shoulder area.
• At this region, lymphatic vessels form ducts that empty into veins.
• Most body lymph drains into left thoracic duct, then into nearby vein.
• Some lymph drains into right thoracic duct, then into vein on right side.
• Chylomicrons traveling in lymph enter bloodstream at these venous junctions.

Key Terms and Definitions

• Small intestine: long, looped organ with high surface area for nutrient absorption.
• Lumen: hollow interior through which food and chyme move.
• Epithelial cells: specialized cells lining intestine that absorb nutrients.
• Lipases: enzymes that hydrolyze fats into glycerol and fatty acids.
• Bile: liver-produced secretion that emulsifies fats in intestine.
• Emulsification: process of dispersing fat into small droplets in water.
• Triacylglyceride (TAG): storage and dietary fat molecule with glycerol plus three fatty acids.
• Fatty acid: carboxylic acid derived from TAG acyl group; can be stored or oxidized.
• Lipoprotein: complex that transports lipids in aqueous body fluids.
• Chylomicron: intestinal lipoprotein carrying dietary TAG and hydrophobic molecules via lymph.
• Lacteal: lymphatic capillary in intestinal villus, takes up chylomicrons.
• Thoracic duct: major lymphatic duct draining lymph into venous system.

Next Steps / What Comes After

• Chylomicrons, once in veins, continue to travel through blood circulation.
• Future content will follow chylomicron journey in blood and delivery of fat to tissues.