Lipid Digestion, Absorption, and Metabolism - Made Simple

What are Lipids (Fats)?

PART 1: DIGESTION - Breaking Fat Down

Step 1 - A Little in the Mouth & Stomach (minor)

• Lingual lipase (from the tongue) and gastric lipase (in the stomach) begin breaking down a small amount of fat (less than 10%)
• The stomach physically churns fat into smaller droplets and slowly releases them into the small intestine - this slow release is controlled by CCK (cholecystokinin), a hormone released when fat hits the duodenum

Step 2 - Emulsification in the Small Intestine (key step)

• Bile (made by the liver, stored in the gallbladder) squirts into the duodenum
• Bile contains bile salts and lecithin (a phospholipid) - these act like detergents
• They surround fat globules, breaking them into tiny droplets = emulsification
• This increases fat surface area up to 1000-fold, allowing digestive enzymes to attack more effectively

Step 3 - Enzyme Digestion

Note on Colipase: Bile salts would normally inactivate pancreatic lipase. Colipase (secreted by the pancreas, activated by trypsin) rescues the situation - it displaces bile salts and anchors lipase to the droplet surface so digestion can proceed.

PART 2: ABSORPTION - Getting Fat Into the Body

• Bile salts wrap around the fat products (monoglycerides, fatty acids, cholesterol) to form tiny disk-shaped particles called micelles (~50 Å diameter)
• The inside of the micelle is fat-soluble; the outside faces the water - perfect for transport

• Micelles float to the brush-border (apical) membrane of intestinal cells (enterocytes)
• The fat products are released from the micelle and diffuse into the cell down a concentration gradient
• The bile salts stay behind in the gut lumen - they travel to the ileum for recycling (enterohepatic circulation)

• Inside the enterocyte, fatty acids and monoglycerides are reassembled into triglycerides, cholesterol esters, and phospholipids on the smooth endoplasmic reticulum

• The re-synthesized lipids are packaged with proteins (especially Apolipoprotein B) into large particles called chylomicrons
• Structure: fatty core (triglycerides + cholesterol) surrounded by a shell of phospholipids + apolipoproteins
• Size: ~1000 Å diameter

• Chylomicrons are too big to enter blood capillaries directly
• They exit via exocytosis through the basolateral membrane into lacteals (lymphatic capillaries in the intestinal villi)
• Lymph carries them through the thoracic duct → directly into the bloodstream (at the junction of jugular and subclavian veins)

Glycerol (the backbone of triglycerides) is water-soluble, so it skips all of this and is absorbed directly into the portal blood.

PART 3: METABOLISM - What Happens to Fat in the Blood

The Chylomicron Phase (after a meal)

1. Chylomicrons enter the bloodstream and circulate
2. Capillaries in adipose tissue, muscle, and heart contain an enzyme called lipoprotein lipase (LPL) on their walls
3. LPL hydrolyzes chylomicron triglycerides → free fatty acids (FFA) + glycerol
4. Fatty acids enter cells - either burned for energy (muscle) or re-stored as fat (adipose tissue)
5. What's left is a chylomicron remnant (enriched in cholesterol) - these are cleared by the liver via remnant receptors

Lipoproteins - The Fat Transport System

Free Fatty Acid Transport (during fasting/exercise)

• When energy is needed, fat cells (adipocytes) break down stored triglycerides via hormone-sensitive lipase
• Released fatty acids bind to albumin in the blood = Free Fatty Acids (FFA)
• These FFAs circulate to muscle, heart, etc., to be burned for energy
• The liver converts them to ketone bodies during prolonged fasting/starvation

The Liver's Role in Fat Metabolism

1. Beta-oxidation - breaks fatty acids into acetyl-CoA for energy
2. Lipogenesis - converts excess carbohydrates/protein into fat (triglycerides)
3. Synthesizes cholesterol, phospholipids, and bile acids
4. Packages excess triglycerides into VLDL for export

Quick Summary Table

If Any Step Goes Wrong = Steatorrhea (fat in stools)

• Pancreatic insufficiency (e.g., chronic pancreatitis, cystic fibrosis) → no lipase → undigested triglycerides in stool
• Low duodenal pH (e.g., Zollinger-Ellison syndrome) → inactivates pancreatic lipase (optimum pH = 6)
• Bile salt deficiency (e.g., ileal resection breaks enterohepatic circulation) → no micelles → fat not absorbed
• Abetalipoproteinemia → can't make ApoB → can't form chylomicrons → fat trapped in enterocytes