Lipid Digestion & Absorption Quiz

Lipid Digestion & Absorption

• Mixed Micelles transport poorly water-soluble lipids like cholesterol and fat-soluble vitamins (A, D, E, K).
• Bile Salts are essential for efficient lipid absorption, solubilizing ingested lipids.
• Chylomicrons are formed in enterocytes and transport fats via lymphatic capillaries into the bloodstream.
• Lipoprotein Lipase, found on blood capillary walls, hydrolyzes triglycerides in chylomicrons into fatty acids and glycerol for energy or storage.

Triglyceride Assimilation

• Dietary Triglycerides must be hydrolyzed into fatty acids, monoglycerides, and glycerol before absorption.
• Micelles form to promote absorption of fatty acids into the intestinal wall.
• Chylomicrons are formed from the resynthesis of triglycerides from absorbed glycerol and fatty acids.

Malabsorption

• Lipid Malabsorption leads to elevated lipid excretion in feces due to impaired digestion or absorption.
• Conditions like cystic fibrosis (CF) and short bowel syndrome can cause lipid malabsorption.
• Steatorrhea refers to significant fat losses (around 30g/day) in stool, including fat-soluble vitamins and essential fats.
• Bile Salt Deficiency caused by liver disease or bile duct obstruction contribute to steatorrhea.

Sources of Fats for Metabolism

• Dietary Fats are the primary source.
• Stored Fat in adipose tissue is another source.
• Synthesized Fat from excess carbohydrates is a third source.

Dietary Composition of Lipids

• We consume approximately 100g of lipids daily.
• Triglycerides make up over 90% of our lipid intake.
• Other lipids: cholesterol, cholesteryl esters, phospholipids, and free fatty acids.

Dietary Sources

• Animal Sources: Dairy products (milk, cheese, butter), meat, eggs, fish, poultry.
• Plant Sources: Cooking oils, nuts, seeds, avocados.

Digestion in Mouth

• Lingual Lipase is secreted by the tongue.
• Works optimally at low pH (2.0-7.5).
• Primarily breaks down short-chain triglycerides (e.g., milk fat).

Digestion in Stomach

• Gastric Lipase is secreted in small quantities.
• Less effective on long-chain fatty acids; mainly works on short to medium chain fatty acids.
• Plays a significant role in lipid digestion for infants and those with pancreatic insufficiency.

Digestion in Small Intestine

• Main Site for fat digestion due to pancreatic lipase and bile salts.
• Bile Salts are emulsifiers for fats, increasing their surface area for pancreatic lipase action.
• Pancreatic Juices are secreted upon stimulation by:
  • Acidic gastric content entering the duodenum
  • Secretin and cholecystokinin (CCK) hormones.

Control of Digestion

• Cholecystokinin (CCK) is released by the duodenum in response to lipids and proteins, stimulating gallbladder bile release, decreasing gastric motility, and promoting satiety.
• Secretin is released in response to low pH of chyme, stimulating pancreatic bicarbonate secretion for neutralization.
• Bile Salts are reabsorbed after aiding lipid absorption.

Cholesteryl Ester Degradation

• Dietary cholesterol is mainly in the free form, with a small portion as cholesteryl esters.
• Pancreatic Cholesteryl Ester Hydrolase (Cholesterol Esterase) hydrolyzes cholesteryl esters to cholesterol and free fatty acids.
• Bile Salts enhance the activity of this enzyme.
• Phospholipase A2 removes one fatty acid from phospholipids, forming lysophospholipids.
• Lysophospholipase further breaks down lysophospholipids.

Lipid Absorption

• Free Fatty Acids, Cholesterol, 2-monoacylglycerol: Products of lipid digestion in the jejunum.
• Mixed Micelles deliver these products to the enterocyte brush border membrane.
• Enterocytes absorb the lipids.
• Bile Salts are largely reabsorbed back into circulation with only a small amount lost in feces.

Resynthesis

• Absorbed lipids are transported to the endoplasmic reticulum of enterocytes.
• Triglycerides and cholesteryl esters are re-synthesized in the enterocytes.