Overview of Lipids

Definition and Biological Importance

Lipids are a heterogeneous group of hydrophobic organic molecules that play essential roles in biological systems. They are generally compartmentalized or transported in association with proteins due to their hydrophobic nature.

• Energy Source: Lipids are a major source of energy for the body.

• Structural Role: They form hydrophobic barriers, allowing compartmentalization within cells and organelles.

• Clinical Relevance: Imbalances in lipid metabolism can result in significant clinical disorders.

Key Lipid Types:

• Fatty acids

• Triacylglycerols (TAGs)

• Glycerophospholipids

• Sterols (e.g., cholesterol)

• Sphingolipids

Digestion, Absorption, Secretion, and Utilization of Dietary Lipids

Dietary Intake and Major Lipid Classes

The typical Western diet is rich in lipids, primarily in the form of triacylglycerol (TAG), which consists of three fatty acids esterified to a glycerol backbone. Other dietary lipids include cholesterol, cholesteryl esters, phospholipids, and nonesterified (free) fatty acids.

• Digestion Initiation: Begins in the mouth and stomach, but is completed in the small intestine.

Digestion in the Stomach

Lipid digestion in the stomach is limited but important, especially in infants.

• Lingual Lipase: Secreted by glands in the tongue; hydrolyzes short- and medium-chain fatty acids from TAGs.

• Gastric Lipase: Produced by the gastric mucosa; also targets short- and medium-chain fatty acids.

• Clinical Note: These enzymes are particularly important in infants (milk fat digestion) and in individuals with pancreatic insufficiency (e.g., cystic fibrosis).

Cystic Fibrosis and Lipid Digestion

Pathophysiology and Clinical Impact

Cystic fibrosis (CF) is a common lethal genetic disorder among Caucasians of Northern European descent. It is an autosomal-recessive disease caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel found in epithelial cells.

• Defective CFTR: Leads to decreased chloride secretion and increased sodium uptake.

• Result: Dehydration of mucosal surfaces, thickened mucus, and blockage of ducts (e.g., pancreatic ducts), causing pancreatic insufficiency and impaired lipid digestion.

Emulsification in the Small Intestine

Mechanisms and Role of Bile Salts

Most dietary lipid emulsification occurs in the duodenum, increasing the surface area for enzymatic action.

• Mechanisms: Detergent action of conjugated bile salts and mechanical mixing (peristalsis).

• Bile Salts: Synthesized in the liver from cholesterol, stored in the gallbladder, and released into the intestine. They consist of a hydroxylated sterol ring with a side chain conjugated to glycine or taurine via an amide linkage.

• Function: Bile salts stabilize lipid droplets, preventing coalescence and facilitating enzyme access.

Enzymatic Degradation of Lipids in the Small Intestine

Pancreatic Enzymes and Their Actions

• Pancreatic Lipase: Hydrolyzes TAGs at the 1 and 3 positions, producing 2-monoacylglycerol (2-MAG) and free fatty acids (FFA).

• Colipase: Secreted by the pancreas, binds to lipase and anchors it at the lipid-water interface, enhancing activity in the presence of bile salts.

• Cholesterol Esterase: Hydrolyzes cholesterol esters to cholesterol and FFA.

• Phospholipase A2: Removes fatty acid from the 2-position of phospholipids, producing lysophospholipids and FFA.

Hormonal Regulation: Cholecystokinin (CCK) stimulates gallbladder contraction and pancreatic enzyme secretion; secretin stimulates bicarbonate secretion to neutralize chyme.

Absorption of Lipid Digestion Products

Micelle Formation and Uptake by Enterocytes

• Mixed Micelles: Composed of bile salts, fatty acids, 2-MAG, cholesterol, and fat-soluble vitamins; facilitate transport of hydrophobic lipids to the brush border of enterocytes.

• Absorption: Most lipid absorption occurs in the jejunum; bile salts are reabsorbed in the terminal ileum.

• Short- and Medium-Chain Fatty Acids: Water-soluble; absorbed directly without micelles.

Re-esterification and Chylomicron Formation

Intracellular Processing in Enterocytes

• Re-esterification: Long-chain fatty acids and 2-MAG are re-esterified to TAGs in the smooth endoplasmic reticulum (SER) via acyltransferases.

• Cholesteryl Esters and Phospholipids: Re-formed by specific acyltransferases.

• Chylomicron Assembly: TAGs, cholesteryl esters, and phospholipids are packaged with apolipoprotein B-48 to form chylomicrons.

• Secretion: Chylomicrons are released by exocytosis into lacteals (lymphatic vessels) and eventually enter the bloodstream.

Lipid Transport and Utilization in Tissues

Chylomicron Metabolism

• Lipoprotein Lipase (LPL): Located on capillary endothelium of muscle and adipose tissue; hydrolyzes TAGs in chylomicrons to FFA and glycerol.

• FFA Fate: Taken up by muscle (oxidized for energy) or adipose tissue (re-esterified for storage).

• Glycerol: Transported to the liver for gluconeogenesis or TAG synthesis.

• Chylomicron Remnants: Cleared by the liver via receptor-mediated endocytosis.

Clinical Correlate: Lipid Malabsorption

Causes and Consequences

• Malabsorption: Can result from impaired digestion (e.g., pancreatic insufficiency) or absorption (e.g., short bowel syndrome).

• Consequences: Steatorrhea (fatty stools), deficiencies in fat-soluble vitamins (A, D, E, K), and essential fatty acids.

Summary Table: Key Steps in Dietary Lipid Digestion and Absorption

Key Equations and Reactions

• General TAG Hydrolysis:

• Cholesterol Ester Hydrolysis:

• Phospholipid Hydrolysis:

Additional info:

• Fat-soluble vitamins (A, D, E, K) are absorbed along with dietary lipids via micelles.

• Orlistat is a drug that inhibits gastric and pancreatic lipases, reducing fat absorption and used in obesity management.