Nutritional Biochemistry: Dietary Fat Composition

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Questions and Answers

What are the primary dietary sources of lipids and their major composition?

The primary dietary sources of lipids include animal sources like dairy products, meat, and fish, and vegetable sources such as cooking oils. Over 95% of dietary lipids are triglycerides.

Explain the role of lingual lipase in fat digestion.

Lingual lipase initiates the hydrolysis of triglycerides in the mouth, forming 1,2-diacylglycerols and free fatty acids. It is active at low pH and optimal for short-chain triglycerides, especially those found in milk fat.

Describe the mechanism by which gastric lipase aids in lipid digestion.

Gastric lipase, secreted by the gastric mucosa, digests triglycerides with short- or medium-chain fatty acids. It complements lingual lipase in lipid digestion for those with pancreatic insufficiency.

What are the complementary mechanisms involved in emulsifying dietary lipids in the small intestine?

Emulsification of dietary lipids in the small intestine involves bile salts that increase the surface area of lipids, allowing lipase to work effectively. Additionally, mechanical mixing in the digestive tract enhances this process. Signup and view all the answers

How does pancreatic insufficiency affect lipid digestion and what treatment is recommended?

Pancreatic insufficiency hinders the action of pancreatic lipase, crucial for lipid digestion, leading to malabsorption. Treatment typically includes enzyme replacement therapy to assist in digesting triglycerides. Signup and view all the answers

What role do bile salts play in lipid digestion?

Bile salts act as emulsifying agents by interacting with dietary lipid particles and aqueous duodenal contents, stabilizing the particles and preventing aggregation. Signup and view all the answers

Describe the mechanism by which pancreatic lipase degrades triacylglycerols.

Pancreatic lipase preferentially removes fatty acids from the 1 and 3 positions of triacylglycerols, yielding 2-monoacylglycerol and free fatty acids. Signup and view all the answers

How is pancreatic cholesterol esterase activated, and what is its function?

Pancreatic cholesterol esterase is activated by bile salts and hydrolyzes cholesteryl esters into cholesterol and free fatty acids. Signup and view all the answers

Explain the role of cholecystokinin (CCK) in lipid digestion.

CCK stimulates the gallbladder to release bile, enhances pancreatic enzyme secretion, and decreases gastric motility to facilitate lipid digestion. Signup and view all the answers

What is the function of secretin in the context of lipid digestion?

Secretin is produced in response to low pH in the intestine and stimulates the pancreas to release bicarbonate, neutralizing chyme and optimizing enzymatic activity. Signup and view all the answers

Study Notes

Dietary Fat Composition

Over 95% of dietary fat is triglycerides, the rest is cholesterol, cholesterol esters, phospholipids, and unesterified fatty acids.

Dietary Sources of Lipids

Animal sources: dairy products, meat, butter, fish, pork, and eggs.
Plant sources: cooking oils, sunflower oil, mustard oil, groundnut oil, and fats from other vegetables.

Digestion in Mouth

Lingual lipase initiates the hydrolysis of triglycerides (TAGs) by attacking the ester bond and forming 1,2-diacylglycerols and free fatty acids.
Lingual lipase is acid-stable and secreted by the dorsal surface of the tongue.
It is active in a low pH range of 2-7.5 with an optimal pH of 4-4.5.
The ideal substrate for lingual lipase is short-chain TAGs.
Milk fat contains short-chain fatty acids esterified at the C-3 position, making it a suitable substrate for lingual lipase.
Lingual lipase activity continues in the stomach.

Digestion in Stomach

Gastric lipase is secreted by the gastric mucosa.
It digests TAG molecules containing short- or medium-chain fatty acids (less than 12 carbons).

Role of Lingual & Gastric Lipase

Both enzymes play a crucial role in lipid digestion for neonates, whose primary calorie source is milk fat.
They also aid individuals with pancreatic insufficiency, such as those with cystic fibrosis, in degrading TAG molecules.
Cystic fibrosis results in the inability of pancreatic enzymes to reach the intestine due to thickened secretions, leading to pancreatic insufficiency.
Enzyme replacement therapy is part of the treatment for cystic fibrosis.

Emulsification of Dietary Lipids in the Small Intestine

Emulsification of dietary lipids occurs in the duodenum.
Lipids are hydrophobic and poorly soluble in the aqueous environment of the digestive tract.
Lipase enzymes are water-soluble and can only work at the surface of lipid droplets.
TAG digestion occurs at lipid-water interfaces, and the rate of digestion depends on the surface area of this interface.
Bile salts are emulsifying agents that increase the surface area of hydrophobic lipids, enabling effective enzyme action.

Emulsification Mechanisms

Bile salts act as detergents, interacting with lipid particles and aqueous duodenal contents to stabilize and prevent aggregation.
Mechanical mixing due to peristalsis contributes to emulsification.

Degradation of Dietary Lipids by Pancreatic Enzymes

Pancreatic juice contains pancreatic lipase, phospholipase A2, and cholesterol esterase (hydrolase).

TAG Degradation

Pancreatic lipase preferentially removes fatty acids at the C-1 and C-3 positions of triglycerides.
The main products of hydrolysis are a mixture of 2-monoacylglycerol and free fatty acids.
Pancreatic lipase is highly concentrated in pancreatic secretions and highly efficient catalytically, making significant fat malabsorption likely only with severe pancreatic deficiency or cystic fibrosis.
Pancreatic colipase causes a conformational change in lipase, exposing its active site. Colipase is secreted as the zymogen, procolipase, and activated in the duodenum by trypsin.

Cholesteryl Ester Degradation

Pancreatic cholesterol esterase hydrolyzes cholesteryl esters, requiring bile salts for optimal activity.
This produces cholesterol and free fatty acids.

Phospholipid Degradation

Pancreatic juice contains proenzyme phospholipase A2, activated by trypsin and requiring bile salts for optimal activity.
Phospholipase A2 removes one fatty acid from the C-2 position of a phospholipid, leaving a lysophospholipid.
The remaining fatty acid at the C-1 position can be removed by lysophospholipase, leaving a glycerylphosphoryl base that can be excreted in feces, further degraded, or absorbed.

Control of Lipid Digestion

Cholecystokinin (CCK) is a small peptide hormone produced by cells in the jejunum and lower duodenum mucosa in response to the presence of lipids and partially digested proteins.
CCK acts on the gallbladder (causing contraction and bile release), the exocrine cells of the pancreas (causing digestive enzyme release), and the gastric motility (reducing gastric motility and slowing down gastric content release into the small intestine).

Secretion Action

Secretin is a small peptide hormone produced in response to the low pH of chyme entering the intestine.
It causes the pancreas and liver to release a watery solution rich in bicarbonate, which neutralizes the intestinal contents and brings them to the appropriate pH for pancreatic enzyme activity.

Absorption of Lipids by Intestinal Mucosal Cells (Enterocytes)

Free fatty acids, free cholesterol, and 2-monoacylglycerol are the primary products of lipid digestion in the jejunum.
They form mixed micelles with bile salts and fat-soluble vitamins, which are soluble in the aqueous environment of the intestinal lumen.
These particles travel to the brush border membrane of the enterocytes, the primary site of lipid absorption.
The hydrophilic surface of the micelles facilitates the transport of hydrophobic lipids through the unstirred water layer to the brush border membrane for absorption.
Short- and medium-chain length fatty acids do not require mixed micelles for intestinal mucosa absorption.

Resynthesis of TAG and Cholesteryl Esters

The absorbed lipid mixture in the enterocytes migrates to the endoplasmic reticulum, where complex lipid biosynthesis takes place.
Most long-chain fatty acids entering the enterocytes are used to form TAGs, phospholipids, and cholesteryl esters.
Short- and medium-chain length fatty acids are not converted to their CoA derivatives and are not re-esterified to 2-monoacylglycerol.
They are released into the portal circulation and carried by serum albumin to the liver.

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Description

Explore the composition and sources of dietary fats in this quiz. Learn about the role of triglycerides, cholesterol, and other lipids in our diet, as well as their digestion processes in the mouth and stomach. Enhance your understanding of nutritional biochemistry.