Lipid Metabolism I: Digestion and Absorption

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Questions and Answers

What is the primary challenge in lipid digestion and absorption?

pH level of the stomach

Lack of lipase in the mouth

Insolubility of proteins in water

Solubility of lipolytic enzymes in an aqueous medium (correct)

What percentage of dietary triglycerides may be digested in the stomach?

10 per cent

20 per cent

50 per cent

30 per cent (correct)

Which of the following is a vegetable source of lipids?

Eggs

Groundnut oil (correct)

Meat

Butter

What is the optimal pH range for lingual lipase?

pH 4.0 to 4.5 Signup and view all the answers

In which phase of lipid digestion and absorption do events take place inside intestinal epithelial cells?

Absorption phase II Signup and view all the answers

What type of fatty acid chains are more susceptible to digestion in the stomach?

Short-chain fatty acids Signup and view all the answers

What is the primary site of fat digestion?

Small intestine Signup and view all the answers

What is the optimum pH for pancreatic lipase?

6 Signup and view all the answers

What is the role of Ca++ in fat digestion?

It facilitates the action of pancreatic lipase Signup and view all the answers

What is the function of secretin?

Regulation of electrolyte and fluid components Signup and view all the answers

What is the role of bile salts in fat digestion?

They help in emulsification of fats and enhance lipase activity Signup and view all the answers

What is the enzyme that breaks down phospholipids?

Phospholipase A2 Signup and view all the answers

What is the primary component of chylomicrons?

Triglycerides Signup and view all the answers

What is the function of apo-B48 in chylomicrons?

To facilitate the transportation of triglycerides Signup and view all the answers

Where are chylomicrons synthesized?

Intestinal wall Signup and view all the answers

What is the approximate diameter of chylomicrons?

0.075-1 mμ Signup and view all the answers

Why can't resynthesised TG pass into the lymphatics or portal blood?

Because it is insoluble in water Signup and view all the answers

What is the difference between nascent and circulating chylomicrons?

Circulating chylomicrons have apo-C, while nascent chylomicrons do not Signup and view all the answers

What is the characteristic of fatty acids that are more readily absorbed?

Short and medium chain FA (6 to 10 C) and unsaturated FA Signup and view all the answers

Where are the products of digestion taken up and resynthesized into TG?

Smooth endoplasmic reticulum Signup and view all the answers

What is the role of the enzyme glycerokinase?

To convert glycerol to α-Glycerol-(P) Signup and view all the answers

What happens to short-chain FA and medium-chain FA (less than 8 to 10 C) and unsaturated FA?

They are absorbed into the portal blood directly Signup and view all the answers

What is the function of the enzyme thiokinase?

To convert FA to Acyl-CoA Signup and view all the answers

What is the ultimate fate of all FFA present in the intestinal wall?

They are reincorporated into TG after activation Signup and view all the answers

What is the final product of fat digestion in the small intestine?

α-monoglycerides Signup and view all the answers

What is the process by which bile salts are returned to the liver?

Enterohepatic circulation of bile salts Signup and view all the answers

What is the purpose of micelles?

Collection of bile salts and fatty acids for absorption Signup and view all the answers

What is the site of fat absorption?

Duodenum and jejunum Signup and view all the answers

What happens to less than 1/4 of the ingested fat?

It is broken down to glycerol and FA Signup and view all the answers

What is the first step of pancreatic lipase action?

Removal of a terminal FA to produce an α, β-diglyceride Signup and view all the answers

Study Notes

Dietary Sources of Lipids

Animal sources: dairy products, meat, fish, and eggs
Vegetable sources: sunflower oil, groundnut oil, cotton seed oil, and mustard oil

Challenges of Lipid Digestion and Absorption

Insolubility of fats in water
Solubility of lipolytic enzymes in an aqueous medium

Phases of Lipid Digestion and Absorption

Preparatory phase: digestion of lipids in the intestine
Absorption phase I: transport of digested fats across the membrane of intestinal villous layer into intestinal epithelial cells
Absorption phase II: events that take place inside intestinal epithelial cells and its passage through lacteals to lymph or portal blood

Preparatory Phase

Little or no fat digestion takes place in the mouth
Lingual lipase (Ebner's gland): pH 4.0 to 4.5
Stomach: about 30% of dietary triglycerides may be digested; more active on triglycerides with shorter or medium fatty acid chains; more specific for ester linkage at 3-position rather than position-1

Digestion in Small Intestine

Major site of fat digestion
Lipase (steapsin) - pancreatic juice
Sequence of events for intestinal fat digestion:
- Secretion of pancreatic juice stimulated by acid gastric contents, GI hormones (secretin and CCKPZ), and bile salts
- Role of bile salts: combination with lipase, protection against inhibitory effects of bile salts, and emulsification of fats
- Role of Ca++: facilitates lipase action and precipitates free fatty acids as 'soaps'

Absorption and Formation of Chylomicrons

Short- and medium-chain fatty acids and unsaturated fatty acids are more readily absorbed
Products of digestion taken up by smooth endoplasmic reticulum and resynthesized into triglycerides
Formation of chylomicrons: composed of triglycerides, cholesterol, phospholipids, and apolipoprotein B-48
Average composition of chylomicron molecule: triglycerides (87-88%), phospholipids (8%), free and esterified cholesterol (3%), and apolipoprotein B-48 (0.05-2%)

Absorption of Resynthesized Triglycerides

α-Monoglycerides are further hydrolyzed by intestinal lipase to produce free fatty acids and glycerol
Fatty acids absorbed and formed from hydrolysis of α-monoglycerides are activated to acyl-CoA
Glycerol is converted to α-glycerol-phosphate by glycerokinase in the presence of ATP
α-Glycerol-phosphate combines with acyl-CoA to form triglyceride molecule

Mode of Action of Pancreatic Lipase

Complete hydrolysis of fats produces glycerol and fatty acids
Digestion steps: removal of terminal fatty acid to produce α, β-diglyceride, removal of terminal fatty acid to produce β-monoglyceride, isomerization of β-monoglyceride to α-monoglyceride
α-Monoglycerides are the major endproducts of fat digestion

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Description

Test your knowledge of lipid metabolism, including dietary sources of lipids, challenges, and phases of digestion and absorption. This quiz is designed for 200-level MBBS students in the Department of Medical Biochemistry at Baze University of Abuja.