Fat Metabolism

The lipid transport can be divided into the exogenous pathway, which refers to the cholesterol metabolism of intestinally derived lipoproteins, and the endogenous pathway, which refers to hepatic-derived lipoproteins.

In fact, dietary cholesterol represents only about one-third of total intestinal cholesterol pool, while endogenous sources, the bile and to some extent intestinal mucosal cholesterol account for the remaining two-thirds.

Fat metabolism - exogenous pathway

• When fat food enters into the duodenum, mucosal cells in duodenum will releases cholecystokinin (CCK).
• CCK stimulates gallbladder contraction and realizes bile acids, into the small intestine.
• Bile acids chemically resemble detergent molecules, which can dissolve fat and disperse it in small packets called as emulsification (mixture of fat and water).
• Small packets of fats are further digested by pancreatic lipase. Enzyme lipase acts only at oil-water interfaces and requires a large surface area. Lipase hydrolyses fat and yield monoglyceride and fatty acids.
• Monoglycerides and fatty acids associated with bile acids, phospholipids to form a complex structure called micelles.
• Micelles present in the border of small intestine enterocytes is taken up into the epithelial cells.
• Intestinal cells resynthesize triglycerides then packaged into Chylomicrons (triglyceride packed with protein). Chylomicrons in enterocytes are transported via the lymphatic system and enters bloodstream for circulation.
• Cholesterol in bloodstream is utilized, otherwise taken up and stored in the liver for future use.

Pile acids recycle in fat metabolism

Adequate quantity of bile acid is a must for fat metabolism, it is maintained by

• The liver, about 0.6 g of new bile acid is produced daily from cholesterol.
• This is added to the total bile acid pool of 3.0 g, which cycles 6 to 10 times daily.
• Approximately 96% of the bile acid is re-absorbed in each cycle; the balance is lost in the stool.

Liver metabolism - endogenous pathway

The liver is capable of removing cholesterol from the blood circulation as well as manufacturing and releasing cholesterol into the blood circulation.

• After a meal, fat is digested and released into the blood stream.
• Liver removes excess chylomicrons (triglyceride or fat packed with protein) from the blood circulation.
• Removed chylomicron is stored into the liver for future requirements.
• In between meals or during fasting, the liver manufactures fat from storage and releases cholesterol back into the blood circulation when needed.
• Even if there is no fat in dietary food, liver can produce the required quantity of fat from carbohydrates or protein foods.

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