The lipid transport can be
divided into the exogenous pathway, which refers to the 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.
Fat metabolism by Liver - 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.
