Divide the lipid transport 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 the 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 the duodenum will release 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).
- Further digestion of small packets of fats is 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 the small intestine (called enterocytes) are absorbed into the epithelial cells.
- Intestinal cells re-synthesize triglycerides then packaged into Chylomicrons (triglyceride packed with protein). Chylomicrons in enterocytes are transport via the lymphatic system and enter the bloodstream for circulation.
- Cholesterol in the bloodstream, if unutilized, then it is stored in the liver for future use.
Pile acids recycle in fat metabolism
Adequate quantity of bile acid is necessary for fat metabolism it is maintained by:
- The liver produces about 0.6 g of new bile acid daily from cholesterol.
- This is adding into 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.
- Digestion of fat and releases it into the blood stream after a meal.
- 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.