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Lipid metabolism is a complex process which involves multiple steps from production within the body or dietary intake to degradation or transformation into several lipid-containing structures in the body.
Lipid metabolism is a complex process that involves multiple steps involving the dietary intake of lipids (exogenous) or the production of lipids within the body (endogenous) to degradation or transformation (catabolism) into several lipid-containing structures in the body. A brief description of metabolism related to fatty acids and cholesterol is provided below.
The dietary fat in the form of triacylglycerol (TAG), cholesterol, cholesteryl esters, and free fatty acids is absorbed by the intestine after going through various steps during digestion from mouth to intestine.
Fatty acids, once absorbed from intestine, are activated in the intestinal wall and eventually resynthesize the TAG. Some of the shorter-chain fatty acids go to the liver after binding to albumin in the blood. TAG, long-chain fatty acids, and cholesterol, after activation, are packaged in the chylomicron particles and moved to the bloodstream via the lymphatic system to reach the entire body. The TAG is mainly broken down in the capillaries of skeletal muscles and adipose tissues. Other organs like heart, lung, liver, and kidneys are also involved. TAG in the chylomicrons is degraded to free fatty acids and glycerol by lipoprotein lipase. This enzyme is primarily produced in the adipose tissues and muscle cells. Thus formed free fatty acids may be either used for energy or may be stored as TAG after reesterification. However, glycerol that is released from TAG is used almost exclusively by the liver to produce glycerol 3-phosphate and this can be used either in the glycolytic pathway to produce energy or in the production of new molecules of glucose (gluconeogenesis). Fatty acids can also be produced by the body and this process occurs mainly in the liver and adipose tissues.
Fatty acids that are stored in the adipose tissue in the form of TAG serve as a major energy storage. The complete oxidation of fatty acids produces 9 kcal/g of fat as compared to proteins and carbohydrates which produce about 4 kcal.
The major degradation (catabolism) of saturated fatty acids occurs in mitochondria, and this process is called beta oxidation. During beta oxidation, two-carbon fragments are removed successively generating high-energy-rich substances (like NADH and FADH2 molecules). After the oxidation of fatty acids, acetyl Co-A is produced and eventually it is used in the formation of ketone bodies. Thus formed ketone bodies can be used as energy source or for production of other important biochemical substrates. However, the oxidation of unsaturated fatty acids results in lesser calories, and this process requires additional enzymes in the metabolic pathway.
Lipid metabolism also includes the formation and degradation of several other important lipid particles like phospholipids which play a very important role in the structure of cellular membranes, prostaglandins, and several hormonal activities in the body.
Cholesterol is not only provided to the body by the dietary intake but also synthesized by virtually all the tissues in the body, although liver, intestine, adrenal cortex, reproductive tissues including ovaries, testes, and placenta produce most of the cholesterol in the body. These sites also utilize cholesterol in the process of production of multiple hormones in the body.
Degradation of the cholesterol happens mainly in the liver, where the sterol nucleus is eliminated from the body after conversion into bile acids and bile salts, which are excreted in the feces, or it may undergo enterohepatic circulation.
The lipoproteins (chylomicrons, high-density lipoproteins, and low-, intermediate-, very-low-density lipoproteins) are involved with transporting these various lipid particles in the body with the apolipoproteins on their surface.
References and Further Reading
- Harvey, R. A., & Ferrier, D. R. (2008). Cholesterol and steroid metabolism. In R. A. Harvey (Ed.), Lippincott’s illustrated reviews biochemistry (pp. 181–200). Philadelphia: Wolters Kluwer/Lippincott Williams and Wilkins.Google Scholar