Fatty acids are rarely found unattached in the body. Their most common partnership occurs in triglycerides or triacylglycerols, known commonly as fats. Triacylglycerol is a three-carbon alcohol to which are attached three fatty acids as esters, as shown:
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In vegetable oils, long-chain fatty acids of 20 or more carbons are most frequently found in the outside positions, one and three. In fish oils and animal fats, the highly unsaturated LC-PUFAs are usually located in the central sn-2 position.
Triglycerides (TG) are the storage form of fatty acids in adipose tissue, food fats, and oils. In order to be digested in the gut they need to be emulsified or made soluble, then broken down to remove the fatty acids from the triglyceride backbone. Emulsification is accomplished in the intestine by bile salts, while cleavage of the fatty acids is the work of the enzyme pancreatic lipase, also released into the intestine. Pancreatic lipase removes fatty acids from the 1 and 3 positions to generate free fatty acids and a TG remnant with one fatty acid in the 2-position, a monoglyceride (2-acylglycerol). Monoglycerides may be further broken down by the enzyme phospholipase A2 into free fatty acids and glycerol. Fatty acids and monoglycerides form complexes with bile salts called micelles, which are absorbed passively into the intestinal cell or enterocyte. Once inside the enterocyte, fatty acids are re-esterified into TG and packaged with cholesterol and lipoproteins into particles called chylomicrons. These rather large units are discharged into the lymph system for eventual disposal into the blood circulation. A substantial portion of the free fatty acids is also transported directly to the liver via the portal vein.
Once in the blood stream, chylomicrons acquire additional proteins and interact with the enzyme lipoprotein lipase found in endothelial cells lining the blood vessels. This enzyme removes some of the fatty acids, which are then taken up by tissues and resynthesized into TG for storage, export, oxidation, or further metabolism. The chylomicron remnants and glycerol backbone are removed by the liver, while the remaining lipids and proteins are transferred to high density lipoproteins in the blood.
The liver is a primary site for fatty acid metabolism. Fatty acids are removed from the circulation, synthesized into TG, and packaged with proteins, cholesterol, and other lipids into very low density lipoproteins (VLDL). Discharged back into the circulation, VLDL are the primary vehicle for delivering TG to tissues, particularly adipose tissue and skeletal muscle. In the circulation, TGs are removed from VLDL through the action of the enzyme lipoprotein lipase, leaving intermediate-sized particles and low density lipoproteins (LDL). LDL are rich in cholesterol which is made available for membrane structure and intracellular storage. Click here for more details on lipoprotein metabolism.