How enzymes digest lipids

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Digestion of edible fats

Fats or lipids are either ingested with food or formed in the body itself. Dietary fats are first digested in the duodenum and upper small intestine. In the presence of proteins, already existing fat breakdown products, lecithin and bile acids, they are converted into emulsified droplets with a diameter of about 5 nm. The enzyme pancreatic lipase accumulates at the interfaces of these droplets and gradually hydrolyzes triacylglycerols to fatty acids and monoacylglycerides. Due to their insolubility in water, these fatty acids and monoacylglycerides are combined to form micelles; this process is promoted by bile acids. The various components of the micelles (monoacylglycerides, fatty acids, cholesterol, fat-soluble vitamins) are then absorbed into the cells of the intestinal mucosa. This type of compartmentalization enables 1000 times more fatty acids to be broken down.

Fat absorption takes place in the mucous membrane of the upper small intestine. It is characterized by the absorption of the mixture of neutral fat, fatty acids, di- and monoglycerides emulsified by bile acids with the formation of micelles, which occurs with the assistance of phosphatides. The chyme in the intestine is almost fat-free after fat absorption. Resorption is followed by the transfer of the neutral fats created by fat synthesis in the mucous membrane cells into the lymphatic system and into the venous circulation or the transport of the - water-soluble - fatty acids via the portal vein circulation to the liver, where the latter are oxidized.

Since fats are insoluble in water, they are bound to certain molecules, the lipoproteins, for transport in the blood.

The job of lipoproteins is to transport the various fats (cholesterol, cholesterol ester, phospholipids and triglycerides) in the blood. Each type of lipid is transported with a special lipoprotein. The lipoproteins of very low density are of particular importance (very low density lipoproteins; VLDL), low-density lipoproteins (low density lipoproteins; LDL) and high density lipoproteins (high density lipoproteins; HDL) too. While the VLDL mainly carry triglycerides, the LDL contain up to about 80% serum cholesterol. The HDL is able to absorb cholesterol and transport it to the liver, where it is then excreted in the bile.

Tab. 1
Types of lipoproteins
HDLhigh density lipoproteins = High density blood lipids
LDLlow density lipoproteins = Low density blood lipids
VLDLvery low density lipoproteins = Blood lipids with a very low density