Notes on Lipid Metabolism
Lipids are one of the body's sources of energy. They are also used for insulation, in the structure of cell membranes and in hormones. Lipids are not water soluble so they are not easily absorbed during digestion. This means that to be completely metabolized and circulate through the blood stream lipids must go through additional chemical reactions.
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Metabolism
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Metabolism includes all of the chemical reactions that either build up or breakdown substances in the body. Reactions that build-up by combining atoms or molecules to form one larger compound are called anabolism. The reactions that cause larger compounds to breakdown are called catabolism. During catabolism the bonds between molecules are broken and energy is produced. On the flip-side, anabolic reactions consume energy. The metabolism of lipids includes their creation (anabolism) and their decomposition (catabolism), but a reference to "lipid metabolism" often carries a focus on digestion and the process of breaking lipids down to be used by the body.
Digestion
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Lipids are digested in the small intestine. They are first broken into their chemical parts---glycerol and fatty acids---which allows them to be absorbed into the intestinal lining. Then they are rebuilt, mixed with cholesterol, and covered in a layer of protein. The resulting droplet---a lipoprotein or chylomicron---is released into the blood. The only way lipids can circulate through the body is in the form of a lipoprotein.
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Catabolism
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Metabolism continues in the liver, where lipid molecules are again broken down into glycerol and fatty acids. The liver converts the glycerol into glucose, which can easily be put through another series of reactions that result in chemical energy used by the cells. Catabolism of the fatty acids requires additional steps called beta oxidation. Liver enzymes break the molecules of fatty acids into smaller fragments. The smaller fragments become acetyl-coenzyme A, and in this form they are available to become energy. Larger fragments are converted into ketone bodies. The liver can't metabolize ketone bodies so they are released back into the blood stream where they enter a cycle that breaks them down for energy.
Anabolism
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Anabolism, or the phase of metabolism that turns glucose or amino acids into lipids, also occurs in the liver. Converting glucose back into a lipid is essentially the reverse of the catabolic process. Amino acids must first be turned into glucose then follow the same process. The purpose of lipid anabolism is to either store excess lipids in the form of fat or to produce lipids for other uses in the body.
Metabolism Disorders
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Lipid metabolism can be disrupted if the enzymes necessary to break them down are deficient or not functioning properly. The results can be an excess of lipids that accumulate in the body or a deficiency in energy. One type of deficiency is an inherited disorder that develops before the age of 3. Left untreated, this deficiency---MCAD Deficiency---can have serious results including delayed mental and physical development, an enlarged heart, and irregular heartbeat. Even though it's rare, according to the Merck Manual, newborn babies have been routinely screened for MCAD since 2007.
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