LIPIDS: CLASSIFICATION, STRUCTURE AND FUNCTIONS AND METABOLISM
Abstract:
The majority of naturally occurring unsaturated fatty acids exist in the cis-conformation. Trans fatty acids occur in some foods and as byproducts of the process of hydrogenating unsaturated fatty acids to make them solids at room temperature, such as in partially hydrogenated vegetable oils. Diets high in trans fatty acids have been associated with an increased risk of cardiovascular disease and development of the metabolic syndrome and have, therefore, been banned from manufactured food products by most major governments.
The majority of fatty acids found in the body are acquired in the diet. However, the lipid biosynthetic capacity of the body (fatty acid synthase and
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The designation of a polyunsaturated fatty acid (PUFA) as an omega-3 fatty acid, for example, defines the position of the first site of unsaturation relative to the omega end of that fatty acid. Thus, an omega-3 fatty acid like α-linolenic acid (ALA), which harbors three carbon-carbon double bonds (i.e. sites of unsaturation), has a site of unsaturation between the third and fourth carbons from the omega end. There are three major types of omega-3 fatty acids that are ingested in foods and used by the body: ALA, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Once eaten, the body converts ALA to EPA and DHA, the two types of omega-3 fatty acids more readily used by the body and which serve as important precursors for lipid-derived modulators of cell signaling, gene expression and inflammatory processes.
It is important to denote that when discussing omega-3 fatty acids, their dietary origin is quite important. Omega-3 fats from plants, such as those in flax seed oil, are enriched in ALA. As indicated above, ALA must first be converted to EPA (requiring three independent reactions) and then to DHA (requiring an additional four reactions). Omega-3 fats from fish are enriched in EPA and DHA and thus do not need to undergo the complex conversion steps required of ALA. In addition, the conversion of ALA to EPA and DHA is inefficient in individuals consuming a typical Western diet rich in animal