Polyunsaturated Fatty Acids
Polyunsaturated fatty acids contain two or more double bonds along their carbon backbones. Polyunsaturated fatty acids are classified into two biologically important subgroups: omega-6 and omega-3 fatty acids. Below is a list of the common and numeric names for omega-6 and omega-3 polyunsaturated fatty acids that may occur in foods.
|Omega-6 Family Common Name||Numeric Name|
|Gamma linolenic acid||18:3n-6|
|Dihomo gamma linolenic acid (DHGLA)||20:3n-6|
|Omega-3 Family Common Name||Numeric Name|
|Alpha linoleic acid (ALA)||18:3n-3|
|Eicosapentaenoic acid (EPA)||20:5n-3|
|Docosahexaenoic acid (DHA)||22:6n-3|
The numeric naming scheme for polyunsaturated fatty acids follows that for saturated and monounsaturated fatty acids. With omega-6 polyunsaturated fatty acids, the last double bond (=) occurs 6 carbon atoms down from the omega or methyl end of the fatty acid, whereas with omega-3 polyunsaturated fatty acids, the last double bond (=) occurs 3 carbon atoms down from the omega end of the fatty acid. In the case of linoleic acid (18:2n-6), there are 18 carbon atoms in the molecule, 2 double bonds (=), and the last double bond is located 6 carbon atoms down from the omega or methyl end of the fatty acid. Below is a schematic diagram of linoleic acid.
As was the case with saturated and monounsaturated fatty acids, the schematic diagram above is not precisely correct because it does not show the correct angle of the carbon-to-carbon bonds, which really is 109 degrees rather than 180 degrees. In the diagram below you see a geometrically correct diagram of linoleic acid.
Let’s now examine the structure of an omega-3 fatty acid, alpha linolenic acid, or 18:3n-3. There are 18 carbon atoms in the molecule, 3 double bonds (=), and the last double bond is located 3 carbon atoms down from the omega or methyl end of the fatty acid. Below is a schematic diagram of alpha linolenic acid.
Alpha linolenic acid (18:3n-3) is the so-called “parent” fatty acid for the omega-3 family of fatty acids because the liver can make other omega-3 fatty acids from it. Similarly, linoleic acid (18:2n-6) is the “parent” fatty acid for the synthesis of other omega-6 fatty acids in the liver. Below is a diagram of how both parent fatty acids can be desaturated and chain elongated in the liver into longer chain polyunsaturated fatty acids.
The conversion of dietary 18 carbon polyunsaturated fatty acids (PUFA) to longer chain ( > 20 carbon atoms in length) PUFA is an inefficient process. Only about 6 % of ALA is converted to EPA and less still (0.5 %) is converted to DHA. Because omega-3 and omega-6 parent fatty acids must compete for the same enzymes of desaturation and elongation, a high dietary intake of omega-6 fatty acids (18:2n-6) can further reduce the conversion of ALA to EPA and DHA by 40 to 50 %.
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There is considerable interest in the potential impact of several polyunsaturated fatty acids (PUFAs) in mitigating the significant morbidity and mortality caused by degenerative diseases of the cardiovascular system and brain. Despite this interest, confusion surrounds the extent of conversion in humans of the parent PUFA, linoleic acid or alpha-linolenic acid (ALA), to their respective long-chain PUFA products. As a result, there is uncertainty about the potential benefits of ALA versus eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA). Some of the confusion arises because although mammals have the necessary enzymes to make the long-chain PUFA from the parent PUFA, in vivo studies in humans show that asymptotically equal to 5% of ALA is converted to EPA and <0.5% of ALA is converted to DHA. Because the capacity of this pathway is very low in healthy, nonvegetarian humans, even large amounts of dietary ALA have a negligible effect on plasma DHA, an effect paralleled in the omega6 PUFA by a negligible effect of dietary linoleic acid on plasma arachidonic acid. Despite this inefficient conversion, there are potential roles in human health for ALA and EPA that could be independent of their metabolism to DHA through the desaturation – chain elongation pathway.
1. Plourde M, Cunnane SC. Extremely limited synthesis of long chain polyunsaturates in adults: implications for their dietary essentiality and use as supplements. Appl Physiol Nutr Metab. 2007 Aug;32(4):619-34.