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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The typical western diet is overloaded with omega-6 fatty acids and contains insufficient omega-3 fatty acids. The current ratio of omega-6 to omega-3 fatty acids in the U.S. diet is about 10:1, whereas in hunter-gatherer diets it is closer to 2:1. This dietary imbalance in fatty acids (excessive omega-6 and insufficient omega-3) is a fundamental underlying cause of many chronic diseases including cardiovascular disease, many cancers, most inflammatory diseases, and many psychological disturbances.
There are multiple physiological reasons why this laundry list of illnesses and diseases can be caused by or exacerbated from an imbalance in omega-6 and omega-3 fatty acids, but one of the most crucial stems from the synthesis of paracrine (nearby) hormones called eicosanoids. Eicosanoids are synthesized from three 20 carbon fatty acids (20:3n-6, 20:4n-6 and 20:5n-5) lying in cell membranes. Eicosanoids have multiple regulatory functions in the body, including regulating the inflammatory response. Eicosanoids synthesized from 20:5n-3 (EPA) tend to be anti-inflammatory, whereas eicosanoids derived from 20:4n-6 (arachidonic acid) may promote inflammation in certain tissues. Because of the imbalance in the (omega-6/omega-3) ratio in the typical western diet, a chronic state of low grade inflammation can exist which in turn may promote many health disorders and diseases.