For the most part, organisms can satisfy their own fatty acid needs. In mammals, however, those enzymes which insert double bonds into the chains of fatty acids cannot insert them into the omega-3 and omega-6 positions. Linoleic and linolenic acid contain omega-6 and 3 bonds and are necessary for mammalian metabolism. These two fatty acids are therefore essential fatty acids (EFA) and must be supplied by the diet.
Variations among species and within species occur in terms of what is or is not essential and at what levels. For example, the cat is limited in its ability to synthesize arachidonic acid (AA, 18:4w6). 1,2 Humans are unable to synthesize sufficient fatty acids with double bonds closer to the methyl end than omega-9. Varying capabilities occur even within species. 3 A diet satisfactory for one individual may not be for another, since enzyme systems regulating synthesis and metabolism vary with individual genetics.
Varying environmental circumstances may also alter needs. A sufficient diet this week may not be so next week if circumstances change. The consumption of alcohol or medications, the presence of disease, atrophy, increased stress, increased consumption of trans- fatty acids (present in all foods containing partially hydrogenated oils), dieting, obesity, allergy, young age, old age, high cholesterol, and so forth can all alter nutrient requirements. 4 There is no such thing as an average requirement which fits everyone all the time.
Some factors affect dietary intake while others affect the ability of essential fatty acids, once consumed, to convert through the steps necessary to form their physiologically important end products. For example, LA (18:2w6) consumed must first be converted to gamma linolenic acid (GLA, 18:3w6) by desaturase enzymes, and then on to other products by enzymes to form important eicosanoids such as prostaglandins. Many factors (stress, disease, allergy, etc.) can interfere with this first step mediated by delta-6-desaturase to create GLA. 5 Therefore, for some individuals, GLA or its progeny may have to be directly supplied to the diet thus by-passing the need for this enzyme. This is the rationale for the use of the supplement evening primrose oil since it contains high levels of GLA. Other oil sources such as black currant seed, borage, and fish also supply fatty acids which skip several steps in these metabolic pathways and thus may be important dietary components for some individuals (animals and people). (Fig. 13)
An important component of fish oil, for example, eicosapentaenoic acid (EPA, 20:5w3) may become a dietary essential if age or other factors limit the ability to convert dietary plant-derived LNA (l8:3w3) into EPA, which is the precursor to several important omega-3 derived eicosanoids.
Although not considered essential fatty acids in the sense of LA and LNA, there are at least four other unsaturated fatty acids in nutrition now recognized as playing key biochemical roles, and for which there may be an essential requirement for some individuals. These are oleic acid (OA, 18: 1w9), gammalinolenic acid (GLA, 18:3w6), eicosapentaenoic acid (EPA, 20:5w8), and docosahexaenoic acid (DHA, 22:6w3). This list is surely incomplete.
Note that OA is a mono-unsaturated compound (one double-bond), GLA has three double bonds, and EPA and DHA have five and six double bonds, respectively. (Fig. 5) Linolenic acid (manufactured only in plant chloroplasts) as well as EPA and DHA belong to the omega-3 family of fatty acids; LA and GLA, as well as arachidonic acid (AA: 20: 4w6), are members of the omega-6 family. Oleic acid is a member of the omega-9 family. All these fatty acids have great biochemical significance and the understanding of their metabolic roles is increasingly demonstrating their link to health and disease.
The essentiality of fatty acids, like that of all other nutrients, is linked to many variables. Required amounts and types of fatty acids can vary among species and even for the same individual. Diets rich in unaltered fatty acids (particularly raw whole foods) and specific properly prepared fatty acid supplements (a second choice) can provide sufficient fatty acid substrate for optimal health.
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