sources of alpha-linolenic acid (ALA), rarely produce polyunsaturated fatty acids longer than 18 carbons and thus are not sources of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Though more genetically complex animals can synthesize EPA and DHA from ALA (Qiu, 2003), the rate of synthesis in most species is low. Fish are good sources of EPA and DHA primarily because their natural diets contain these fatty acids, not because they are able to synthesize them de novo. Organisms low on the food chain consume the algal and microbial sources of EPA and DHA, which become concentrated in the lipid stores of those species higher up in the food chain.
Omega-3 fatty acids are long-chain polyunsaturated fatty acids that are characterized by the presence of a double bond at the omega position (3 carbon atoms from the methyl end) in the carbon chain. This position is what identifies them as omega-3 fatty acids. EPA and DHA are not endogenously synthesized from saturated, monounsaturated, or omega-6 fatty acids; they can only be made from the precursor omega-3 fatty acid, ALA. Figure A-1 shows the synthesis pathways for omega-3 fatty acids.
The omega-3 fatty acids include:
Alpha-linolenic acid, 18:3 n-3, a plant-derived source of fatty acid. ALA can be converted to the omega-3 fatty acids EPA and DHA through a series of desaturation and chain elongation events, but the conversion in humans is inefficient and varies with the content of other fatty acids in the diet (see discussion below for more information about conversion efficiency);
Eicosapentaenoic acid, 20:5 n-3, a fatty acid synthesized from ALA and found primarily in fatty fish. EPA is a precursor molecule in the human synthesis of one family of eicosanoids, including prostaglandins, thromboxane, leukotrienes, hydroxy fatty acids, and lipoxins. These compounds serve as modulators of cardiovascular, pulmonary, immune, reproductive, and secretory functions at the cellular level;
Docosahexaenoic acid, 22:6 n-3, a fatty acid synthesized from ALA and found primarily in fatty fish. It is a component of all membrane structural lipids in neural and retinal tissues and spermatozoa. The developing brain accumulates large amounts of DHA late in fetal life. This accumulation continues through at least the first 2 postnatal years.
Selenium is an element classified within Group VIA in the periodic table following oxygen and sulfur but preceding tellurium and polonium.