Vegetarians, Essential Fatty Acids and DHA
Getting the right essential fatty acids (EFAs) may pose a dilemma for many vegetarians. The essential fatty acids linoleic acid (LA, omega-6) and alpha-linolenic acid (LNA, omega-3) are utilized by the body as sources of energy, as functional components of cell membranes and for the production of various compounds in the body such as eicosanoids that either promote or decrease inflammation. These fatty acids are essential because the human body cannot synthesize them from other fats. Therefore, they must be taken in through foods.
Linoleic acid is easy to get in the diet, in fact too easy because it is found in high amounts in most vegetable oils. Because Americans eat lots of fried foods and margarine, our LA intake tends to be higher than our bodies are designed to handle. The current dietary ratio of omega-6 to omega-3 is 2030:1 whereas in pre-modern times the ratio was on the order of 12:1.  The omega-3 fatty acids such as LNA are in short supply because they are not widely distributed in the foods most Americans eat. LNA is found in the greatest amounts in canola oil, flax oil and walnut oil.
Although LA is essential for our health, an excess of it in the diet causes its conversion into arachidonic acid (AA), a fatty acid also found in meat. In the body, AA can be converted into inflammation-promoting eicosanoids such as the 2-series prostaglandins that contain two double bonds in their side chains. LNA acid is more likely to be converted into eicosapentaenoic acid (EPA) and docosohexaenoic acid (DHA), which then convert to inflammation-reducing eicosanoids such as the prostaglandins in the 3-series that have more unsaturated side chains containing three double bonds. But the conversion of LNA to EPA is limited and conversion to DHA is negligible. They are both easily overwhelmed by AA derived from linoleic acid. 
Many vegetarians take flax oil supplements to increase their intake of omega-3 fatty acids. What they may not know, however, is that flax oil, because of low conversion, is unlikely to increase their own production of DHA.  DHA concentrates in the brain, retina and sperm cells and is important for the development of vision and neurological function in infants. Nonmacrobiotic vegetarians have significantly lower serum levels of DHA than omnivores, who derive their DHA from fish. Eating at least two portions of fatty fish per week, or using a fish oil supplement, can provide sufficient DHA as well as EPA. 
Lactoovovegetarians can get small amounts of DHA from the yolks of regular eggs (0.018 g or 18 mg) and even more from special DHA-enriched eggs from chickens fed a DHA-rich algae supplement (100175 mg). But vegans and raw foods dieters are at risk of having insufficiencies of DHA unless they supplement. Fortunately, DHA supplements that derive the fatty acid from marine algae are now available.
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2. Gerster H. Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)? Int J Vitam Nutr Res 1998;68(3):159-73.