EFAs Docosahexaenoic acid

In common with other vertebrates, fish cannot synthesize linoleic or linolenic acids de novo and vary considerably in abilities to convert 18-carbon unsaturated fatty acids to longer-chain, more highly unsaturated fatty acids of the same series. The EFA requirements for selected species of fish are presented in Table 16 (11). In general, freshwater fish require either linoleic acid (18 2n-6) or hnolenic acid (18 3n-3) or both, whereas stenohaline marine fish (those unable to withstand a wide variation in water salinity) require dietary eicosapentaenoic acid (EPA, 20 5n-3) and/or docosahexaenoic acid (DHA, 22 6n-3). 

A double bond within the terminal seven carbon atoms can be present at o>-3 or co-6. y-Linolenic acid is an a>-6 EFA and a-linolenic acid an rw-3 EFA. Other co-3 EFA are eicosapentaenoic acid (EPA) and docosahexaenoic acid (EX)HA), both abundant in edible fish tissues. Vegetable oils are rich in rw-6 EFA (Table 18-4). Plants contain a-linolenic acid, which can be converted in the body to EPA and DOHA, but it is found within chloroplast membranes and not in seed oils hence, it may not be available in significant quantities in the diet. The a>-3 and o)-6 EFA have different metabolic effects (see below). Particularly rich sources of EPA are fishes (e.g., salmon, mackerel, blue fish, herring, menhaden) that live in deep, cold waters. These fishes have fat in their muscles and their skin. In contrast, codfish, which have a similar habitat, store fat in liver rather than muscle. Thus, cod liver oi I is a good source of EPA, but it also contains high amounts of vitamins A and D, which can be toxic in large quantities (Chapters 38 and 37, respectively). Shellfi.sh also contain EPA. Plankton are the ultimate source of EPA. 

The experimental study of FA deficit has been characterized by investigations that utilize food deprivation or restrictions on nutritional intake, and by designs that have provided for dietary supplementation of the FA and/or their metaboUtes (especially DHA and its precursors EPA and LNA). Metabolic studies continue to address many of the unexplained complexities associated with the behavior performance observations in the laboratory. Among the questions of interest are How do the EFAs get into the brain and other organs What is the basis for the apparent selectivity of various organs, cells, and subcellular organelles for particular lipids and FA Why is DHA (docosahexaenoic acid 22 6n-3) concentrated in the brain How can the adult brain maintain its DHA even when there is little support in the diet How much can the metabolism of the precursors of DHA (e.g., LNA, EPA, etc.) support DHA composition in the brain in comparison to the incorporation of preformed DHA taken in the diet In addition to their basic science value, these issues have practical implications for public health policy, such as the design of infant formulas. 

Important in this respect is that 18 2, n-6 18 3, n-3 20 3, n-9 FA and varions cis-trans isomers show competitive inhibition with 20 3, n-6 20 4, n-6 and 20 5, n-3, which are the precursors for the known biologically active prostaglandins and lencotrienes. Administration of an EFA-deficient diet leads to partial replacement of AA by 20 3, n-9, which is a substrate for lipoxygenase bnt not for cyclooxygenase. Feeding a diet high in PUFA of the n-3 type (e.g., linseed oil, fish oil) resnlts in an enrichment of membrane phospholipids with EPA and docosahexaenoic acid (DHA). This may explain why the FA composition of tissue lipids, including membrane PF, can be modified to some extent by the amount of each class of FA in the diet. 

As in other marine finfish larvae, first feeding in YTK larvae is a major hurdle and adequate nutrition is critical to the success of this phase. Also in common with larvae of most other marine finfish, essential fatty acids (EFAs), in particular docosahexaenoic acid (DHA), are critical for normal development (Masuda et al., 1998,1999). DHA is accumulated in the central nervous system of YTK larvae and is essential not only for activity and vigour but also for the development of schooling behaviour in juveniles. Studies on the effect of the different EFAs on the growth and survival of kingfish larvae such as those of S. quinqueradiata have shown that the growth and survival rate of those fed DHA-enriched Anemia at 2.1-2.5 % dry wt day is up to ten times better (88%) than larvae fed Anemia enriched with other EFAs including eicosapentaenoic acid (EPA), arachi-donic acid (AA) or oleic acid (OA). 

In both EFA families 20 and 22 carbon chain length derivatives with 3,4,5, and 6 double bounds are produced. Linoleic acid ((o-6), a-Unolenic add (cd-3), and oleic acid (oj-9) compete for elongation and desaturation by the same enzyme system, which has preferential affinities as follows a-linolenic > linoleic > oleic add [9]. Arachidonic acid (AA) (C20 4 co-6) is a principle component in the phospholipids of cell membranes and serves as a major precursor for prostaglandin and leukotriene synthesis. Eicosapentaenoic acid (EPA) (C20 5 co-3) is the corresponding 20 carbon chain length derivative of a-linolenic acid, but it can be further converted to the docosahexaenoic acid (DHA) (C22 6 to-3). The latter is particularly concentrated in highly active sites, e. g., the synaptic junction and the outer segments of the rods in the retina [21]. 

In addition to the three main EFA, linoleyl alcohol (Turpeinen, 1938), docosahexaenoic acid, and two hexahydroxystearic acids (linusic and... 

An outstanding feature of the composition of brain phospholipids is its remarkable consistency, irrespective of species or diet. The concentrations of the precursor EFA are extremely low (18 2, n-6,0.1-1.5% and 18 3, n-3, 0.1-1.0%) while arachidonic (20 4, n-6) and docosahexaenoic (22 6, n-3) acids predominate at 8-17% and 13-29% respectively in all species. This contrasts with the liver lipids where there is much greater variation between species. The precursor EFA are present in much greater concentrations than they are in brain and there are major differences in the product EFA. For example, 22 5 is the major n-3 fatty acid in the liver lipids of ruminants and other herbivores while 22 6 predominates in the carnivores and omnivores. Fatty acids of the n-6 family usually predominate in liver phosphoglycerides, even when the overwhelming dietary intake is in favour of the n-3 fatty acids. Thus, zebra and dolphin, both species that have an overwhelming excess of n-3 fatty acids in the diet, attain a preponderance of n-6 acids in the liver phosphoglycerides (Table 5.14). 

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