FEATURE
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Essential fatty acids in aquaculture
by Christopher C. Parrish, Department of Ocean Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, A1C 5S7 Canada s the fastest growing food sector, aquaculture is evolving in terms of technological innovation and adaptation to meet changing requirements. One of these is to reduce the dependence on fishmeal and fish oil, which provide feed ingredients for many aquaculture species. Currently, around 15 percent of the landings from fisheries are used for the production of fishmeal and fish oil or for direct feeding. This level of use is unsustainable since capture fishery production has been relatively static since the 1980s. Aquaculture’s reliance on threatened marine resources constrains its profitability and growth and has led to the development of diets that incorporate a significant amount of terrestrial plant oils. Plant oils and fish oils contain different types of fatty acids in terms of omega (ω) family and in terms of chain-lengths. Fatty acids are the building blocks of fats and oils and among the many that occur in cultured organisms and their food, the focus has been on the long-chain fatty acids, docosahexaenoic acid (DHA, 22:6ω3), eicosapentaenoic acid (EPA, 20:5ω3) and, to a lesser extent, arachidonic acid (ARA, 20:4ω6). These fatty acids contain 20 or 22 carbons and between four and six double bonds and they belong to two families, the ω3 series and the ω6 series. Because they have more than one double bond they are called polyunsaturated fatty acids (PUFA) and because they are required by organisms for optimal health they are deemed essential. They maintain membrane structure and function and are precursors of bioactive compounds in vertebrates, invertebrates and plants.
Definition of essential fatty acids
The use of the term ‘essential fatty acid’ originated from work with rats more than a half century ago when it was shown that the 18 carbon PUFA, linoleic acid (LNA, 18:2ω6) and α-linolenic acid (ALA, 18:3ω3) could eliminate deficiency states in rats that had been fed fat free diets. The ensuing search for fatty acids with essential fatty acid activity showed there were a variety of PUFA of the ω6 and ω3 series that could remove deficiency symptoms, some of which are shown in Figures 1 and 2. When provided with sufficient ω3 and ω6 fatty acids in the diet, most animals can make other ω3 and ω6 fatty acids by inserting double bonds and by chain elongation, but the ω3 and ω6 series are not interconvertible in animals except in the case of transgenic animals. So, while LNA and ALA have physiological roles in and of themselves, their major role is as precursors of long-chain PUFA. The extent to which a given species can convert one ω3 fatty acid to another or one ω6 fatty acid to another leads to degrees of essentiality. Thus, while DHA, EPA, and ARA are the most important PUFA which could be supplied in the diet, many
animals have at least some ability to synthesise them when sufficient quantities of suitable shorter chain precursors, such as LNA and ALA, are available. This capability in marine fish appears to be low and so they need to be provided one or more of the three important long-chain PUFA at some minimal level, which may change according to age or growth temperature.
Effects of essential fatty acids
The long-chain ω3 fatty acid content of seafood has implications for seafood quality. In this context, public interest in ω3 fatty acids has increased dramatically over recent years since consumption of EPA and DHA decreases cardiovascular risk factors and has beneficial effects on several diseases and mental health. However, for many, the distinction between ω3 fatty acids such as EPA and DHA and shorter chain ones such as ALA is less clear. While ALA is a ω3 fatty acid, it has fewer carbons than EPA or DHA, and humans, like marine fish, are inefficient at converting it to EPA and then on to DHA. Many people know that EPA and DHA are recommended by the American Heart Association, for example, and that they are derived from seafood. However, few people realise that the 18:3
20:5
22:6 Figure 1: tStructures of some ω3 polyunsaturated fatty acids present in aquatic animals. Fatty acid notation gives the ratio of carbon atoms to double bonds. α-linolenic acid (ALA, 18:3ω3), eicosapentaenoic acid (EPA, 20:5ω3), and docosahexaenoic acid (DHA, 22:6ω3) are all related biochemically because of the location of the first double bond 3 carbons from the methyl end of the chain. 18:2
20:4
Figure 2: Structures of some ω6 polyunsaturated fatty acids present in aquatic animals. Fatty acid notation gives the ratio of carbon atoms to double bonds. Linoleic acid (LNA, 18:2ω6) and arachidonic acid (ARA, 20:4ω6) are related biochemically because of the location of the first double bond 6 carbons from the methyl end of the chain.
16 | January 2018 - International Aquafeed