J.R. Sargent

The role of polyunsaturated fatty acids in fish

The physical properties of polyunsaturated and saturated fatty acids are compared in relation to melting points and fluidity. The role of polyunsaturated fatty acids on membrane fluidity and membrane bound enzyme activity is discussed. The influence of the environment, particularly temperature, on poikilothermic animals is considered in relation to membrane fatty acid composition and metabolism. The metabolic role of polyunsaturated fatty acids of the (n-3) series and their interaction with arachidonate metabolism is discussed.

DEVELOPMENT OF FARMED FISH : A NUTRITIONALLY NECESSARY ALTERNATIVE TO MEAT

The projected stagnation in the catch from global fisheries and the continuing expansion of aquaculture is considered against the background that fishmeal and fish oil are major feed stocks for farmed salmon and trout, and also for marine fish. The dietary requirement of these farmed fish for high-quality protein, rich in essential amino acids, can be met by sources other than fishmeal. However, the highly-polyunsaturated fatty acids eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) present in high concentrations in fish oil are essential dietary constituents for marine fish and highly-desirable dietary constituents for salmonids. Currently, there is no feasible alternative source to fish oil for these nutrients in fish feeds. Vegetable oils rich in linoleic acid (18:2n-6) can partially substitute for 20:5n-3 and 22:6n-3 in salmonid and marine-fish feeds. However, this is nutritionally undesirable for human nutrition because the health-promoting effects of fish-derived 20:5n-3 and 22:6n-3 reflect a very high intake of 18:2n-6 relative to linolenic acid (18:3n-3) in Western diets. If partial replacement of fish oils in fish feeds with vegetable oils becomes necessary in future, it is argued that 18:3n-3-rich oils, such as linseed oil, are the oils of choice because they are much more acceptable from a human nutritional perspective, especially given the innate ability of freshwater fish, including salmonids, to convert dietary 18:3n-3 to 20:5n-3 and 22:6n-3. In the meantime, a more judicious use of increasingly-expensive fish oil in aquaculture is recommended. High priorities in the future development of aquaculture are considered to be genetic improvement of farmed fish stocks with enhanced abilities to convert C18 to C20 and C22 n-3 polyunsaturated fatty acids, enhanced development of primary production of 20:5n-3 and 22:6n-3 by single-cell marine organisms, and continuing development of new species.

Fish oils and human diet

Trends in global fish catches are described together with fish landings and fish consumption in the UK. The importance of n-6 and n-3 polyunsaturated fatty acids as essential constituents of human diets is considered and the role of oily fish as a dietary source of the long-chain n-3 polyunsaturates, docosahexaenoic acid and eicosapentaenoic acid, is emphasized. The origin of n-3 polyunsaturates in, the marine phytoplankton and their transmission via zooplankton to fish is described as a means of understanding the composition of different fish body oils. The ease with which the fatty acid composition of fish body oils can be manipulated by altering the fatty acid composition of their feeds is emphasized and the dietary requirements of marine and freshwater fish for n-3 and n-6 polyunsaturates considered. Farming fish on diets containing principally fish meal and fish oil, as used in salmon production in Scotland, generates a high quality product with levels of long-chain n-3 polyunsaturates equalling or exceeding those of wild fish. Farming fish on high quality marine oils rich in docosahexaenoic and eicosapentaenoic acids is an efficient means of delivering these essential nutrients in human diets and also efficiently exploiting a strictly limited marine bioresource.

Changes in the fatty acid composition of phospholipids from turbot (Scophthalmus maximus) in relation to dietary polyunsaturated fatty acid deficiencies

Young turbot (1-20 g) were maintained for not less than 14 weeks on three diets: (1) a control diet containing normal amounts of polyunsaturated fatty acids (PUFA); (2) a diet totally deficient in PUFA; (3) a diet deficient in the (n-6) series of PUFA but containing (n-3) PUFA. At 14 weeks the fatty acid compositions of the phospholipids from liver, gut, gills and muscle were analysed. Large changes in the amounts of PUFA in the phospholipids were found. Fish maintained on the totally PUFA deficient diet 2 had retained arachidonic acid, 20:4(n-6), and docosahexaenoic acid, 22:6(n-3), at the expense of eicosapentaenoic acid, 20:5(n-3). Fish maintained on the (n-6) PUFA-deficient diet (3) contained decreased amounts of 20:4(n-6) and 22:6(n-3) while retaining 20:5(n-3). In all cases phosphatidylinositol had the lowest n-3/n-6 ratios. These results are discussed in terms of PUFA function.

Biosynthesis of docosahexaenoic acid in trout hepatocytes proceeds via 24-carbon intermediates.

The role of 24:5n-3 and 24:6n-3 as intermediate in the formation of 22:6n-3 in trout liver was examined. Microsomes prepared from trout liver converted [1-14C]-eicosapentaenoic acid (20:5n-3) to 24: 5n-3 and 24:6n-3 but not docosahexaenoic acid (22:6n-3). The radiolabeled 24:5n-3 and 24:6n-3 were isolated from the microsomal incubations by argentation chromatography and used as substrates in incubations with hepatocytes isolated from trout liver. Both 14C-labelled 24:6n-3 and 22:6n-3-were produced by hepatocytes incubated with radiolabelled 24:5n-3. When hepatocytes were incubated with radiolabelled 24:6n-3, the amount of radioactivity recovered in 22:6n-3 over 6 hr increased in direct relation to the decrease observed in the amount of radioactivity recovered in 24:6n-3. The results suggest that the formation of 22:6n-3 in trout liver does not involve delta 4 desaturation of 22:5n-3 but rather proceeds via the delta 6 desaturation of 24:5n-3 with the subsequent chain shortening of the 24:6n-3 produced.

Marine wax esters

The composition, biosynthesis and role of wax esters in zooplankton including calanoids and krill is reviewed together with their transformation to triacylglycerols in zooplanktonivorous fish. A special role for 22:I(n—11) alkyl units is indicated.

Lipid nutrition in fish

Abstract 1. 1. Two main forms of neutral lipid are available to fish in the natural environment, namely triacylglycerols and wax esters. 2. 2. There is evidence that triacylglycerols can be hydrolysed completely to free fatty acids and glycerol in the gastro-intestinal tract and absorbed as such. Fatty alcohols resulting from wax ester hydrolysis are oxidized to the corresponding acid and thereafter follow pathways of fatty acid metabolism. 3. 3. Polyunsaturated fatty acids in fish tissues are predominantly of the ω3 series. 4. 4. Fatty acids of the ω3 series have essential fatty acid activity for fish. Some species have the ability to convert linolenic acid (18:3ω3) rapidly to longer chain polyunsaturated acids (20:5ω3, 22:6ω3) that have full essential fatty acid activity. Other species lack this ability and the polyunsaturated ω3 acid must be supplied preformed in the diet for maximal growth and freedom from pathology.

POLYUNSATURATED FATTY ACIDS IN NEUTRAL LIPIDS AND PHOSPHOLIPIDS OF SOME FRESHWATER INSECTS

The fatty acid compositions of total neutral lipids and total polar lipids from eight species of freshwater insects were determined: stonefly nymphs (Plecoptera), beetle larvae (Coleoptera), Chironomidae (Diptera), water boatmen (Corixidae and Notonecta; Heteroptera) and mayfly nymphs (Ecdyonurus venosus, Caenis, Ephemerella; Ephemeroptera). In addition, the compositions of individual phosphoglycerides were determined for four of the species (Plecoptera, Corixidae, Ecdyonurus venosus and Emphemerella). Saturated and monounsaturated fatty acids together represented up to 85% of the fatty acids of total neutral lipids with 16:0 (18–31%) being the most abundant saturated fatty acid and 16:1n-7 (10–28%), 18:1n-9 (6–12%) and 18:1n-7 (3–12%) the most abundant monounsaturates. Polyunsaturated fatty acids (PUFA) accounted for between 16% and 33% of the total fatty acids of neutral lipids, with 20:5n-3 (4–12%), 18:3n-3 (3–30%) and 18:2n-6 (1–8%) all being major components. Arachidonic acid, 20:4n-6 (0.4-1.0%) and 22:6n-3 were, respectivety, minor and insignificant components of total neutral lipids. PUFA were major fatty acids (34–56% of the total) in total polar lipids and in phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. The major PUFA present were 20:5n-3 (14–27%) and 18:3n-3 (6–23%). The most abundant n-6 PUFA, especially in phosphoglycerides from Corixidae, was 18:2n-6 (3–11%). Arachidonic acid, 20:4n-6, was present in all phosphoglycerides accounting for 1–4% of the total fatty acids, except in the phosphatidylinositol of Corixidae where it accounted for 12% of the total. 22:6(n-3) was not present in significant amounts in any phosphoglyceride in any species. 18:1n-9 (8–20%) and 18:1n-7 (2–14%) were the most abundant monounsaturated fatty acids, especially in phosphatidylethanolamine. 16:0 was abundant in phosphatidylcholine (11–21%), and 18:0 (17–23%) was abundant in phosphatidylserine. The results are discussed in relation to the functions and origins of PUFA in freshwater insects.

Marine (n-3) polyunsaturated fatty acids

Marine fish oils have been intensively studied for many years because of their commercial value and their special chemical properties that stem from their high content of very long-chain polyunsaturated fatty acids (PUFA) of the (n-3) series. The high content of (n-3) PUFA gives marine fish oils the special drying properties that were traditionally exploited for the production of paints and varnishes. However, in modern times, the major commercial outlet for fish oils was and continues to be as foodstuffs for man and farmed livestock. In the 1970s and 1980s, marine oils accounted for only 2% of the total world production of edible fats and oils, the remaining 98% being accounted for by vegetable oils (68%) and animal fats (30%).1 In 1992, one million metric tons of fish oil were produced worldwide against a global total fat and oil production of 84 million metric tons, 10% of which was animal tallow and grease. Commercial fish oils are consumed in human foods mostly as partially hydrogenated fish oil (PHFO) in margarines, shortenings and fillers. For example, in 1981 fish oils accounted for 56% of the total oils used for margarine production in the UK.2 Partial hydrogenation eliminates problems of instability due to peroxidation of PUF A in the original oils and also generates a final product with useful properties of plasticity and phase transition temperature (melting point).

The biosynthesis of docosahexaenoic acid [22:6(n‐3)] from linolenic acid in primary hepatocytes isolated from wild northern pike

Primary hepatocytes from wild northern pike Esox lucius were incubated with radiolabelled linolenic acid ([l-14 C]-18:3(n-3)) to assess their ability to synthesize docosahexaenoic acid [22:6(n-3)]. The distribution of radioactivity in lipid classes and hepatocyte polyunsaturated fatty acids (PUFA) was measured over the time-course of 24h. The majority of radioactivity from [l-14 C]-18:3(n-3) was recovered in hepatocyte triacylglycerols (TAG) and phosphatidylcholine (PC). The levels of radioactivity in TAG and in most of phospholipids, including PC, increased significantly over the incubation period. Radioactivity from [1-14 C]-18:3(n-3) was recovered in several hepatocyte PUFA, including 22:6(n-3), and the Δ6 and Δ5-desaturation products 18:4(n-3) and 20:5(n-3). The presence of radioactivity in C24 (n-3) PUFA may be evidence that the biosynthesis of 22:6(n-3) in pike proceeds via a pathway independent of Δ4-desaturation. Analysis by radio gas chromatography revealed that radiolabelled 24:6(n-3) was present among the desaturation and elongation products of [l-14 C]-18:3(n-3). The results establish that, under the in vitro conditions employed, pike hepatocytes are able to convert linolenic acid to 20:5(n-3) and 22:6(n-3).