Lipids and essential fatty acids
Lipids are a group of fat-soluble compounds occurring in the tissues of plants and animals and broadly consist of fats, phospholipids, sphingomyelins, waxes and sterols. Fats are the fatty acid esters of glycerol and are the principal form of energy storage. They contain more energy per unit weight than any other biological product – it is estimated that they provide 8.5 kcal metabolizable energy (ME) per gram. Natural diets may contain as much as 50 per cent fat. Phospholipids are the esters of fatty acids and phosphatic acid. These are the main constituent lipids of cellular membranes, determining the hydrophobic or hydrophylic properties of the membrane surfaces. Sphin-gomyelins are present in the brain and nerve tissue compounds.
Waxes are fatty acid esters of long-chain alcohols and can be metabolized for energy. Sterols are polycyclic, long-chain alcohols and are components of several hormone systems, especially those related to sexual maturation and reproductive functions. The protein-sparing function of lipids has already been referred to. Fatty acids are described as saturated when they contain no double bonds, and unsaturated when they contain one (mono-unsaturated) or more (polyunsaturated) double bonds.They are composed of carbon, hydrogen and oxygen and are generally acyclic, unbranched molecules containing an even number of carbon atoms.
The polyunsaturated fatty acids (PUFA) are divided into three families, named after the shortest chain length fatty acid representing each, namely oleic, linoleic and linolenic acids. The omega (w) system of nomenclature is used to identify the families. (In some recent litera ture w is replaced by n.) Those belonging to the oleic family are referred to as w9, linoleic as w6 and linolenic as w3 fatty acids. An abbreviated form is used to refer to the structure of the fatty acid, for example linoleic acid is expressed as 18:2w6, where 18 is the number of carbon atoms in the fatty acid molecule, 2 is the number of double bonds and w6 is the location of the first double bond relative to the methyl (CH3) end of the molecule.
Fish in general contain more w3 than w6 PUFA, but fresh-water fish appear to have higher levels of w6 fatty acids than marine species. The requirement for w3 fatty acids in diets may, therefore, be greater in salt-water species. Besides salinity, there are other environmental factors that affect the fatty acid composition, particularly PUFA, of fish: temperature is an important factor and the effectsof temperature on fatty acid composition have been clearly demonstrated. There is a general trend towards a higher content of long-chain PUFA at lower temperatures. The w6/w3 ratio decreases with decrease in temperature. If this trend in fatty acid composition can be taken as clues to the essential fatty acid (EFA) requirements, the w3 requirements of fish grown at lower temperatures would be greater, and conversely those grown in warmer waters may do better with a mixture of w6 and w3 fatty acids (Halver, 1980). The melting point of fat, related to the degree of unsaturation, has an important bearing on digestibility. Liquid fats are more readily digested and used by fish, whereas high-melting-point fats are not effectively utilized. Fats that solidify at relatively low environmental temperatures may be poor lipid sources for
aquaculture diets, especially during cold weather, as the temperature of the animal will be about the same as that of the environment. Other significant factors that affect EFA are depth, season, diet and reproduction, and possibly also genetic variation.
The known EFA requirements of a number of species of fish are presented in Table 7.5. Most of the estimates of EFA requirements for crustaceans are suggestive rather than conclusive. There are very few reports on the effects of purified fatty acids in semi-purified test diets for crustaceans. Kanazawa et al. (1977) found that both 18:2w6 and 18:3w3 improved the growth of Penaeus japonicus compared to diets containing 18:1w9 as the sole lipid. It has also been shown that, even though the chain elongation/desaturation ability of the prawn (Pjaponicus) was less than that of rainbow trout,they were able to convert 18:3w3 into 20:5w3 and 22:6w3 at a faster rate than marine fish like red seabream (Kanazawa et al., 1979b). Fresh-water crustaceans will probably have a requirement for one or both of w3 and w6 fatty acids, depending on culture temperature. Herbivores will probably be more capable of utilizing the 18 carbon w3 and w6 fatty acids than carnivores (Castell and Tiews, 1980)
In addition to EFA, crustaceans require other dietary lipids. The sterols are very important in many essential hormonal functions, besides being membrane lipids. The level of sterol required varies from 0.5–2 per cent of the dry weight diet or 5–30 per cent of the dietary lipid. Phospholipid phosphatidyl choline has been observed to have growth-promoting properties. The addition of 1 per cent lecithin from the short-necked clam (Tapes sp.) to a semipurified test diet resulted in optimum growth of P. japonicus in studies made by Kanazawa et al. (1979a). Similarly, optimum growth and survival of juvenile lobsters (Homarus americanus) were obtained when 7–8 per cent lecithin from soybean lipids was added to the casein/albumin-based test diet (Conklin et al., 1980; D’Abramo et al., 1981). Though the precise role of dietary lecithin is not yet fully known, it is believed that it may have an important role in the transport of lipids in crustacean haemolymph.
As mentioned earlier, there is very little information on the nutritional requirements of molluscs, and this applies also to the requirements of lipids and EFA. Marine molluscs tend to have relatively high levels of 20:5w3 and 22:6w3. The content of 18:2w6, 20:4w6 and 18:3w3 tend to be higher in fresh-water than in marine molluscs and higher in warm-water than in cold-water species (Ackman et al., 1974; Hoskin, 1978). Based on these similarities, it is predicted that the range of EFA requirements of molluscs will be similar to those of finfish and crustaceans. But unlike them, the ability to synthesize sterols de novo from acetate and mevalonate appears to vary from species to species in molluscs.
The lipid contents of most commercial aquaculture diets are less than 10 per cent, mainly due to processing problems. Though the optimum levels have not yet been determined, higher levels do not appear to result in highergrowth. Excessive dietary lipid levels can cause nutritional diseases such as fatty liver. From the point of view of product quality also this may not be desirable, as high lipid levels may cause greater deposit of visceral fat.
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