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fatty acids starting from the carboxyl end; however, for biological activity it is interesting to number from the methyl carbon, which is done by using the symbol w. Three different types of fatty acids can be distinguished, the oleic type with one double bond removed nine carbons from the methyl end (18:la>3). Three different types of unsaturated fatty acids can be distinguished: the oleic type with one double bond removed nine carbon atoms from their methyl end (18:lct»9 or oleic acid type); the linoleic type with two double bonds removed six carbon atoms from the methyl end (18:2c«6 or linoleic acid type); and the linoleic type with three double bonds removed three carbon atoms from the methyl end (18:3®3 or linoleic acid type). The end structure is usually retained even if additional double bonds are introduced or if additional carbon atoms are added. Thus, linoleic acid (18:2o6) may be changed into arachidonic acid (20:4co6) while retaining the to6 structure which confers essential fatty acid character to the molecule. The latter two types are now often referred to as n-6 and n-3 fatty acids. The high content of polyenoic fatty acids makes fish oils highly susceptible to autoxidation. The component fatty acids of some marine and freshwater fish oils are listed in Table 4 (5).

Considerable interest has developed recently in the health effect of certain n-3 fatty acids, especially eicosa-pentaenoic acid (EPA) (20:5a>3) and docosahexaenoic acid (DHA) (22:6co3). These fatty acids can be produced slowly from linoleic acid by herbivore animals, but not by humans. EPA and DHA occur in major amounts in fish from cold deep waters, such as cod, mackerel, tuna, swordfish, sardines, and herring (6,7). Arachidonic acid is the precursor in the human system of prostanoids and leukotrienes.

The vegetable oils and fats can be divided into three groups on the basis of their fatty acid composition. The first group comprises oils containing mainly 16- and 18-carbon fatty acids and includes most of the seed oils—cottonseed oil, peanut oil, sunflower oil, corn oil, sesame oil—as well as palm oil. The second group comprises seed oils containing erucic acid (docos-13-enoic), and includes rapeseed and mustard oil. The third group is that of the vegetable fats, comprising coconut oil and palm kernel oil, which are highly saturated and also known as lauric fats, as well as cocoa butter. The component fatty acids of some of the common vegetable oils and fats are listed in Table 5. Palmitic is the most common saturated fatty acid. Oils containing high levels of linolenic acid are susceptible to rapid oxidative deterioration.

The Crucifera seed oils, including rapeseed and mustard oil, are characterized by high levels of erucic acid (docos-13-enoic) and smaller amounts of eicos-ll-enoic acid. Plant breeders have succeeded in replacing virtually all of these fatty acids by oleic acid, resulting in what is now known as canola oil (8).

Cocoa butter is unusual in that it contains only three major fatty acids, palmitic, stearic, and oleic, in approximately equal proportions.

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