Unsaturated fatty acids consist of monounsatu-rates and polyunsaturates. There are two classes of PUFA: n-6 and n-3. The distinction between n-6 and n-3 fatty acids is based on the location of the first double bond, counting from the methyl end of the fatty acid molecule. In the n-6 fatty acids, the first double bond is between the 6th and 7th carbon atoms and in the n-3 fatty acids the first double bond is between the 3rd and 4th carbon atoms. Monounsaturates are represented by oleic acid an n-9 fatty acid, which can be synthesized by all mammals including humans. Its double bond is between the 9th and 10th carbon atoms (Figure 1).
n-6 and n-3 fatty acids are also known as essential fatty acids (EFAs) because humans, like all mammals, cannot make them and must obtain them in their diet. n-6 fatty acids are represented by linoleic acid (LA; 18:2n-6) and n-3 fatty acids by a-linolenic acid (ALA; 18:3n-3). LA is plentiful in nature and is found in the seeds of most plants except for coconut, cocoa, and palm. ALA, on the other hand, is found in the chloroplasts of green leafy vegetables and in the seeds of flax, rape, chia, perilla, and in walnuts (Tables 1, 2, and 3). Both EFAs are metabolized to longer chain fatty acids of 20 and 22 carbon atoms. LA is metabolized to arachidonic acid (AA; 20:4n-6) and LNA to eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3), increasing the chain length and degree of unsaturation by adding extra double bonds to the carboxyl end of the fatty acid molecule (Figure 2).
Humans and other mammals, except for carnivores such as lions, can convert LA to AA
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