Structure Function and Nutritional Requirements

Omega-6 (n-6) fatty acids are a class of polyunsaturated fatty acids (PUFA). They have two or more cis double bonds, with the position of the first double bond six carbon atoms from the methyl end of the molecule. The general formula of n-6 fatty acids is CH3 (CH2)4(CH=CHCH2)x(CH2)yCOOH [where x = 2-5]. Linoleic acid (cis-9, cis-12-octadecadienoic acid, 18:2n-6, LA) and a-linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic acid, 18:3n-3, ALA) are the precursor fatty acids of the n-6 and omega-3 (n-3) fatty acids, respectively. These two fatty acids cannot be made by mammals and are therefore termed essential fatty acids (EFA). In addition, mammals are unable to interconvert LA and ALA, or any of the n-6 and n-3 fatty acids, because mammalian tissues do not contain the necessary desaturase enzyme. Plant tissues and plant oils tend to be rich sources of LA. ALA is also present in plant sources such as green vegetables, flaxseed, canola, and some nuts. Once consumed in the diet, LA can be converted via chain elongation and desaturation to 7-linolenic acid (GLA, 18:3n-6), dihomo-7-lino-lenic acid (DGLA, 20:3n-6), and arachidonic acid (AA, 20:4n-6) (Figure 1). The same enzymes involved in elongation and desaturation of the n-6 fatty acids are common to the n-3 series of fatty acids (Figure 1). Thus, ALA can be converted to eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). EPA and DHA are found in relatively high proportions in marine oils.

The n-6 and n-3 fatty acids are metabolically and functionally distinct and often have important opposing physiological functions. Indeed, the balance of EFA is important for good health and normal development. Historically, human beings evolved on a diet in which the ratio of n-6 to n-3 fatty acids was about 1:1. In contrast, Western diets have a ratio of

Linoleic Acid (18:2 w 6)

Linoleic Acid (18:2 w 6)

A 6Desaturase a-Linolenic Acid (18:3 w 3)

Octadecatetraenoic Acid (18:4 w 3)

a-Linolenic Acid (18:3 w 3)

Octadecatetraenoic Acid (18:4 w 3)

+ 2C Elongation

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