PUFA (polyunsaturated fatty acid) structure, nomenclature, sources and intake
The general structure of a fatty acid is a hydrocarbon chain with a carboxy group at one end and a methyl group at the other. The most abundant fatty acids have straight chains of an even number of carbon atoms. Fatty acid chain lengths vary from 2 to 30 or more and the chain may contain double bonds. Fatty acids containing double bonds in the acyl chain are referred to as unsaturated fatty acids; a fatty acid containing two or more double bonds is called a PUFA. Unsaturated fatty acids are named by identifying the number of double bonds and the position of the first double bond counted from the methyl terminus (with the methyl, or w, carbon as number 1) of the acyl chain. Therefore an 18-carbon fatty acid with two double bonds in the acyl chain and with the first double bond on carbon number six from the methyl terminus is notated as 18:2w–6, often shown as 18:2n–6 (Figure 1). The common name of this fatty acid is linoleic acid and it is the simplest member of the w–6 or n–6 family of fatty acids. Linoleic acid can be further desaturated by insertion of a double bond between carbons 3 and 4 (counted from the methyl carbon) to yield a-linolenic acid (18:3n–3), the simplest member of the w–3 or n–3 family of fatty acids. Mammals, but not plants, lack the desaturase enzymes necessary to synthesize linoleic and a-linolenic acids.
Although mammalian cells cannot synthesize linoleic and a-linolenic acids, they can metabolize them by the introduction of further double bonds (desaturation) and by lengthening the acyl chain (elongation). Thus linoleic acid can be converted into g-linolenic acid (18:3n–6), and dihomo-g-linolenic acid (20:3n–6) into arachidonic acid (20:4n–6) (Figure 1). Using the same series of enzymes as those used to metabolize n–6 PUFAs, a-linolenic acid is converted into EPA (eicosapentaenoic acid; 20:5n–3) (Figure 1). Further conversion of EPA into DHA (docosahexaenoic acid; 22:6n–3) involves the addition of two carbons to form docosapentaenoic acid (22:5n–3), the addition of two more carbons to produce 24:5n–3, desaturation to form 24:6n–3 and removal of two carbons by limited b-oxidation to yield DHA . Arachidonic acid can also be metabolized by the same series of enzymes. In mammals, the pathway of desaturation and elongation occurs mainly in the liver.
It is evident from the pathway shown in Figure 1 that there is competition between the n–6 and n–3 fatty acid families for metabolism. Although the preferred substrate for D6-desaturase is a-linolenic acid, because linoleic acid is much more prevalent in most human diets when compared with a-linolenic acid, the metabolism of n–6 fatty acids is quantitatively more important.
Plant seed oils are frequently rich in PUFAs. For example, corn, sunflower, safflower and soya-bean oils are rich in linoleic acid, which may comprise as much as 75% of the fatty acids present. Thus these oils and foods made from them (e.g. margarines) are important dietary sources of linoleic acid. Some plant oils (e.g. soya-bean oil) also contain a-linolenic acid in smaller amounts; green plant tissues are also a source of this fatty acid. The main PUFA in the Western diet is usually linoleic acid followed by a-linolenic acid. Typical intakes of these two fatty acids in the U.K. are 10–15 and 0.75–1.5 g/day respectively . As to longer chain PUFAs, they are consumed in smaller amounts than linoleic and a-linolenic acids. Estimates of the intake of arachidonic acid in Western populations vary between 50 and 300 mg/day for adults. Fish, especially oily fish (salmon, herring, tuna and mackerel) and fish oils are a rich source of EPA and DHA. In the absence of oily fish or fish oil consumption, a-linolenic acid is by far the principal dietary n–3 PUFA. Average intake of the long-chain n–3 PUFAs in the U.K. is estimated at 2].