Nomenclature Synthesis and Dietary Sources of Fatty Acids

Because of the wide range of foods consumed, the human diet contains a great variety of fatty acids. The most abundant fatty acids have straight chains of an even number of carbon atoms. The chain lengths vary from four (e.g. in milk) to 30 (e.g. in some fish oils) and may contain double bonds (unsaturated fatty acids) (Fig. 4.1). It is the nature of the constituent fatty acids (their chain length and degree of unsaturation) that gives a fat its physical properties. Fatty acids are often referred to by their common names, but are more correctly identified by a systematic nomenclature (Table 4.1). This nomenclature indicates the number of carbon atoms and the number and position of double (unsaturated) bonds in the chain (see Fig. 4.1). It is the position of the first double bond in the hydrocarbon chain that is indicated by the n-7, n-9, n-6 or n-3 part of the shorthand notation for a fatty acid. Note that n-6 and n-3 are sometimes referred to as omega-6 and omega-3.

Mammalian cells are able to synthesize (from non-fat precursors) saturated fatty acids and unsaturated fatty acids of the n-9 and n-7 series but lack the delta-12 and delta-15 desaturase enzymes (found in most plants) for insertion of a double bond at the n-6 or n-3 position (Figs 4.1 and 4.2). Thus, mammalian cells cannot synthesize n-6 or n-3 PUFAs de novo. The n-6 and n-3 fatty acids are essential substrates for many of the major regulatory lipids in the body and, as they cannot be synthesized in the body, the body must obtain them from the diet. The commonly consumed PUFAs are linoleic acid (18:2n-6) and a-linolenic acid (18:3n-3). Once consumed, these fatty acids can be converted to the longer-chain, more unsaturated derivatives (Fig. 4.2). Thus linoleic acid is converted via 7-linolenic (18:3n-6) and dihomo-7-linolenic

Methyl end H3C

Carboxyl end

COOH

Stearic acid 18:0

COOH

Oleic acid 18:1n-9

h3o 3

COOH

Linoleic acid 18:2n-6

COOH

a-Linoleic acid 18:3n-3

Mammals cannot insert double bonds in here Fig. 4.1. Structure of some fatty acids.

Table 4.1. Fatty acid nomenclature and sources.

Systematic name

Trivial name

Shorthand notation Sources

Decanoic Dodecanoic Tetradecanoic Hexadecanoic

Octadecanoic

9-Hexadecenoic

9-Octadecenoic

9,12-Octadecadienoic

9,12,15-Octadecatrienoic

6, 9,12-Octadecatrienoic 11,14,17-Eicosatrienoic 8,11,14-Eicosatrienoic 5, 8,11,14-Eicosatetraenoic

Capric 10:0

Lauric 12:0

Myristic 14:0

Palmitic 16:0

Stearic 18:0

Palmitoleic 16:1n-7

Oleic 18:1n-9

Linoleic 18:2n-6

a-Linolenic 18:3n-3

7-Linolenic 18:3n-6

Mead 20:3n-9

Dihomo-7-linolenic 20:3n-6

Arachidonic 20:4n-6

5, 8,11,14,17-Eicosapentaenoic Eicosapentaenoic 20:5n-3 7,10,13,16,19-Docosapentaenoic Docosapentaenoic 22:5n-3 4,7,10,13,16,19-Docosahexaenoic Docosahexaenoic 22:6n-3

De novo synthesis; coconut oil De novo synthesis; coconut oil De novo synthesis; milk

De novo synthesis; milk; eggs; animal fats; meat; cocoa butter; palm oil (other vegetable oils contain lesser amounts); fish oils De novo synthesis; milk; eggs; animal fats; meat; cocoa butter Desaturation of palmitic acid; fish oils

Desaturation of stearic acid; milk; eggs; animal fats; meat; cocoa butter; most vegetable oils, especially olive oil Cannot be synthesized in mammals; some milks; eggs; animal fats; meat; most vegetable oils, especially maize, sunflower, safflower and soybean oils; green leaves

Cannot be synthesized in mammals; green leaves; some vegetable oils, especially rapeseed, soybean and linseed oils Synthesized from linoleic acid; borage and evening primrose oils Synthesized from oleic acid; indicator of essential fatty acid deficiency Synthesized from 7-linolenic acid

Synthesized from linoleic acid via 7-linolenic and dihomo-7-linolenic acids; meat

Synthesized from a-linolenic acid; fish oils

Synthesized from a-linolenic acid via eicosapentaenoic acid

Synthesized from a-linolenic acid via eicosapentaenoic acid; fish oils

Ol CD

A12-desaturase

Diet

1

j Plants only

18:2n-6-18:3n-3

A15-desaturase

Plants only

A6-desaturase.

A6-desaturase

y

20:3n-6

18:4n-3

18:3n-6

20:3n-6

A5-desaturase

18:4n-3

20:4n-3

A5-desaturase t

20:4n-6 Arachidonic acid

20:5n-3

Eicosapentaenoic acid +

22:6n-3 Docosahexaenoic acid

Fig. 4.2. Outline of the pathway of biosynthesis of polyunsaturated fatty acids.

(20:3n-6) acids to arachidonic acid (20:4n-6) (Fig. 4.2). Likewise, a-linolenic acid is converted to eicosapentaenoic acid (EPA) (20:5n-3) (Fig. 4.2). There is some controversy about the extent to which docosahexaenoic acid (DHA) (22:6n-3) can be synthesized from EPA in humans.

In the past 40 years, the absolute consumption of saturated fatty acids in Western diets has declined. For example, in the UK saturated fatty acid intake has declined by 40% since 1970, while the consumption of monounsaturated fatty acids has declined by 30% (Department of Health, 1994). The consumption of PUFAs increased by 25% over this period of time (Department of Health, 1994). This was largely the result of increased consumption of linoleic acid, which became generally available in margarines and cooking oils. This has also resulted in an alteration in the amounts of n-6 and n-3 PUFAs consumed, with the n-6 to n-3 PUFA ratio of the diet increasing. According to the UK Adult Survey conducted in 1986, the daily diet of the average adult male in the UK contains 42 g saturated fatty acids, 31 g monounsaturated fatty acids (mainly oleic acid) and 15.8 g PUFAs (Department of Health, 1994). The main PUFA in the diet is linoleic acid (intake is approximately 14 g day-1 for adult males), with a-linolenic acid contributing approximately 2 g day-1 (British

Nutrition Foundation, 1999). Adult females show a similar pattern of fatty-acid consumption to that of males, but the absolute amounts of each type of fatty acid consumed are about 70% of those consumed by males. Fat intakes are similar in North America to those in the UK, with the exception that the intake of n-3 fatty acids may be even lower (Kennedy et al., 1999; Cavadini et al., 2000). Longer-chain PUFAs are consumed in lower amounts than are linoleic and a-linolenic acids. Estimates of the intake of arachidonic acid intakes in Western populations vary between 50 and 300 mg day-1 for adults (Sinclair and O'Dea, 1993; Jonnalagadda et al., 1995; Mann et al., 1995). EPA and DHA are found in high quantities in many marine (e.g. herring, mackerel, fresh (i.e. not tinned) tuna, sardines) oils and in the oils extracted from the livers of fish that live in warmer waters (e.g. cod). EPA and DHA comprise 20-30% of the fatty acids in a typical preparation of fish oil, which means that a 1 g fish oil capsule can provide 200-300 mg of EPA plus DHA. In the absence of oily fish or fish oil consumption, a-linolenic acid is the main dietary n-3 PUFA. Average intake of the long-chain n-3 PUFAs in the UK is estimated at 250 mg day-1 (British Nutrition Foundation, 1999).

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