N6 and n3 Fatty Acids Sources Desaturation and Elongation

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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

O II

Palmitic acid (16:0)

"OH

C Linoleic acid (18:2n-6)

C Arachidonic acid (20:4n-6)

O II

Linolenic acid (18:3n-3)

"OH

C Eicosapentaenoic acid (20:5n-3)

Figure 1 Structural formulas for selected fatty acids.

C Docosahexaenoic acid (22:6n-3)

and ALA to EPA and DHA. This conversion was shown by using deuterated ALA. There is competition between n-6 and n-3 fatty acids for the desaturation enzymes. However, both A-4 and A-6 desaturases prefer n-3 to n-6 fatty acids. But a high LA intake interferes with the desaturation and elongation of ALA. Trans-fatty acids interfere with the desaturation and elongation of both LA and ALA.

Table 1 Polyunsaturated oils high in n-6 and n-3 fatty acids

n-6 oils

n-3 oils

Corn oil

Fish oil

Safflower oil

Chia oil

Sunflower seed oil

Perilla oil

Cottonseed oil

Flaxseed oil

Soybean oil

Canola oil

Peanut oil

Walnut oil

Sesame oil

Soybean oila

Grapeseed oil

Borage oil

Primrose oil

anote: soybean oil is higher in n-6 fatty acids than most n-3 oils, so it belongs in both categories.

anote: soybean oil is higher in n-6 fatty acids than most n-3 oils, so it belongs in both categories.

A-6 desaturase is the limiting enzyme and there is some evidence that it decreases with age. Premature infants, hypertensive individuals, and some diabetics are limited in their ability to make EPA and DHA from ALA. These findings are important and need to be considered when making dietary recommendations. EPA and DHA are found in the oils of fish, particularly fatty fish (Table 4). AA is found predominantly in the phospholipids of grain-fed animals and eggs.

LA, ALA, and their long-chain derivatives are important components of animal and plant cell membranes. In mammals and birds, the n-3 fatty acids are distributed selectively among lipid classes. ALA is found in triglycerides, in choles-teryl esters, and in very small amounts in phos-pholipids. EPA is found in cholesteryl esters, triglycerides, and phospholipids. DHA is found mostly in phospholipids. In mammals, including humans, the cerebral cortex, retina, and testis and sperm are particularly rich in DHA. DHA is one of the most abundant components of the brain's structural lipids. DHA, like EPA, can be

Table 2 Comparison of dietary fats (fatty acid content normalized to 100%)

Dietary fat Saturated fat Polyunsaturated fat Monounsaturated fat Cholesterol

Table 2 Comparison of dietary fats (fatty acid content normalized to 100%)

Dietary fat Saturated fat Polyunsaturated fat Monounsaturated fat Cholesterol

LA

ALA

LA:ALA

Flaxseed oil

10

16

53

(0.3)

20

0

Canola (rapeseed) oil

6

22

10

(2.2)

62

0

Walnut oil

12

58

12

(4.8)

18

0

Safflower oil

10

77

Trace

(77)

13

0

Sunflower oil

11

69

-

(69)

20

0

Corn oil

13

61

1

(61)

25

0

Olive oil

14

8

1

(8.0)

77

0

Soybean oil

15

54

7

(7.7)

24

0

Margarine

17

32

2

(16)

49

0

Peanut oil

18

33

-

(33)

49

0

Palm oila

51

9

0.3

(30)

39

0

Coconut oila

92

2

0

(2.0)

7

0

Chicken fat

31

21

1

(21)

47

11

Lard

41

11

1

(11)

47

12

Beef fat

52

3

1

(3.0)

44

14

Butter fat

66

2

2

(1.0)

30

33

apalm oil has arachidic of 0.2 and coconut oil has arachidic of 0.1.

Data on canola oil from data on file, Procter & Gamble. All other data from Reeves JB and Weihrauch JL (1979) Composition of Foods, Agriculture Handbook No. 8-4. Washington, DC: US Department of Agriculture.

apalm oil has arachidic of 0.2 and coconut oil has arachidic of 0.1.

Data on canola oil from data on file, Procter & Gamble. All other data from Reeves JB and Weihrauch JL (1979) Composition of Foods, Agriculture Handbook No. 8-4. Washington, DC: US Department of Agriculture.

derived only from direct ingestion or by synthesis from dietary EPA or ALA.

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