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Food Lipids: Chemistry, Nutrition, and Biotechnology

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VI. SYNTHESIS AND ABUNDANCE OF PUFA<br />

Despite the considerable variation that is possible in PUFA structure, there are only<br />

about 20 unsaturated fatty acids of nutritional importance to humans. These consist<br />

primarily of monounsaturated <strong>and</strong> methylene-interrupted PUFAs in the cis configuration.<br />

Virtually all of the unsaturated fatty acids consumed in normal diets are<br />

members of the n-3, n-6, n-7, or n-9 families of fatty acids. Non-methylene-interrupted<br />

<strong>and</strong> trans-configured fatty acids were historically consumed in very small<br />

quantities, but the advent of modern food production may have enriched these fatty<br />

acids in the food supply. The primary unsaturated fatty acids in human nutrition are<br />

reviewed below.<br />

A. n-7 Fatty Acids<br />

1. Palmitoleic Acid (16:1n-7)<br />

Palmitoleic acid is a minor component of both animal <strong>and</strong> vegetable lipids. Fish oil<br />

is particularly enriched in palmitoleic acid, <strong>and</strong> some seed oils also represent a<br />

significant source of the fatty acid. Palmitoleic acid is produced de novo by plants<br />

<strong>and</strong> animals by the �9 desaturation of palmitic acid.<br />

2. Vaccenic (18:1n-7)<br />

Vaccenic acid is a major product of bacterial fatty acid synthesis <strong>and</strong> is also present<br />

at lower concentrations in plant <strong>and</strong> animal lipids. Vaccenic acid is produced by the<br />

elongation of palmitoleic acid. The true content of vaccenic acid in the diet may be<br />

underestimated due to difficulty in separating it from its n-9 isomer. In animals,<br />

vaccenic acid appears to be concentrated in the mitochondrial lipid cardiolipin [103].<br />

B. n-9 Fatty Acids<br />

1. Oleic Acid (18:1n-9)<br />

Oleic acid is a �9 desaturase product of stearic acid <strong>and</strong> is produced de novo in<br />

plants, animals, <strong>and</strong> bacteria. Oleic acid is the most common unsaturated fatty acid<br />

<strong>and</strong> is the precursor for the production of most other PUFAs. Plants produce both<br />

n-3 <strong>and</strong> n-6 PUFAs from oleic acid, <strong>and</strong> animals can elongate <strong>and</strong> desaturate oleic<br />

acid into a variety of n-9 fatty acids. Olive oil is a particularly rich dietary source,<br />

<strong>and</strong> most foods, especially nuts <strong>and</strong> butter, are rich in oleic acid.<br />

2. Erucic Acid (22:1n-9)<br />

Erucic acid is a long chain monounsaturated fatty acid found in plants, particularly<br />

in rapeseeds. It is an elongation product of oleic acid, <strong>and</strong> is an uncharacteristically<br />

long chain unsaturated fatty acid for plants. Mildly toxic, erucic acid has been bred<br />

out of rapeseeds used for food oil production. In animals, dietary erucic acid can be<br />

retroconverted to form oleic acid via peroxisomal oxidation.<br />

3. Mead Acid (20:3n-9)<br />

Mead acid is a hallmark of essential fatty acid deficiency <strong>and</strong> has the distinction of<br />

being the only major PUFA produced de novo by animals. In the absence of dietary<br />

n-6 <strong>and</strong> n-3 fatty acids, the �6 desaturase converts oleic acid to 18:2n-9, which is<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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