Food Lipids: Chemistry, Nutrition, and Biotechnology

further elongated and �5-desaturated to form mead acid [50]. It has been speculated
that mead acid compensates for the loss of n-3 and n-6 PUFAs by increasing the
unsaturation of animal cell membranes.
4. Other n-9 Fatty Acids
The family of n-9 fatty acids is derived exclusively from the production of oleic acid
but can be converted by elongation, desaturation, � oxidation, etc. Other rare but
naturally occurring n-9 fatty acids include 18:2, 20:1, and 22:3 [104].
C. n-6 Fatty Acids
1. Linoleic Acid (18:2n-6)
Linoleic acid, along with �-linolenic acid, is a primary product of plant PUFA synthesis.
Linoleic acid is produced de novo by plants and in particular is enriched in
seed oils. Although nature produces linoleic acid at concentrations fairly equitable
with those of �-linolenic acid, modern agriculture has greatly enriched linoleic acid
in the food supply. Although animals are incapable of producing linoleic acid, livestock
are fed diets particularly rich in this fatty acid, and thus humans acquire a
large portion of their linoleic acid from meats. Linoleic acid serves as a precursor
for the production of the essential fatty acid arachidonic acid, as well as other n-6
acyl species.
2. �-Linolenic Acid (18:3n-6)
�-Linolenic acid (GLA) is produced in animals and lower plants by the �6 desaturation
of linoleic acid. Natural sources include evening primrose oil, borage oil, and
black current oil, and minute amounts can be found in animal tissue [105]. In animals,
dietary linoleic acid is desaturated by the �6 desaturase to produce �-linolenic
acid as an intermediate in the production of arachidonic acid. Interestingly, dietary
�-linolenic acid is accumulated in animal tissue largely as its direct elongation product
20:3n-6, and not substantially converted to arachidonic acid. There has been a
great deal of recent interest in dietary �-linolenic acid for its antagonistic action on
arachidonic acid metabolism.
3. Dihomo-�-Linolenic Acid (20:3n-6)
The elongation product of linoleic acid, dihomo-�-linolenic acid (DGLA), is a minor
component of animal phospholipids. DGLA serves as a precursor to the formation
of the essential fatty acid arachidonic acid as well as for the prostaglandin G1 series.
Dietary DGLA does not appear to be rapidly converted to arachidonic acid, and
because prostaglandins of the G1 series have anti-inflammatory properties, DGLA
has received attention as a potential therapeutic agent.
4. Arachidonic Acid (20:4n-6)
Arachidonic acid is the product of desaturation and elongation of linoleic acid in
animals. Arachidonic acid is also produced in quantity in marine algae. Dietary
linoleic acid is converted to arachidonic acid in animals by the concerted activity of
the �6 desaturase, a microsomal elongase and the �5 desaturase. Arachidonic acid
is referred to as an essential fatty acid for its action as the precursor for the production
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.