Food Lipids: Chemistry, Nutrition, and Biotechnology

of physiological and cellular functions. Inadequate intake or defective metabolism
leads to various dysfunctions due to deficiencies of these fatty acids in particular
cellular locations. In addition, dietary fatty acids have been well correlated with
metabolic and physiological alterations associated with heart disease and cancer [1–
3]. Unsaturated fatty acids in particular play an important role in these non-energyproducing
metabolic functions.
Dietary fatty acids have the singular ability among macromolecules to incorporate
into tissue intact, thereby altering tissue acyl compositions. Proteins and nucleic
acids, while providing energy and building blocks for metabolism, are incapable
of remodeling the protein and nucleic acid compositions of tissues in their own
image. Consequently, fatty acids occupy a unique and important role in human nutrition.
With the recent advances in basic knowledge of plant fatty acid biosynthesis
and genomics, it is possible to produce virtually any fatty acid in significant quantities.
The availability of fatty acids for supplementation and the ability to engineer
agricultural products provide opportunities to significantly modify the lipid content
of the food supply. Unfortunately, knowledge concerning fatty acid function in physiology
lags far behind the ability to modify dietary fatty acid compositions. Developing
an understanding of specific fatty acids and interactions among fatty acids and
how they affect individual metabolism and health will be critical for nutrition and
agriculture in the next decade.
II. FATTY ACID BIOSYNTHESIS
The primary product of fatty acid synthase in both plants and animals is palmitic
acid. However, many plant and animal fatty acids are longer and more unsaturated
than palmitic acid; consequently, acyl modification systems are a critical component
in the regulation of cellular acyl composition. Products of fatty acid synthase or
dietary fatty acids may be modified by a chain elongation or the insertion of double
bonds. Both elongation and desaturation reactions are organism-, tissue-, and cellspecific,
allowing individual cells to maintain their compositional identities largely
independent of diet. The possibilities and limitations imposed on the fatty acid composition
of a cell are intimately associated with these enzymatic reactions. Figure 1
provides an overview of the most common mammalian fatty acid modifications.
Prior to discussing the biosynthesis of fatty acids, it is useful to describe the
standard nomenclature for fatty acids. The systematic method of naming fatty acids
provides information on acyl chain length and the degree and position of desaturation.
Standard nomenclature describes a double bond occurring between the ninth
and tenth carbons from the carboxyl end of a 16-carbon fatty acid as a �9 double
bond (Fig. 2). The same bond, when viewed from the methyl end of the fatty acid,
is referred to as an n-7 double bond. The usefulness of two nomenclature systems
has grown out of a need to view fatty acids from multiple perspectives. Customarily,
the n system is used when fatty acids are discussed with respect to nutrition, whereas
the � designation is more useful when observing the biochemical reactions of fatty
acids. In many ways, the n system simplifies investigation into the nutritional relevance
of fatty acids. Animals are not capable of desaturating on the methyl side of
a previously formed double bond; thus, in humans and other animals, fatty acids of
a particular n designation will remain so permanently. This greatly simplifies the
analysis of fatty acid metabolism in animals. In contrast, because many organisms,
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.