Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 Pathways of metabolic fate of eicosanoic acids.
activity and may increase the requirement for EFA. Endogenously synthesized unsaturated
fatty acids are of the �9 series and cannot be converted to the EFA precursors.
EFA-deficient animals have an increase in 20:3 �9 (derived from oleic acid)
in tissue, and the triene/tetraene ratio in plasma lipids is often used clinically for
diagnosis of this condition. This ratio essentially reflects the relative proportions of
20:3 �9 to 20:4 �6. Individuals with fat malabsorption (especially cystic fibrosis
patients) or those on long-term parenteral nutrition are most likely to display signs
of EFA deficiency. Diet-derived fatty acids are transported from the intestine to all
tissues of the body, first via chylomicrons and then via other lipoproteins to all tissues
in the body, usually becoming incorporated into the structurally important phospholipids
of cell membranes.
Phospholipids of plasma membranes from mammals tend to be relatively enriched
in long chain polyunsaturated fatty acids. Some of these serve as the precursors
for synthesis of eicosanoids, principally arachidonic acid (20:4, �6),
eicosapentaenoic aid (EPA, 20:5 �3) and dihomo-�-linolenic acid (20:3, �6). Although
�-linolenic acid (18:3 �3) is the first fatty acid of the �3 series, the biological
effects of the 20-carbon fatty acids are usually stronger, presumably because of their
direct steric competition with arachidonic acid as substrates for the enzymes of further
metabolism. Most, if not all, terrestrial animals have limited ability to
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