Food Lipids: Chemistry, Nutrition, and Biotechnology

acids synthesized de novo, primarily either in liver (in chickens and fish) or in
adipose tissue (in pigs) [85,86]. In general, de novo synthesis of saturated fats is
decreased by dietary fats [81]. Therefore, fats from the diet constitute the greatest
source of variation in the composition of storage fats. Within this framework, metabolic
control can be seen. For example, short and medium chain fatty acids are not
incorporated into storage lipids of most animals. These pass into the liver where they
are either elongated or oxidized for fuel [87]. Although monounsaturated and linoleic
acid, 18:2, are readily incorporated, in most animals long chain (greater than C18),
highly unsaturated fatty acids are not esterified into triacylglycerols [85]. However,
fish will accumulate HUFAs, notably the n-3 PUFAs 20:5 and 22:6, but only if they
or their precursors are present in the diet and only at low water temperatures [88].
Fish actually require n-3 PUFAs in their diets but are unable to synthesize them
[89,90]. Alternatively, very high concentrations (>50%) of saturated fats are not
found in storage lipids due to the well-regulated activities of the �9 desaturase that
produces oleic acid from stearic acid [85]. In ruminant animals, the rumen microorganisms
hydrogenate unsaturated fatty acids in the diet, which has an overriding
influence on the composition of the storage fats [91]. However, when PUFAs such
as 18:2 are protected from ruminant microorganisms, they accumulate in storage
lipids in beef comparably to accumulation in nonruminants [50]. Finally, mammals
absorb fat into the lymph, whereas fish and poultry absorb fat directly into the portal
vein. As a result, adipose tissue can access incoming fatty acids directly in mammals,
but fat passes by liver first in avians and fish. Thus, there is considerably more
hepatic metabolism of ingested fatty acids in avian and fish tissues.
C. Effect of Agriculture on the Composition of the Food Supply
Fatty acids occupy a unique position in nutrition in that they have the ability to
survive digestion intact, enabling them to replace the fatty acid content of the consumer.
It is reasonable then to expect that the lipid composition and, correspondingly,
the physiology of individuals who consume particular fats and oils to be reflective
of the fatty acid composition of their diet. Interestingly, it has been postulated that
the dietary PUFA composition of an average human diet has changed markedly with
modern advances in agriculture [92–94]. Wild foods are typically much higher in n-
3 PUFAs than crops successfully developed by agriculture. There is a variety of
evidence to suggest that the changing ratio of n-3 to n-6 fatty acids has affected
human physiology adversely and that humans may have developed major classes of
pathologies as a result of this change. The lower rates of coronary heart disease and
cancer in populations consuming a higher n-3 to n-6 PUFA ratio are well documented
[95–97]. Despite mounting evidence that human populations would benefit from an
increased consumption of n-3 fatty acids, it is unlikely that this change will occur
in the near future. The primary reason for this is the agricultural success of crops
rich in n-6 fatty acids. The n-6 fatty acids are most typically found in seed crops,
which are not only consumed directly but are also used in animal feed. In addition,
n-6-rich crops are generally more stable than n-3-rich crops, leading to their preferential
cultivation and use as food ingredients. Thus, the increases in coronary artery
disease, cancer, and autoimmunity may be a direct consequence of the advance of
modern agriculture.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.