IV. DIETARY SOURCES OF UNSATURATED FATTY ACIDS

production of commercially viable oil sources, wax esters and eicosapentaenoic acid

have been produced at a commercially relevant scale in bacteria [70,71].

Microbes as a broad class of single-cell (or relatively undifferentiated collections of cells) organisms have the obligate biosynthetic capability of generating fatty

acids necessary for membrane synthesis and other processes. Historically these have

not been considered an important source of fat in the diet, although it is recognized

that these organisms add small quantities to certain foods, whose presence may be

an important contribution, e.g., for flavor. Nevertheless, even for microbes that produce large quantities of storage triacylglycerides, the economics of growing and

obtaining the oils has been noncompetitive with the traditional sources of edible oils.

However, recently this area has seen a considerable resurgence in interest both academically and in commercial application [72]. This change is largely due to the

improved efficiency and capabilities of large-scale microbial fermentation, to the

identification of therapeutically useful edible oils, and to the capability of microbes

to produce unusual fatty acids or unusual concentrations of fatty acids and glycerides

[73]. Fatty acids that have raised the ante, as it were, for edible oils include dihomo␥-linoleic acid, eicosapentaenoic acid, and DHA, due to their ability to alter arachidonic acid metabolism and hence thrombosis, inflammation, cancer, and autoimmune diseases; DHA for inclusion in infant formulas; nervonic acid for its potential

in treating neuropathies; long chain monounsaturated fatty acids for adrenoleukodystrophy; and stearculic acid as a possible treatment for bowel cancer [74,75].

Several factors mitigate in favor of microbial production for high-value lipids. The

greater potential for aggressive recombinant approaches to manipulate microbial lipid

metabolism to obtain novel fatty acids is likely to increase the growth of this cottage

industry for fatty acid production. Higher plants and animals are also somewhat

limited in the glyceride forms that they will produce. For example, most plants do

not place a saturated fatty acid in the sn-2 position of a triglyceride. Similarly, fish

tend to place virtually all of the long chain n-3 PUFAs in the sn-2 position. This

both limits the total range of glycerides available using these plants and animals as

sources of lipids and imposes structural effects on digestion and absorption of the

fatty acids in nutritional applications. These limitations are both less well defined

and more mutable in microbial fermentation applications. This area, though coming

under intense regulatory scrutiny, may reach a significant segment of the food industry, at least in the short term [76,77].

Single-cell eukaryotes have, as a class, a remarkably wide variety of lipid

metabolic capabilities. For example, some yeast produce only a single desaturase,

the stearoyl or ⌬9 enzyme, and neither produces or requires PUFA for growth. At

the other end of the spectrum, some fungi and algae can produce very high amounts

of arachidonic, dihomo-␥-linolenic, eicosapentaenoic, and DHA [78]. An additional

and synthetically useful attribute of these organisms is their ability to take up fatty

acids from the medium and either to incorporate them into triacylglycerides and

phospholipids (even with unusual stereospecificity) or to further metabolize them

prior to esterification [78]. These various properties were known previously but were

not thought to warrant commercialization. This is changing. Already, microbial lipid

sources are proving to be a cost-effective feedstock for shrimp and fish aquaculture.

The ability of the microbial feedstock to also elaborate valuable pigments and antioxidants is used to advantage, so this entire technology and its biotechnological

elaborations are likely to increase in impact in the future.



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



B.



Agricultural Products



The extended metabolism of 18-carbon PUFAs to longer chain, more unsaturated

fatty acids [frequently referred to as highly unsaturated fatty acids (HUFAs)] in

animals means that even though animals and plants may contribute similar families

of PUFAs to the diet, the precise form of these fatty acids will differ. As a result,

an important consequence of consuming animal in contrast to vegetarian foods is

that in the latter, linoleic and linolenic acids are the fatty acids ingested from the n6 and n-3 families, whereas in animal foods, their metabolic products, preformed

arachidonic, eicosapentaenoic, and DHA, are also ingested. It is now clear that these

are significant nutritional, biochemical, and physiological differences.

Furthermore, whereas animal sources of fat are often grouped as similar, avian,

aquatic, and ruminant or nonruminant mammalian storage lipids are very different

in the quantity of depot triacylglycerols, their distribution and their fatty acids, as

well as their composition and arrangement on the glycerol. The final content of fatty

acids in storage triacylglycerols is the result of diet, metabolism, and de novo synthesis. In this respect, each of the major animal fat sources differs in important ways,

which tends to distinguish each as a fat-rich commodity. These differences have

important effects on the texture, flavor, and caloric density of the muscles as consumed directly [79–81] and also on processed foods prepared from them [82].

Although the differences among species in the quantity and distribution of fat

are associated with the particular commodities, they are not necessarily all innate to

them. These differences also reflect the historical development of the particular muscle food as a commodity. Even among ruminants, the fat content of modern beef

muscle is higher and more saturated than that of comparable wild ruminant muscle

[83]. Breeding and feeding practices allow for the production of meat at a specific

fat concentration [84]. If different properties were perceived to be beneficial, the fat

content could arguably be altered to various extents accordingly. Thus, when examining the content of storage fat in muscle tissue that is used as food, one is looking

at a rather narrow window of a wide range of possibilities. As commodity needs

become more defined and the fat functionality better understood, the means to arrive

at these targets will need to be explored.

In addition to the differences in total quantity of fat and its tissue distribution,

the composition of storage triacylglycerols in animal species differs as well. Red

meats tend to be relatively higher in saturated fatty acids and lower in PUFAs than

poultry or fish. Poultry and fish differ significantly in the chain length of monounsaturated fatty acids and in the content of n-3 PUFAs.

Once again, a consistent observation of the lipid content in different animal

tissues is the variability within species. Even within ruminant animals in which the

dietary PUFAs are largely hydrogenated by rumen flora, there is a significant range

of composition. Among monogastric animals, the variability in fat composition

within species due to muscle type, diet, environment, and age is typically greater

than the differences noted among species [81].

An important question becomes, what unique properties of the metabolism of

the three animal types lead to the observed or apparent differences in lipid composition and behavior? In all animals, the storage triacylglycerols both in adipose and

individual muscle cells can be assembled from both dietary fatty acids and fatty



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



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 n3 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.



V.



NUTRITIONAL EFFECTS OF UNSATURATED FATTY ACIDS



The field of PUFA biochemistry has only begun to develop convincing molecular

models for the effects of fatty acids on physiology. Having lagged somewhat behind,

lipid biochemistry is now poised to develop in the same way that protein and nucleic

acid biochemistry has over the last 20 years. There is considerable information

known about fatty acid synthesis as well as a massive collection of data on the fatty

acid composition of foods. It has recently even become feasible to modify the fatty

acyl content of foods through genetic manipulation. Yet there are very few data on

how and why particular fatty acids modulate physiology. An understanding of the

molecular basis of fatty acid nutrition will be important for the design of diets appropriate to the individual. PUFAs are thought to exert their physiologic effects

through a variety of mechanisms, ranging from acting as precursors for signal molecule formation to modulating membrane structure. The remainder of this chapter

will review what is known about the non-energy-producing functions of PUFA and

how individual fatty acids modulate these functions.

A.



Role of PUFA in Cell Physiology



Whereas all fatty acids contribute hydrophobicity to membranes, unsaturated fatty

acids provide several unique functionalities. Unsaturated fatty acids and particularly

PUFA can form a vast array of chemical structures, each of which has unique physiochemical properties. Cells can use fatty acids to modulate their membrane properties and the activities of membrane-associated enzymes, and for the production of

potent signal molecules. Knowledge of the effect of fatty acids on membrane properties has been impeded by the fact that it is difficult to greatly modulate membrane

fatty acid composition. Although the difficulty associated with modifying cell membrane composition suggests that membrane homeostasis is critical to cells, it makes

investigation on the effects of individual fatty acids difficult. The true successes in

this area have come from the discovery of small but highly active phospholipid pools

and the enzymes that maintain them. The regulation of the ether-linked arachidonatecontaining phospholipid pools is the best described example of how small changes

in membrane compositions can have significant effects on cell physiology [98]. Future research in this area will likely have to focus on developing controllable models

in which the contributions of specific fatty acids are determined.

1.



Unsaturated Fatty Acids and Membrane Structure



A critical feature of the production of unsaturated fatty acids by eukaryotes is that

the carbon–carbon double bonds exist in the cis configuration. Pi-bonded carbon–

carbon double bonds may exist in two conformations, cis and trans (see Fig. 4), yet

nature has carefully preserved the production of the cis isomer to the virtual exclusion

of trans isomers. The reason for this is best explained by viewing fatty acids as

important structural components of cells. Brenner [99] and Cook [50] reviewed the

key physiochemical features of double bonds in fatty acids. Most important among

the changes imparted to a fatty acid by a cis double bond is the rigid ‘‘kink’’ or

bend in the acyl chain.

Membranes are largely held together by London–van der Waals forces between

adjacent fatty acyl chains [50]. Because these interaction forces are significantly

diminished with even a slight increase in the distance between acyl chains, fatty acid



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