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Dietary Lipids and Health

Bruce A. Watkins, 1 Yong Li, 1

Bernhard Hennig, 2

and Michal Toborek 2

1 Purdue University

West Lafayette, Indiana

2 University of Kentucky

Lexington, Kentucky

1. INTRODUCTION

Lipids support multiple biological functions in the body. They serve as the structural

building material of all membranes of cells and organelles. Lipids are the most

efﬁcient fuel for living organisms containing more than twice the energy content

compared with carbohydrates and proteins on a weight basis. Lipids and their deri-

vatives also serve as signaling molecules that facilitate a variety of physiological

functions. In addition, lipids are recognized as important biomarkers of disease

and are involved in several pathological conditions. The cellular activities in tissues

and organs are to some extent a result of biological actions of fatty acids mediated

by changes in the membrane bilayer structure to impact the processes of membrane-

associated receptors and signal transduction systems and ion channels. Recent

literature also demonstrates a speciﬁc role of fatty acids in gene modulation and

protein expression to inﬂuence risk of chronic disease.

In contrast to the shorter chain and more saturated fatty acids, the essential fatty

acids (EFAs), linoleic acid (LA, an omega-6 fatty acid, 18:2n-6), and a-linolenic

acid (LNA, an omega-3 fatty acid, 18:3n-3) serve as substrates for the production

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.

Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

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DIETARY LIPIDS AND HEALTH

of polyunsaturated fatty acids (PUFAs) used in cellular structures and as precursors

for the biosynthesis of many of the body’s regulatory biochemicals [glycerolipids,

long-chain (LC) PUFAs, and eicosanoids] (1). The eicosanoids are powerful, short-

lived, hormone-like compounds synthesized from speciﬁc PUFA. In addition,

formation and dietary sources of the LC-PUFA, arachidonic acid (AA, 20:4n-6),

eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3)

balance prostanoid production. An absolute requirement of DHA is necessary for

normal neural and retinal development in the infant and young child. Biochemical

and clinical studies indicate that sources of LNA and stearidonic acid (SDA, 18:4n-3)

alter the balance of their respective LC-PUFA, principally EPA. A short-term

human study indicated that vegetable oils containing SDA could be a dietary source

of n-3 fatty acids that would be more effective in increasing tissue EPA concentra-

tions than with LNA-containing seed oils (2). However, the concentration of DHA

in tissues was not affected by dietary SDA supplementation, and this should be a

concern when DHA is intended to be enhanced, for example, in infant nutrition.

This chapter introduces the contemporary understanding of food lipids in nutri-

tion and health. The health aspects of dietary lipids and the underlying cellular and

molecular mechanisms of PUFA actions are also discussed. A primary focus is

placed on the role of dietary fat in cardiovascular disease and atherosclerosis.

1.1. The Dietary Reference Intakes

New dietary reference intakes (DRIs) for healthy individuals and populations were

recently published by the Institute of Medicine of the National Academies in 2002.

Along with setting the Adequate Intake (AI) of LA levels at 17 g/day for young

men and 12 g/day for young women, the new report provides the ﬁrst comprehen-

sive recommendations for n-3 PUFA in the United States (3, 4). The DRIs for n-3

PUFAs place a primary emphasis on adequate consumption of LNA to satisfy the

principle requirement for all ages and both genders. To a lesser extent, provisions

for modest recommended intakes are made for the long-chain n-3 PUFAs, EPA, and

DHAs. The daily AI for LNA are 1.6 and 1.1 g/d for adult men and women, respec-

tively. The acceptable macronutrient distribution range (AMDR) for LNA is 0.6%

to 1.2% of daily energy intake (4). Both EPA and DHA can satisfy 10% of the

AMDR (0.06% to 0.12% energy) for n-3 PUFAs, and an optimal ratio of LA/

LNA (n-6/n-3 fatty acids) is proposed to range from 5 to 10. The AMDR for total

fat is set at 20% to 35% of energy. No DRIs are set for saturated fat and trans-fatty

acids because of their perceived adverse effects on health and the tolerable upper

intake levels (ULs) were not set for these fatty acids because of practical issues. In

light of these DRIs for fatty acids, the food supply may provide a reasonable way

for all healthy individuals and at-risk groups to achieve these intakes.

1.2. General Nutrition and Health of Lipids

Lipids (fatty acids, glycerol lipids, and cholesterol) are vital nutrients that serve as

an energy source, structural components of the living organisms, and essential to

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INTRODUCTION

biological activities of homeostasis. Beyond their traditional role as nutrients, lipids

or fatty acid molecules also facilitate critical biochemical and physiological func-

tions as modulators of cell actions and genes. The n-6 and n-3 PUFAs have been

recognized as ligands for peroxisome proliferator-activated receptors (PPARs). The

PPARs regulate gene expression in lipid and carbohydrate metabolism (5, 6), but

emerging evidence indicates that different PPARs are involved in a much broader

capacity as biological regulators (7).

Long-chain PUFAs and their derivatives, such as prostaglandins and leuko-

trienes, are PPAR activators. The n-6 and n-3 PUFAs are the most potent PPAR

ligand fatty acids. These natural ligands work on all three types of PPARs

(PPARa,PPARg, and PPARd) by binding to PPARs, however, the afﬁnity to bind

and expression of PPARs vary depending on the type of tissue and cell. The PUFA

ligands (LA, AA, EPA, and DHA) are known agonists or antagonists of COX-2

expression through the activation of PPARs (8).

1.3. n-3 PUFA

The biological effects of dietary lipids on human health remain a primary focus of

nutrition research as consumption recommendations are continually updated in

response to new information obtained through epidemiological, clinical, and animal

investigations. The role of n-3 PUFAs (DHA) in the development of the infant ner-

vous system and retina is clearly established (9). Moreover, implications of a ther-

apeutic effect (10, 11) on reducing cardiovascular disease and cancer risk and

actions of their derivatives as biological effectors of human pathologies further

drive biochemical and molecular investigations to elucidate the health beneﬁts of

dietary fatty acids (12). In addition to their beneﬁcial impact on cardiovascular

pathologies and cancers, n-3 fatty acids are also known to lessen the severity and

minimize symptoms of chronic inﬂammatory diseases (13, 14), including rheuma-

toid arthritis (15) and inﬂammatory bowel disease (16), and may even beneﬁtin

correcting psychological disorders (17).

There are three major n-3 PUFA species present in food. These are LNA in vege-

tables, oilseeds, and nuts, and EPA and DHA in cold water ﬁshes and algae.

Another n-3 PUFA receiving greater attention recently is SDA, which is high in

some plant oils (such as hempseed oil and black currant seed oil) but can be isolated

and concentrated from marine ﬁsh. SDA may function as an important human diet-

ary component for people with deﬁcits in 6-desaturase activity. In human and

other mammals, fatty acids of the n-3 series longer than 18 carbons cannot be

synthesized from common carbon sources. Thus, n-3 PUFAs in the human body

are either ingested directly or formed from LNA, which renders LNA as the essen-

tial fatty acid of the n-3 series PUFA. Although it is known that the human can

make LC n-3 PUFAs starting with LNA, some evidence suggests that a supplement

of preformed LC-PUFA is beneﬁcial, especially in early infancy (formula-feeding)

and under certain metabolic disease conditions.

James et al. (2) reported recently that consuming encapsulated SDA oil (ethyl

ester) increased EPA and DPA (n-3) (22:5n-3) in healthy male and postmenopausal

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DIETARY LIPIDS AND HEALTH

female subjects (n ¼ 15/group) in a double-blind, parallel-group design study. The

results clearly showed that SDA was more efﬁciently converted to biologically

active 20 and 22 carbons n-3 PUFAs and implies that vegetable oils containing

SDA could be a dietary source of n-3 fatty acids that would be more effective in

increasing tissue EPA concentrations than LNA-containing oils. The introduction of

SDA-containing oils in food manufacturing could provide a wide range of dietary

alternatives for increasing tissue n-3 PUFA concentrations to fulﬁll the beneﬁts

proposed for a variety of chronic health problems.

1.4. Conjugated Linoleic Acid

Conjugated linoleic acid (CLA) is the name given to describe a group of positional

and geometric fatty acid isomers of octadecadienoic acid. The CLA isomers are

reported to have antioxidant capacity, reduce carcinogen-DNA adduct formation,

induce apoptosis, modulate tissue fatty acid composition and prostaglandin E 2

(PGE 2 ) formation, and alter the expression and action of cytokines and growth fac-

tors (18). Though numerous biological actions of CLA have been reported, the most

consistent ﬁndings include anticancer effects in rodents and cancer cells, and reduc-

tion of body fat in growing animals. In some cases, the biological responses

observed from CLA isomers were inﬂuenced by the amounts of dietary n-6 and

n-3 PUFAs (19–21).

CLA isomers have been recognized as effective anticarcinogenic agents for sev-

eral types of cancers. The cytotoxic effects of CLA isomers on growth of various

human and animal-derived cancer cells seem to be mediated by lowering the

expression of the gene transcription factor Bcl-2 family members that inhibit apop-

totic cell death or induce caspase-dependent apoptosis (22–26). CLA also prevented

basic ﬁbroblast growth factor-induced angiogenesis (27), a critical process for

growth and metastasis of cancers. Evidence for the anticarcinogenic effects of

CLA isomers indicates a modifying role in PPARa action (28).

Research demonstrated that CLA isomers reduce body fat in growing animals

(29–31) and its actions on fat and energy metabolism may, in part, be directed

through changes in both PPARa and PPARg (32, 33). In addition, speciﬁc effects

of CLA isomers on activity and expression of enzymes associated with anabolic

pathways of lipid metabolism are reported (34). For example, CLA was observed

to decrease the mRNA level of the 9-desaturase enzyme in both liver tissue and

hepatocyte cultures (35).

CLA may also modulate immune function by diminishing the production of an

array of pro-inﬂammatory products in macrophages through activation of PPARg

(32) and lowering basal- and lipopolysaccharide-stimulated IL-6 and basal tumor

necrosis factor (TNF) production in rat resident peritoneal macrophages (19).

Through activation of PPARg, CLA decreased interferon-g-induced mRNA

expression of cyclooxygenase (COX)-2, inducible nitric oxide synthase (iNOS),

TNFa, and pro-inﬂammatory cytokines [interleukin (IL)-1b and IL-6] in RAW

macrophage cell cultures (32). Dietary CLA isomers also reduced ex vivo PGE 2

production in rat bone organ cultures (20). Similar effects of CLA on PGE 2

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INTRODUCTION

production in various biological systems have been demonstrated (36–38). Based

on these aforementioned actions of CLA isomers, the likely biochemical and mole-

cular targets that integrate their potential impact on biology include PPARs, COX

enzymes, and other transcription factors.

1.5. Food Fortiﬁcation

Although signiﬁcant strides have been directed at reducing fat content in food pro-

ducts, certain lipid ingredients and sources of fatty acids are used to enhance the

health and nutritional quality of foods. For example, CLA isomers were enriched

in both dairy and nondairy products to convey its anticancer and antiobesity effects

that were reported repeatedly in animal studies (39). Sources of n-3 PUFAs are also

added directly to infant formula to provide sufﬁcient DHA for normal development

of the nervous system during early infancy. In the United States, DHA was

approved by the FDA in 2001 to be added into infant formula (40, 41).

Biological enrichment of n-3 PUFAs has been reported in lamb muscle (42),

chicken meat (43–46), eggs (43, 47), ewe milk (48–50), and pork (43). Sources

of n-3 PUFA in the above mentioned food products included ﬁsh meal, ﬁsh oil,

vegetable oils rich in n-3 PUFA (linseed oil and canola oil), algae and algal oil,

and oilseeds. Clinical trials clearly showed improvement in visual function in

infants fed LC-PUFA (DHA and AA)-enriched formula matching that of breast-

fed infants (9). In most cases, addition of LC-PUFA did not dramatically affect

the physical and sensory quality of the food products. However, enhancing the level

of LC-PUFA resulted in elevated susceptibility to lipid peroxidation both in the

food system and in the human body. To counter this problem, increased amounts

of Vitamin E were tested and found to be satisfactory in reducing lipid peroxidation

(47, 51). Several human studies indicated that n-3 PUFA fortiﬁcation provides an

effective means to increase n-3 PUFA intakes to satisfy the new DRIs for this group

of health-enhancing fatty acids (52, 53).

Numerous studies have been conducted to enrich dairy products with CLA and

n-3 PUFAs by providing sources containing high amounts of LC n-3 PUFAs to the

ration of dairy cows. Feeding ﬁsh oil (200 ml and 400 ml/head/d) to dairy cows

consistently increased milk CLA yield by as much as three-fold compared with

the control group (54). Milk from multiparous Holstein cows supplemented with

2% added menhaden oil contained higher concentrations of CLA, transvaccenic

acid, and total unsaturated fatty acids (0.68 and 2.51; 1.42 and 6.28; and 30.47

and 41.71 g/100 g of fat, respectively) compared with the milk from controls

that consisted of a 50:50 ratio of forage to concentrate. Butter made from milk

enriched with CLA inherently had higher concentrations of CLA (55). Similar ﬁnd-

ings were also reported by Donovan et al. (56) that by feeding lactating cows men-

haden oil at 2% dry-matter basis, the CLA content in milkfat increased 356% (to

2.2 g/ 100 g fatty acids) compared with the milkfat of control cows. Aside from

changes in CLA content, the n-3 PUFA also increased from a trace amount to

over 1 g/100 g of milk fatty acids (LNA 0.22, EPA 0.40, and DHA 0.20 g/100g)

when a diet with 3% ﬁsh oil was fed. In another study, concentrations of n-3 PUFAs

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