Potential Synergy of Phytochemicals in Cancer Prevention

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International Research Conference on Food,Nutrition, and Cancer

Potential Synergy of Phytochemicals in Cancer Prevention:

Mechanism of Action1

Rui Hai Liu2

Department of Food Science, Cornell University, Ithaca, NY 14853

ABSTRACT Epidemiological studies have consistently shown that regular consumption of fruits and vegetablesis strongly associated with reduced risk of developing chronic diseases, such as cancer and cardiovasculardisease. It is now widely believed that the actions of the antioxidant nutrients alone do not explain the observedhealth benefits of diets rich in fruits and vegetables, because taken alone, the individual antioxidants studied inclinical trials do not appear to have consistent preventive effects. Work performed by our group and others hasshown that fruits and vegetable phytochemical extracts exhibit strong antioxidant and antiproliferative activitiesand that the major part of total antioxidant activity is from the combination of phytochemicals. We proposed that

the additive and synergistic effects of phytochemicals in fruits and vegetables are responsible for these potentantioxidant and anticancer activities and that the benefit of a diet rich in fruits and vegetables is attributed to thecomplex mixture of phytochemicals present in whole foods. This explains why no single antioxidant can replace thecombination of natural phytochemicals in fruits and vegetables to achieve the health benefits. The evidencesuggests that antioxidants or bioactive compounds are best acquired through whole-food consumption, not fromexpensive dietary supplements. We believe that a recommendation that consumers eat 5 to 10 servings of a widevariety of fruits and vegetables daily is an appropriate strategy for significantly reducing the risk of chronic diseasesand to meet their nutrient requirements for optimum health. J. Nutr. 134: 3479S3485S, 2004.

KEY WORDS: diet and cancer fruits vegetables antioxidant cancer prevention phytochemicals

Epidemiological studies have consistently shown that a

high dietary intake of fruits and vegetables as well as wholegrains is strongly associated with reduced risk of developingchronic diseases, such as cancer and cardiovascular disease(CVD), which are the top 2 causes of death in the UnitedStates and in most industrialized countries (13). It is esti-mated that one third of all cancer deaths in the United Statescould be avoided through appropriate dietary modification(35). This suggests that change in dietary behavior, such asincreasing consumption of fruits, vegetables, and whole grains,and related lifestyles is a practical strategy for significantlyreducing the incidence of cancer.

In 1982 the National Academy of Sciences of the UnitedStates included guidelines in their report on diet and cancer,emphasizing the importance of fruits and vegetables (6). The

value of adding citrus fruits, carotene-rich fruits and vegeta-

bles, and cruciferous vegetables to the diet for reducing the riskof cancer was specifically highlighted. In 1989 a report fromthe National Academy of Sciences on diet and health recom-mended consuming 5 or more servings of fruits and vegetablesdaily for reducing the risk of both cancer and heart disease (7).The Five-a-Day program was developed as a tool to increasepublic awareness of the health benefits of fruits and vegetableconsumption and to promote adequate intakes of known vi-tamins. Plant-based foods, such as fruits, vegetables, and wholegrains, which contain significant amounts of bioactive phyto-chemicals, may provide desirable health benefits beyond basicnutrition to reduce the risk of chronic diseases (8).

Phytochemicals

The phyto- of the word phytochemicals is derived fromthe Greek word phyto, which means plant. Therefore, phyto-chemicals are plant chemicals. Phytochemicals are defined asbioactive nonnutrient plant compounds in fruits, vegetables,grains, and other plant foods that have been linked to reducingthe risk of major chronic diseases. It is estimated that 5000individual phytochemicals have been identified in fruits, veg-etables, and grains, but a large percentage still remain un-known and need to be identified before we can fully under-stand the health benefits of phytochemicals in whole foods

1 Published in a supplement to The Journal of Nutrition. Presented as part ofthe International Research Conference on Food, Nutrition, and Cancer held inWashington, DC, July 1516, 2004. This conference was organized by the Amer-ican Institute for Cancer Research and the World Cancer Research Fund Inter-national and sponsored by BASF Atkiengesellschaft; Campbell Soup Company;The Cranberry Institute; Danisco USA Inc.; DSM Nutritional Products, Inc.; HillsPet Nutrition, Inc.; Kellogg Company; National Fisheries Institute; The SolaeCompany; and United Soybean Board. An educational grant was provided by TheMushroom Council. Guest editors for this symposium were Helen A. Norman, VayLiang W. Go, and Ritva R. Butrum.

2 To whom correspondence should be addressed. E-mail: [email protected].

0022-3166/04 $8.00 2004 American Society for Nutritional Sciences.

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(8). However, more and more convincing evidence suggeststhat the benefits of phytochemicals in fruits and vegetablesmay be even greater than is currently understood, because theoxidative stress induced by free radicals is involved in theetiology of a wide range of chronic diseases (9).

Phytochemicals can be classified as carotenoids, phenolics,alkaloids, nitrogen-containing compounds, and organosulfurcompounds (Fig. 1). The most studied of the phytochemicalsare the phenolics and carotenoids.

Phenolics. Phenolics are compounds possessing one ormore aromatic rings with one or more hydroxyl groups and

generally are categorized as phenolic acids, flavonoids, stil-benes, coumarins, and tannins (Fig. 1). Phenolics are theproducts of secondary metabolism in plants, providing essen-tial functions in the reproduction and the growth of the plants;acting as defense mechanisms against pathogens, parasites, andpredators, as well as contributing to the color of plants. Inaddition to their roles in plants, phenolic compounds in ourdiet may provide health benefits associated with reduced riskof chronic diseases. Among the 11 common fruits consumed inthe United States, cranberry has the highest total phenoliccontent, followed by apple, red grape, strawberry, pineapple,banana, peach, lemon, orange, pear, and grapefruit (10).Among the 10 common vegetables consumed in the UnitedStates, broccoli possesses the highest total phenolic content,

followed by spinach, yellow onion, red pepper, carrot, cabbage,potato, lettuce, celery, and cucumber (11). It is estimated thatflavonoids account for approximately two thirds of the phe-nolics in our diet and the remaining one third are fromphenolic acids.

Flavonoids. Flavonoids are a group of phenolic com-pounds with antioxidant activity that have been identified infruits, vegetables, and other plant foods and that have beenlinked to reducing the risk of major chronic diseases. Morethan 4000 distinct flavonoids have been identified. They com-monly have a generic structure consisting of two aromaticrings (A and B rings) linked by 3 carbons that are usually in anoxygenated heterocycle ring, or C ring (Fig. 2). Differences inthe generic structure of the heterocycle C ring classify them as

flavonols, flavones, flavanols (catechins), flavanones, antho-cyanidins, and isoflavonoids (Fig. 1 and Fig. 3). Flavonols

(quercetin, kaempferol, and myricetin), flavones (luteolin andapigenin), flavanols (catechin, epicatechin, epigallocatechin,epicatechin gallate, and epigallocatechin gallate), flavanones(naringenin), anthocyanidins, and isoflavonoids (genistein)are common flavonoids in the diet (Fig. 1 and Fig. 4). Fla-vonoids are most frequently found in nature as conjugates inglycosylated or esterified forms but can occur as aglycones,especially as a result of the effects of food processing. Manydifferent glycosides can be found in nature; 80 differentsugars have been discovered bound to flavonoids (12). Antho-cyanidins give the red and blue colors in some fruits and

vegetables.Human intake of all flavonoids is estimated at a few hun-

dred milligrams (13) to 650 mg/d (14). The total averageintake of flavonols (quercetin, myricetin, and kaempferol) andflavones (luteolin and apigenin) was estimated as 23 mg/d, ofwhich quercetin contributed 70%; kaempferol, 17%; myr-icetin, 6%; luteolin, 4%; and apigenin 3% (15).

Phenolic acids. Phenolic acids can be subdivided into twomajor groups, hydroxybenzoic acids and hydroxycinnamic ac-ids (Fig. 5). Hydroxybenzoic acid derivatives include p-hy-droxybenzoic, protocatechuic, vannilic, syringic, and gallicacids. They are commonly present in the bound form and aretypically a component of a complex structure like lignins andhydrolyzable tannins. They can also be found in the form of

sugar derivatives and organic acids in plant foods.Hydroxycinnamic acid derivatives include p-coumaric, caf-

feic, ferulic, and sinapic acids (Fig. 5). They are mainly presentin the bound form, linked to cell-wall structural components,

FIGURE 1 Classification of dietary

phytochemicals.

FIGURE 2 The generic structure of flavonoids.

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such as cellulose, lignin, and proteins through ester bonds.Ferulic acids occur primarily in the seeds and leaves of plants,mainly covalently conjugated to mono- and disaccharides,plant-cell-wall polysaccharides, glycoproteins, polyamines, lig-nin, and insoluble carbohydrate biopolymers. Wheat bran is agood source of ferulic acids, which are esterified to hemicellu-lose of the cell walls. Free, soluble-conjugated, and boundferulic acids in grains are present in the ratio of 0.1:1:100 (16).Food processing, such as thermal processing, pasteurization,fermentation, and freezing, contributes to the release of thesebound phenolic acids (17).

Caffeic, ferulic, p-coumaric, protocatechuic, and vannilicacids are present in almost all plants. Chlorogenic acids andcurcumin are also major derivatives of hydroxycinnamic acids

present in plants. Chlorogenic acids are the ester of caffeicacids and are the substrate for enzymatic oxidation leading tobrowning, particularly in apples and potatoes. Curcumin ismade of two ferulic acids linked by a methylene in a diketonestructure and is the major yellow pigment of mustard.

Carotenoids. Carotenoids are natures most widespreadpigments and have also received substantial attention becauseof both their provitamin and antioxidant roles. More than 600different carotenoids have been identified in nature. Theyoccur widely in plants, microorganisms, and animals. Carote-noids have a 40-carbon skeleton of isoprene units (Fig. 6).The structure may be cyclized at one or both ends, may havevarious hydrogenation levels, or may possess oxygen-contain-ing functional groups. Lycopene and -carotene are examples

of acyclized and cyclized carotenoids, respectively. Carotenoidcompounds most commonly occur in nature in the all-trans

FIGURE 3 Structures of main classes of dietary flavonoids.

FIGURE 4 Chemical structures of common dietary flavonoids.

FIGURE 5 Structures of common phenolic acids: (a) benzoic acidand derivatives; (b) cinnamic acid and derivatives.

SYNERGY OF PHYTOCHEMICALS 3481S

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form. The most characteristic feature of carotenoids is the longseries of conjugated double bonds forming the central part ofthe molecule. This gives them their shape, chemical reactivity,and light-absorbing properties. -Carotene, -carotene, and-cryptoxanthin are able to function as provitamin A. Zeax-anthin and lutein are the major carotenoids in the macularregion (yellow spot) of the retina in humans.

Orange vegetables and fruits, including carrots, sweet po-tatoes, winter squash, pumpkin, papaya, mango, and canta-loupe, are rich sources of the carotenoid -carotene. Toma-toes, watermelons, pink grapefruits, apricots, and pink guavas

are the most common sources of lycopene; 85% of Americanlycopene intake comes from processed tomato products such asketchup, tomato paste, and tomato soup.

Carotenoid pigments play important functions in photo-synthesis and photoprotection in plant tissues. The photopro-tection role of carotenoids originates from their ability toquench and to inactivate reactive oxygen species such assinglet oxygen formed from exposure of light and air. Thisphotoprotection role is also associated with its antioxidantactivity in human health. Carotenoids can react with freeradicals and become radicals themselves. Their reactivity de-pends on the length of the chain of conjugated double bondsand the characteristics of the end groups. Carotenoid radicalsare stable by virtue of the delocalization of the unpaired

electron over the conjugated polyene chain of the molecules.This delocalization also allows addition reactions to occur at

many sites on the radical (18). Astaxanthin, zeaxanthin, andlutein are excellent lipid-soluble antioxidants that scavengefree radicals, especially in a lipid-soluble environment. Caro-tenoids at sufficient concentrations can prevent lipid oxida-tion and related oxidative stress.

Role of phytochemicals in the prevention of cancer

Cells in humans and other organisms are constantly ex-posed to a variety of oxidizing agents, some of which arenecessary for life. These agents may be present in air, food, andwater, or they may be produced by metabolic activity withincells. The key factor is to maintain a balance between oxidantsand antioxidants to sustain optimal physiological conditions.Overproduction of oxidants can cause an imbalance, leadingto oxidative stress, especially in chronic bacterial, viral, andparasitic infections (19). Oxidative stress can cause oxidativedamage to large biomolecules such as lipids, proteins, andDNA, resulting in an increased risk for cancer and CVD(9,19,20). To prevent or slow the oxidative stress induced byfree radicals, sufficient amounts of antioxidants need to beconsumed. Fruits, vegetables, and whole grains contain a wide

variety of antioxidant compounds (phytochemicals), such asphenolics and carotenoids, and may help protect cellular sys-tems from oxidative damage and also may lower the risk ofchronic diseases (10,11,16,2123).

Strong epidemiological evidence suggests that regular con-sumption of fruits and vegetables can reduce cancer risk. Blocket al. (24) reviewed 200 epidemiological studies that exam-ined the relationship between intake of fruits and vegetablesand cancer of the lung, colon, breast, cervix, esophagus, oralcavity, stomach, bladder, pancreas, and ovary. In 128 of 156dietary studies, the consumption of fruits and vegetables wasfound to have a significant protective effect. The risk of cancerwas 2-fold higher in persons with a low intake of fruits andvegetables than in those with a high intake. Significant pro-

tection was found in 24 of 25 studies for lung cancer. Fruitswere significantly protective in cancer of the esophagus, oralcavity, and larynx. Fruits and vegetable intake was protectivefor cancer of the pancreas and stomach in 26 of 30 studies andfor colorectal and bladder cancer in 23 of 38 studies. Aprospective study involving 9959 men and women in Finlandshowed an inverse association between the intake of fla-vonoids and incidence of cancer at all sites combined (25).After a 24-y follow-up, the risk of lung cancer was reduced by50% in the highest quartile of flavonol intake. Consumptionof quercetin from onions and apples was found to be inverselyassociated with lung cancer risk (26). The effect of onions wasparticularly strong against squamous-cell carcinoma. Boyle etal. (27) showed that increased plasma levels of quercetin after

a meal of onions was accompanied by increased resistance tostrand breakage by lymphocyte DNA and decreased levels ofsome oxidative metabolites in the urine.

Carcinogenesis is a multistep process, and oxidative damageis linked to the formation of tumors through several mecha-nisms (19,20). Oxidative stress induced by free radicals causesDNA damage, which, when left unrepaired, can lead to basemutation, single- and double-strand breaks, DNA cross-link-ing, and chromosomal breakage and rearrangement (20). Thispotentially cancer-inducing oxidative damage might be pre-vented or limited by dietary antioxidants found in fruits andvegetables. Studies to date have demonstrated that phyto-chemicals in common fruits and vegetables can have comple-mentary and overlapping mechanisms of action (Table 1),

including antioxidant activity and scavenging free radicals;regulation of gene expression in cell proliferation, cell differ-

FIGURE 6 Chemical structures of common dietary carotenoids.

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entiation, oncogenes, and tumor suppressor genes; inductionof cell-cycle arrest and apoptosis; modulation of enzyme activ-ities in detoxification, oxidation, and reduction; stimulation ofthe immune system; regulation of hormone metabolism; andantibacterial and antiviral effects (10,11,28,29).

Health benefits of phytochemicals in whole foodsfood synergy

The hypothesis that dietary antioxidants lower the risk ofchronic disease was developed from epidemiological studies.These have consistently shown that consumption of wholefoods, such as fruits, vegetables, and whole grains, is stronglyassociated with reduced risk of chronic diseases, especiallycancer and CVD. Therefore, it is reasonable for scientists toidentify the bioactive compounds responsible and hope to findthe magic bullet to prevent those chronic diseases. The keyquestion here is whether a purified phytochemical has thesame health benefit as the compound when its source is a food

or a mixture of foods. It is now widely believed that the actionsof the dietary supplements alone do not explain the observedhealth benefits of diets rich in fruits, vegetables, and wholegrains, because, taken alone, the individual antioxidants stud-ied in clinical trials do not appear to have consistent preven-tive effects (30 32). The isolated pure compound either losesits bioactivity or may not behave the same way as the com-pound in whole foods. For example, numerous investigationshave shown that the risk of cancer is inversely related to theconsumption of green and yellow vegetables and fruits. Be-cause -carotene is present in abundance in these vegetablesand fruits, it has been extensively investigated as a possiblecancer-preventive agent. However, the role of carotenoids asanticancer supplements has recently been questioned as a

result of several clinical studies (30,3335). In one study, theincidence of nonmelanoma skin cancer was unchanged in

patients receiving a -carotene supplement (33). In otherstudies, smokers gained no benefit from supplemental -caro-tene with respect to lung cancer incidence and may even havesuffered a significant increase in lung cancer and total mortal-ity (30,35). In The Heart Outcomes Prevention Evaluation(HOPE) Study, patients at high risk for cardiovascular eventswere given 400 IU vitamin E or a placebo daily for 4.5 y. Nodifference was found in deaths from cardiovascular causes,

myocardial infarctions, or deaths from CVD or strokes be-tween the 2 groups (32). In the Cambridge Heart AntioxidantStudy (CHAOS), patients with CVD were given 400 IU or800 IU -tocopherol or a placebo for a median of 510 d.-Tocopherol intake was associated with a significantly re-duced risk of myocardial infarction; however, it insignificantlyincreased risk of cardiovascular death (31). Vitamin E supple-mentation had no effect on the end points of death, myocar-dial infarction, or stroke for the patients who had recentlysuffered a myocardial infarction (36). Vitamin C supplementsalso failed to lower the incidence of cancer or CVD (37,38).

Phytochemical extracts from fruits and vegetables wererecently shown to have potent antioxidant and antiprolifera-tive effects, and the combination of phytochemicals from fruits

and vegetables was proposed to be responsible for the potentantioxidant and anticancer activity of these foods (10,11,39).The total antioxidant activity of phytochemicals in 1 g appleswith peel is equivalent to 83.3 mol vitamin C equivalents; toput it another way, the antioxidant value of 100 g apples isequivalent to 1500 mg vitamin C (39). This is far higher thanthe total antioxidant activity of 0.057 mg vitamin C (theamount of vitamin C in 1 g apples with peel) that is equivalentto 0.32 mol vitamin C equivalent. In other words, vitamin Cin apples contributes 0.4% of its total antioxidant activity.Thus, most of the antioxidant activity comes from phyto-chemicals, not vitamin C. The natural combination of phy-tochemicals in fruits and vegetables is responsible for its potentantioxidant activity. Apple extracts also contain bioactive

compounds that inhibit tumor cell growth in vitro. Phyto-chemicals in apples with peel (50 g/L on a wet basis) inhibitcolon cancer cell proliferation by 43%. However, this wasreduced to 29% when apple without peel was tested (39).

Different species and varieties of fruits, vegetables, andgrains have different phytochemical profiles (10,11,16,23).The combination of orange, apple, grape, and blueberry dis-played a synergistic effect in antioxidant activity (Fig. 7). Thedoseresponse curve of antioxidant activity was shifted to theleft after the combination of 4 fruits. The median effectivedose (EC50) of each fruit after combination was 5 times lowerthan the EC50 of each fruit alone, suggesting synergistic effects

TABLE 1

Proposed mechanisms by which dietary phytochemicals may

prevent cancer

Antioxidant activityScavenge free radicals and reduce oxidative stress

Inhibition of cell proliferationInduction of cell differentiation

Inhibition of oncogene expressionInduction of tumor suppress gene expressionInduction of cell-cycle arrestInduction of apoptosisInhibition of signal transduction pathwaysEnzyme induction and enhancing detoxification

Phase II enzymeGlutathione peroxidaseCatalaseSuperoxide dismutase

Enzyme inhibitionPhase I enzyme (block activation of carcinogens)Cyclooxygenase-2Inducible nitric oxide synthase

Xanthine oxideEnhancement of immune functions and surveillance

AntiangiogenesisInhibition of cell adhesion and invasionInhibition of nitrosation and nitrationPrevention of DNA bindingRegulation of steroid hormone metabolismRegulation of estrogen metabolism

Antibacterial and antiviral effects

FIGURE 7 Doseresponse of antioxidant activity of orange, apple,

grape, blueberry and 4-way combination.


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after the combination of the 4 fruits. Therefore, consumersshould obtain their phytochemicals from a wide variety offruits, vegetables, and whole grains for optimal health benefits.In 2003 Temple and Gladwin (40) reviewed 200 cohort andcase-control studies that provided risk ratios concerning intakeof fruits and vegetables and risk of cancer. They concludedthat cancer prevention is best achieved by consumption of awide variety of fruits and vegetables, although one group of

fruits and vegetables may dominate for a particular cancer. Toimprove their nutrition and health, consumers should be ob-taining antioxidants from their diet and not from expensivedietary supplements, which do not contain the balanced com-bination of phytochemicals found in fruits and vegetables andother whole foods. More importantly, obtaining antioxidantsfrom dietary intake by consuming a wide variety of foods isunlikely to result in consumption of toxic quantities, becausefoods originating from plants contain many diverse types ofphytochemicals in various quantities. Fruits and vegetableseaten in the recommended amounts (510 servings of fruitsand vegetables per day) are safe. Furthermore, health benefitsfrom the consumption of fruits and vegetables extend beyondlowering the risk of developing cancers and CVD: benefits also

include preventive effects on other chronic diseases such ascataracts, age-related macular degeneration, central neurode-generative diseases, and diabetes.

The additive and synergistic effects of phytochemicals infruits and vegetables have been proposed to be responsible fortheir potent antioxidant and anticancer activities. The benefitof a diet rich in fruits and vegetables is attributed to thecomplex mixture of phytochemicals present in these and otherwhole foods (10,11,39). This partially explains why no singleantioxidant can replace the combination of natural phyto-chemicals in fruits and vegetables in achieving the observedhealth benefits. Thousands of phytochemicals are present inwhole foods. These compounds differ in molecular size, polar-ity, and solubility, which may affect the bioavailability and

distribution of each phytochemical in different macromole-cules, subcellular organelles, cells, organs, and tissues. Thisbalanced natural combination of phytochemicals present infruits and vegetables cannot simply be mimicked by pills ortablets.

Research progress in antioxidant and bioactive compoundshas boosted the dietary supplement and nutraceutical indus-tries. The use of dietary supplements is growing, especiallyamong baby-boomer consumers. However, many of these di-etary supplements have been developed based on the researchresults derived from biochemical and chemical analyses andstudies, in vitro cell culture studies, and in vivo animal exper-iments and not from human intervention studies. The healthbenefits of natural phytochemicals at the low levels present in

fruits and vegetables does not mean that these compounds aremore effective or safe when they are consumed at a higherdose, even in a pure dietary supplement form. Generally speak-ing, higher doses increase the risk of toxicity. The basic prin-ciple of toxicology is that any compound can be toxic if thedose is high enough, and dietary supplements are no excep-tion. Therefore, a thorough understanding of the efficacy andthe long-term safety of many dietary supplements needs furtherinvestigation.

It is also important to differentiate the pharmacologicaldose from the physiological (or nutritional) dose. Pharmaco-logical doses are used clinically to treat specific diseases incertain situations and require a doctors prescription; physio-logical (or nutritional) doses are used to improve or maintain

optimal health, such as in dietary supplements or in foods. Inthe case of antioxidant nutrients, the proper physiological (or

nutritional) dose should follow the recommended dietary al-lowances. The pharmacological dose is not equal to the phys-iological (or nutritional) dose and, in some cases, can be toxicfor long-term use. Currently, there are no recommended di-etary allowances for phytochemicals. Therefore, it is not wiseto take megadoses of purified phytochemicals as dietary sup-plements before the appearance of strong supporting scientificevidence.

Summary

Dietary modification by increasing the consumption of awide variety of fruits, vegetables, and whole grains daily is apractical strategy for consumers to optimize their health and toreduce the risk of chronic diseases. Use of dietary supplements,nutraceuticals, and functional foods is increasing as industry isresponding to consumers demands. However, more informa-tion about the health benefits and the possible risks of dietarysupplements is needed to ensure their efficacy and safety.Phytochemical extracts from fruits and vegetables have strongantioxidant and antiproliferative activities, and the major partof total antioxidant activity is from the combination of phy-

tochemicals. The additive and synergistic effects of phyto-chemicals in fruits and vegetables are responsible for theirpotent antioxidant and anticancer activities. The benefit of adiet rich in fruits, vegetables, and whole grains is attributed tothe complex mixture of phytochemicals present in these andother whole foods. This explains why no single antioxidantcan replace the combination of natural phytochemicals infruits and vegetables and achieve their health benefits. There-fore, the evidence suggests that antioxidants are best acquiredthrough whole food consumption, not from expensive dietarysupplements. Further research on the health benefits of phy-tochemicals in whole foods is warranted.

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