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Phytochemicals in soybeansTheir potential health benefits

Soybeans have been a valuableresource for mankind by provid-ing excellent protein and other

nutrients. There is strong evidencethat dietary soy protein can lowerserum cholesterol levels. which in tumreduces the risk of heart disease.There also is increasingly convincingevidence that consumption of soyfoods can prevent various cancers.

Research has credited the phyto-chemicals in soybeans for these bene-ficial effects. Literally. phyrochemi-eals are "chemicals from plants."However, phytochemicals are definedby researchers as minor non-nutritivecomponents from plants. Soybeanshave many unique phytochemicalsincluding isoftevones, saponins, phy-tates, phytosterols, phenolic acids, andtrypsin inhibitors. All of these com-pounds have been cited as importantin prevention of degenerative condi-tions such as heart diseases and can-cers. For a long time, phytochemicalshave been regarded as irrelevant to

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HEALTH Be NUTRITION

This article ;$ by Chl,"yall,Mbn,. tUsocUJl~/HfJ/euor. andlWglna -. ..... ,." .. weI-...... the lk_ of NIIlrl-.... and Food _ ., SolllltDoMta -. UnIwniI)< Brook-..... SoIIIIt DotDt4. hu JfJIUM/P_ 3086of the SoIIIIt DoMta~~-BmoIin,s, SD 57007. witll ... R. ........,.,." SJI(1ptNf«J by the ~ of_and Food _. the_ DoMta Soybomt b_ andP_ c-.:i1. and the S-hDaI:ota Agricu/twal Ex,MriIItmt StatUm. 71W ptJper was pe~N'rt1iewedprior topob/krItIoft.

nutrition, since they neither yieldenergy nor function as vitamins. Fur-thermore, many phytochemlcals havebeen considered antinutrients in tradi-tional nutritional theory. However,antinutrients may play critical roles inprevention of diseases that result fromover-nutrition.

Soy IsonavonesIsoflavones are a class of phenoliccompounds, including daidzein,glycitein, and genistein (Figure I).lsoflavones exist in soybeans either asglucosides or in free fonn (aglucones).The glucosides of daidzein, glycitein,and genistein are called daidzin, glyc-

H 0

0 OH

IOH 0

Genistein

HO ,0

OH OHOCH3

H 0Glyciteln

H

Figure 1. Struc1ures of.oy lsonavone agluconesSource; I.E.IJener, Irnpliclltions01Antinutrltional Compounds in Soybean Foods, CtiticaJ Revfews In Food ~.nd Nutrilloo 34(').31~7 (1994).

H

H 0

Daidzein

INFORM, Vol. 10, no. 4 (Apr111999)

316

itin, and genistin, respectively. Twoderivatives for each glucoside alsoexist in soybeans: 6'-O-acctyl-daidzin,-glycitin, and -genistin; and 6'-O-ma\-onyl-daidzin. -glycittn, and -genistin.These isoflavone compounds, previ-ously considered antinutrients, playsuch significant roles in prevention ofheart diseases and cancers that theymay come \0 be regarded as the vita-mins of the 21st century.

The risk for breast cancer amongnative Chinese is about 10% of that ofNorth Americans. The risk for prostatecancer of the Chinese population isonly 2% of that of the North Ameri-cans. Although many factors contributeto these differences, soy consumptioncould be a major [actor. A study onJapanese and Finnish women showed astrong correlation between high urinaryisoflavone content and a low breastcancer rate. Among Asian women, hightofu consumption was found to be asso-ciated with a reduced risk for breastcancer. In vitro studies have shown thatgenistein has inhibitory effects on many

HEALTH & NUTRITION

cancer cell lines, including breast,prostate, thyroid, and gastrointestinal.The anticarcinogenic properties ofgenistein also were demonstrated in fatbreast cancer models.

The exact mechanism by whichisoflavones act as anticarcinogens isnot known. However, there are severalproposed mechanisms. First, someisoflavones. especially genistein, cancompete for estrogen receptors withestrogen, the female sex hormone.Therefore, Isoffavones function asantiestrogens and are protectiveagainst hormone-related cancers. Sec-ond, isoflavones inhibit some of theenzymes, such as tyrosine kinase,associated with carcinogenesis. Third,some isoflavones are found to inhibitangiogenesis, which is essential forthe development of tumors.

It has been well-established thatsoy protein products are hypocholes-terolemic. lsoflavones are thought tobe at least partially responsible for thisproperty. Reduced cholesterol levelsin turn can reduce the risk of cardio-

vascular disease for individuals whoconsume soy ingredients and soyfoods. Isoflavones, being potentantioxidants, also may reduceatherosclerosis by inhibiting oxidationof low-density lipoprotein (LDL)cholesterol. In addition, the weakestrogenic properties of isotlavonesalso could protect postmenopausalwomen against heart diseases.Isof1avones have been studied fortheir other potential benefits withregard to bone health and diabetes.

The concentration of isoflavones insoy foods and soy ingredients variesdramatically. First, the genetics ofsoybean cultivars and the environmentwhere the soybeans are raised have asignificant effect on the quantity andtype of isoflavones in the soybeans.However, there has been no conclu-sion as to which environmental factorspromote which isoflavones. Second,processing techniques also will affectthe concentration of isoflavones. Astudy on the effects of processing onisotlavones during preparation of soy

For in!ormatlon circ;:la '105 on form on lasl page

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INFORM. Vol. 10. no. 4 {April 1999)

I

317

R,~

OR,

R~rCH,OH

R R,A group

JIa GIc(1-2)gal(1-2)glcUA- -OH 2,3,4-tri Ac.xy1(1-3)ara-Ab GIc(1-2)gal(1-2)glcUA- -OH 2,3,4,6-tetra Ac.glc(1-3)ara-Ac Rha(1-2)gal(1-2)glcUA- -OH 2,3,4,6-tetra Ac.glc(1-3)ara-Ad GIc(1-2)ara(1-2)gleUA- -OH 2,3,4,6-tetra Ac.glc(1-3}ara-k GaI(1-2)glcUA- -OH 2,3,4-tri Ac.glc(1-3)ara-/oj GaI(1-2)glcUA- -OH 2,3.4,6-tetra Ac.glc(1-3)ara-Ag Ara(1-2)glcUA- -OH 2,3,4,6-tetra Ac.glc(1-3)ara-Ah Ara(1-2)gleUA- -OH 2,3,4-trl Ac.xy1(1-3)ara-

8group

Sa GIc(1-2)gal(1-2)gleUA- -HBb Rha(1-2)gal(1-2)gleUA- -HBe Rha(1-2)ara(1-2)glcUA- -HBb' GaI(1-2)gleUA- -HBe' Ara(1-2)gleUA- -H

Figure 2. Structure ot soyasaponins.Source: K. Okubo, S. Kudou, T. Uchida, V. Yoshlkl. M. Yoshlkoshl. and M. TonomurB, Soybean Saponin and lS/:Iflslltlnoid5 Structure and AntMral Activity aga;nsl HumanImmunodeficiency Virus In VIIro, ACS Symposium Senes 546, 330-339 (1994).

protein isolates revealed that only23% of the total isoflavones in soyflour remained after the soy flour hadbeen processed to yield soy proteinisolates. The washing step caused thegreatest loss among the processingsteps. New processing technologies ormodifications of existing processesare needed to minimize the loss ofiscflavones during processing.

SaponinsSaponins are a group of glycosides oftriterpenoids or steroids. Soybeans area major dietary source for saponins.Soyasaponins are classified intoGroups A, B, and E. As shown in Fig-ure 2, Group A soyasaponins, knownas bis-desmosides, contain two ether-linked sugar chains attached to C-3and C-22. Groups Band E, known asmono-desmosides, contain sugarsattached to C-3 alone. There are threepredominant forms in raw soybeanseeds: soyasaponin ag, soyasaponinI3g, and soyasaponin Ba, in concenrra-

lions ranging from [ to 3 mg/g. Thesaponin compound is umphiphilic. thetriterpene or steroid portion ishydrophobic. and the sugar portion ishydrophilic. These surface character-istics give saponins their biologicalactivities, including hemolytic, hypo-chclesterolemic, immunostimulatory,and antitumor properties.

Saponins are hemolytic due to theirinteractions with cell membranes. Forthis reason, they are considered anti-nutrients and toxins, although thehemolytic properties of saponins varyaccording to their specific structures.Some seponins are extremely toxic tofish and other cold-blooded animals.but saponins administered orally 1.0

mammals seem to have no toxiceffects. Nevertheless, toxicity shouldbe considered when the potentialhealth benefits are explored.

The hypocholesterolemic propertyof saponins is the major mechanismfor their preventive effects against car-diovascular diseuses. Dietary saponins

can lower plasma cholesterol directlyby inhibition of cholesterol absorptionor indirectly by inhibition of bile acidreabsorption, thereby increasing fecalexcretion of bile acids.

The anticarcinogenic property ofsaponias has been studied using ill vitroand ill vivo studies. In one study. soyas-aponins were shown to have dose-dependent growth inhibitory effectsagainst human colon carcinoma (HCf-15) cells. The same researchers laterinvestigated the effects of soybeansaponins on colon cancer in the mouse.Soyasaponins at 3% by weight of thediet suppressed azoxymethane-inducedmouse colon carcinogenesis. Saponinsmay function as anticarcinogens inthree modes. First, saponins may func-tion directly as cytotoxins to cancercells. Second. saponins may alter themetabolism of bile acids and reduceproduction of secondary bile acids andtherefore reduce the risk of colon can-cer. Third, saponins may Function as

[continued 011 page 319)

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HEALTH Be NUTRITION

[continued from JNlge 3/7)

free-radical scavengers. Due to theirrelatively poor absorption, saponins arebelieved to affect only colon cancerrisk. However, they might indirectlyaffect other forms of cancers by reduc-ing serum cholesterol and enhancingimmune functions.

Some nonsay saponins-c-For-mosanin-C from a Chinese herbal drugand ginsenoside from ginseng-havebeen shown to have immune-stimulat-ing effects. It is not known if soyas-aponins have similar effects.

Unheated and healed full-fat flourscontain approximately the same levelsof saponins, indicating soybeansaponins are heat-stable. Saponins lendto remain with the protein productsderived from soybeans during process-ing. Fermentation results in somedegradation of saponins. Details on themass balance of saponins during pro-cessing of soy products are no( known.

Phytic acidPhytic acid (myo-inositol 1,2,3,4,5,6.-hexakis-dihydrogen phosphate) (Figure3) is a storage form of phosphorus inplant seeds. Its concentrations in soy-beans and in soy products are high(about 1-2%). IIhas been viewed as anantinutritional factor due to its ability tochelate minerals. It also functions as ananti nutrient by binding protein andstarch, reducing their digestibility.These properties could be considered asdesirable in prevention of cancers, car-diovascular diseases. and diabetes.

A number of in vitro and animalstudies have demonstrated potentialpreventive effects of phytic acid

Figure 3. Structure. 0' phytlc ecld.Soon::.: I.E. Liene<, Impbtlonl of MIlnu\ritIon.aICompounds .. Soybean Foods,CrilicalR~ ..Food Sdenc:e and Nutrition 3<I(1):31~1 (1994).

HO

HOsitosterol

22 24

"'-stigmasterol

cholesterol

Figure 4. Struetu .... of three msJor phytosterol. In soybAn. end that of cholHtemlSource: AV. Reo and SA JanazIc:, The Role 010Ieury ~eroIt .. COlen CIllt:iuol/eusis.NIIIr. Cancer18:~(1992)

damage by free radicals.The potential preventive effect of

phytic acid against cardiovascular dis-eases is likely due to its hypolipidemicand hypochoiesteroiemic properties.Phytic acid provides a clear benefit forpeople with diabetes by delaying gfu-case release from starch. Long-termexposure 10 phytic acid also may bebeneficial in prevention of diabetes byreducing peak glucose concentrationand preventing development of tissueresistance to insulin.

Soy protein products (flours, con-centrates. and isolates) contain 1-2%phytic acid. The effects on phytic acidcontent of growing conditions of soy·bean crops and of processing condi-lions of soy products need to be sys-tematically studied.

PhytosterolsThe phytosterols comprise a numberof compounds structurally related tocholesterol. ~-Sitosterol, campesterol,and stigmasterol are three major phy-tosterols in soybeans. Their structuresand that of cholesterol are shown inFigure 4. The total phytosterol contentof soybeans is 0.3 to 0.6 mg/g.

HO

against cancers in colon. mammaryglands. and other tissues. The additionof 1% phytic acid to the drinkingwater of rats one 10 two weeks prior toadministration of azoxymethane. acarcinogen, and up to 12 months after-ward significantly decreased tumorsize and number. In a mammary can-cer study. addition of 2% phytic acidto the diet after administration of thecarcinogen dimethylbenz(a)anthra-cence (DMBA) in rats significantlyincreased the number of survivors anddecreased the tumor size.

The binding of phytic acid withstarch, proteins, and minerals mightbe its major mechanism as an anticar-cinogen. Fermentation of unabsorbedstarch due to phytic acid producesshort-chain fatty acids. This wouldcause a low pH environment in thecolon, which in turn would provideprotection against colon cancer byinsolubilizing bile acids and neuuuliz-ing ammonia. Binding of zinc byphytic acid also may be enncercino-genic. as zinc is essential during DNAsynthesis. Phyric acid also functionsas an antioxidant by chelating theprooxidant Fe3+ and reducing DNA

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HEALTH & NUTRITION

1080

110 120

130

180

Kunitz inhibitor

FIgure 5. Amino acid sequences of Kunitz and Bowman-Birk trypsin InhibitorsSource: T, Kokla and T, Iltenaka, Eur. J. Biochem. 32:417 (1973)

Although phytosrerols are structurallyrelated to cholesterol, they have many"enttcholesterot" properties, whichhave been demonstrated in many ani-mal and human studies. III vitro andanimal studies also have demonstratedamicarcinogenic properties of phytos-terols. Phytosterots are also a feed-stock for the manufacture of pharma-ceutical steroids.

By the early 19505, the hypo-cholesterolerruc effect of phytosterolsin chicks had been demonstrated. Thiseffect was confirmed in humans by anumber of studies. The hypocholes-terolemic effect most likely is due to

Trypsin inhibitorsTrypsin inhibitors are prot e iumolecules that bind with trypsinsand interfere with protein hydrolysisduring digestion. Kunirz and Bow-rnan-Birk are the two major trypsin

decreased cholesterol absorption. Phy-tosterols inhibit the formation of bile-cholesterol micelles and compete forthe enzymes required for cholesteroluptake. Although only a minimalamount of phytosterols (5%) isabsorbed by humans, the absorbedphytosterol may adversely affectcholesterol synthesis.

Phytosterols have been shown to beanticarcinogenic in animal models ofcolon cancer. This effect is related tothe hypcchclesterolemic properties ofphytosterols. Low cholesterol levelscould indi.rectly lower colon cancerrisk. More importantly. phytosterots

competitively inhibit cholesterol dehy-drogenase and other bacterial enzymesin the colon and therefore reduce theproduction of the secondary biles thatmay cause colon cancer.

It is known thai refining soybeanoil is detrimental to the phytosterolcontent. However, the detailed massbalance is not understood.

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321

CH30

HO P- CHO

CH30

SyringaJdehyde

() OH

-,CH =CH-COOH

c-Coumanc acid

OH

/S) COOH

HO

Gentistic acid

Figure 6. Structures of three major phenolic acids In soybeansSoor(:ll: P. Ribereau-Gayon. Plant Phenalic:s, Hafner Publishing Co.,1972, pp. &-15, 82-83.

inhibitors in soybeans (Figure 5),They are partially responsible forlow protein digestibility of raw soy-beans. The Bowman-Birk inhibitor(BBI) has 71 amino acids with sevendisulfide bonds and two bindingsites: a trypsin-reactive site (Lys 16and Ser 17) and a chymotrypsin-reactive site (Leu 43 and Ser 44),The Kunitz inhibitor, which has 181amino acids, possesses only twodisulfide bonds and binds stoichio-metrically to trypsin and inhibitsonly trypsin. The Kunitz inhibitor isthermally unstable and is destroyedduring most processing procedures.However, Bowrnan-Birk is relativelythermal stable and can survive nor-mal thennal treatment. Nevertheless,heat treatment has been the mosteffective way of inactivating trypsininhibitors and improving the soyprotein digestibility.

Besides causing low proteindigestibility. trypsin inhibitors havebeen shown to cause pancreasenlargement in rats. Physiologically,when trypsin inhibitors from soy-beans bind with trypsin then the pan-creas responds by producing moreenzyme via increased activity of the

hormone cholecystokinin. However,this effect has not been observed inguinea pigs, calves. pigs, or monkeys.

Recently, there has been renewedinterest in trypsin inhibitors due totheir reported anticarcinogenic prop-erties. BBI has been shown to bevery effective in preventing carcino-genesis in the liver, lung, and gas-trointestine in mouse models. Themechanism of prevention is not clear,however, it has been suggested thatprotease inhibitors suppress both ini-tiation and promotion stages of car-cinogenesis.

BibliographyJournals

Journal of Agricultural Food Chem-istry

Wang, H.J., and P.A. Murphy,lsoflavcne Content of CommercialSoybean Foods (42:1666-1673.1994).

Journal of NutritionAnderson, R.I" and W.J. Wolf, Com-

positional Changes in TrypsinInhibitors, Phytic Acid, Saponinsand Isoflavones Related to Soy-bean Processing (125:58IS-588S,1995).

Carroll, K.K., and E.M. Kurowska,Soy Consumption and CholesterolReduction; Review of Animal andHuman Studies(/25:594S-597S.1995).

Rao, A.V., and M.K. Sung, Saponins asAmicarcinogens (/25:717S-724S,1995).

Journal of the American OilChemists' Society

Mounts, T.L., S.L. Abidi, and K.A.Rennick, Effects of Genetic Modifi-cation on the Content and Composi-tion of Bioactive Constituents inSoybean Oil (73:581-586,1996).

Wang, c., Q. Ma, P. Krishnan, and M.Self, Changes of lsoflavones Dur-ing Processing of Soy Protein Iso-lates (75:337-341, 1998).

OtherHuang, M.T., T. Osawa, C.T. Ho, and

R.T. Rosen, in Food Phytochemi-cats for Cancer Prevention I:Fruits and Vegetables, ACS Sym-posium Series 546, AmericanChemical Society, Washington,DC,I994.

Shahidi, F., Antsnutrients and Ptiyto-chemicals in Food. ACS SymposiumSeries 662, American ChemicalSociety, Washington, DC, 1997. •

Phenolic acidsPhenolic acids are another class ofccmpli cated phytochemicals.Syringaldehyde, o-coumaric acid. andgentistic acid (Figure 6) are the threemajor ones in soybeans. Many in vitroand ill vivo studies have demonstratedanticarcinogenic properties of pheno-lic acids, including ferulic acid,chlorogenic acid, caffeic acid, andellagic acid. However, no data areavailable on the three major phenolicacids in soybeans. Knowledge onanalysis, presence, and processing ofphenolic acids is also very limited.

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