Prevention of cancer: Vegetables and plants

Comp. Biochem. PhysioL Vol. 93B, No. 2, pp. 201-212, 1989

0305-0491/89 $3.00 + 0.00 © 1989 Pergamon Press pie

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REVIEW PREVENTION OF CANCER: VEGETABLES A N D PLANTS GABRIEL HOCMAN Research Institute of Preventive Medicine, 14 Limbova Street, CS 83301 Bratislava, Czechoslovakia

(Received 15 August 1988) Abslraet--l. Results of epidemiological studies indicate that a human diet rich in vegetables may lower the incidence of cancer. 2. This preventive effect of the vegetable diet against cancer could be ascribed to lowered intake of energy (joules) and its content of vitamins and carotene. 3. The consumption of vegetables means also less meat and fats as well as increased fiber content and specific chemopreventive compounds (indoles, plant phenols) present in such a diet. 4. The supposed mechanisms of prevention may include enhanced enzymatic detoxification of harmful compounds, and inhibition of their binding to cellular DNA, their adsorption on fiber, detoxification of radical forms of carcinogens by natural antioxidants in plants and probably many other ways too.

INTRODUCTION

In the history of mankind people quite often sought remedies for their ailments in plants. They found in plants not only food, but medicine, too; however, a number of harmful substances and poisons were also discovered in their leaves, roots and flowers. Even today plants represent a large but as yet almost untapped source of substances with medical properties. Compounds originating from plants were used in curing cancer, too. A number of such compounds also exhibited properties preventing the occurrence of this disease. It is well known that diets consisting mainly, or even exclusively, of vegetables are more healthy than foods containing predominantly sugar, meat or fat. The consumption of edible plants and vegetables is supposed, therefore, to prevent the occurrence of a

Abbreviations used: 2-AA 2-aminoanthracene, carcinogen; AFB t aflatpxin B,, hepatocarcinogen; AHH aryl hydrocarbon hydroxylase (cytochrome P-450-dependent monoxygenase, a PAH metabolizing enzyme); BaP benzo(a)pyrene, carcinogen; BOP N-nitroso-bis-(2-oxopropyl)-amine, carcinogen; DI 3,3'-diindolylmethane, supposed preventive agent; DMAB 3,2'-dimethyl-4aminobiphenyl, carcinogen; DMBA 7,12-dimethylbenz(a)anthracene, carcinogen; DMSO dimethylsulfoxide, solvent; DMH 1,2-dimethylhydrazine, colon carcinogen; ECD 7-ethoxyeoumarin-O-deethylase, detoxicating enzyme; HPLC high-performance liquid chromatography; IC indole-3-carbinol, supposed preventive agent; IN indole-3-acetonitrile, supposed preventive agent; MCA methylcholanthrene, carcinogen; MNNG methyl-Nnitro-N-nitrosoguanidine, carcinogen; MNU methylnitrosourea, carcinogen; ODC omithine decarboxylase (an indicator of tumor promotional activity); PAH polycyelic aromatic hydrocarbons, group of compounds involving many overt carcinogens; RR relative risk; SCE sister-chromatid exchange, indicator of cellular damage; TPA 12-O-tetradecanoyl-phorbol-13-acetate, a tumor promoter. CBP(B)93/2--A

number of diseases in humans and animals, among them such killers as cardiovascular diseases or cancer. Since the vegetables contain less nutritional energy (joules) than do, for example, sugar or fat, this in itself seems to be one of the factors preventing diseases (see Hocman, 1988a). Any food rich in vegetables and fruits contains usually huge amounts of indigestible fibrous components which seem to prevent the cancer of the gastrointestinal tract. Moreover, vegetables, fruits and their seeds are rich sources of Vitamins A, C, and E, or, perhaps, inhibitors of proteases as well, compounds which might protect the organism against cancer. Some plants may contain specific compounds which exhibit chemopreventive properties against neoplasms, such as ellipticine, indole derivatives, plant phenols, etc. However, other plants might contain compounds showing mutagenic, and, perhaps, carcinogenic properties, too, such as quercetin or rutin, as well as some constituents of exotic spices or mushrooms. Therefore one should be careful in choosing the constituents of foods of vegetable origin. In some vegetables there are harmful substances induced into them by man, such as pesticide residues or excess nitrate fertilizers. Mouldy vegetables or seeds could contain carcinogenic mycotoxins, too. In some cases mutagenic substances of natural origin, such as in onions might be present in human food. Nevertheless, the presence of potentially chemopreventive compounds in edible plants and vegetables merits the attention of researchers seeking to find and use chemopreventive compounds against cancer.

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The lowered incidence of various kinds of cancers in groups of the population consuming exclusively, or at least predominantly vegetables and fruits was confirmed in a number of epidemiologic studies. For example, the overall cancer mortality rate of Seventh-Day Adventists (SDA) is only 50-70% of

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that of the general population. The members of the sect, in general, abstain from smoking and drinking, avoid the use of coffee, tea, hot condiments and spices, lead a rather conservative sexual life and about 50% of them are lacto-ovo-vegetarians. They consume vegetables, fruits, whole grains and nuts in abundance. This lifestyle seems to protect them against cancer. The consumption of meat is considered to be a serious risk factor for colon cancer, raising the relative risk of this disease to a value almost three times higher than that of vegetarians (Phillips, 1975). From a cohort of 122,261 Japanese men followed for 16 years Hirayama (1985) selected a group of non-smoking, non-drinking men with no daily consumption of meat, but eating green and yellow vegetables daily. The lifestyle of these men resembled closely that of the members of the Seventh-Day Adventist sect (SDA-like group). Comparing the age-adjusted, specific mortality rates in this group of people with those of the opposite lifestyles, the author established much lower risks of cancer death in the SDA-like group. The risk of overall cancer mortality was less than one-half in the group of men with SDA-like lifestyles as compared to those with opposite lifestyles. Similarly, the risks of stomach and liver cancers, as well as those of peptic ulcer and heart disease were less than one-half in the SDA,like group. The risk of cancer of mouth, pharynx, esophagus and lung, as well as that of subarachnoid hemorrhage, were one-fifth or less in the group of men with SDA-like lifestyle. Altering the habits connected with the SDA-lifestyle, e.g. smoking or drinking alcohol elevated the risk of cancer substantially. Consuming meat also resulted in increased risk of heart disease and that of the cancers of pancreas, esophagusand lung, states Hirayama (1985). Considering the relative risk of a given site-specific cancer in the non-smoking, non-drinking, no daily meat consuming, but eating green and yellow vegetables group of people as 1.0, the relative risks of certain cancers in the group with opposite lifestyles were: cancer of mouth and larynx, 9.11; esophagus, 5.76; stomach, 2.02; liver, 3.58; lung, 8.54; in general, for cancers of all sites the relative risk of this group was 2.49 (Hirayama, 1985). The sister-chromatid exchange (SCE) in lymphocytes is considered to be an indicator of previous exposure of a person to mutagens or carcinogens. Those members of the SDA sect who are lacto-ovovegetarians and neither drink nor smoke, exhibited a mean value of SCE of 5.54 + 0.07 (SEM) per cell, against the control group of non-vegetarians, having the number of SCE per cell of 8.00 + 0.15, their difference being statistically highly significant (P < 0.00001). The authors (Wulf et aL, 1986) ascribe this difference mainly to the increased consumption of meat among the persons in the control group. From the above results it follows that altering the lifestyle might influence the relative risk of cancer mortality significantly. When the people at risk start eating green and yellow vegetables, and consume, consequently, less meat, their relative risks of cancer decline substantially. The beneficial effect of increased consumption of green and yellow vegetables

is ascribed mainly to the increased intake of vitamins (mainly those of A, C and E). The chemopreventive influence of these compounds will be discussed in a subsequent part of these reviews on the prevention of cancer. In an evaluation of the effect of vegetable consumption upon the risk of lung cancer adjusted to smoking habits Ziegler et al. (1986) found that men in the lowest quartile of vegetable consumption had a higher (1.4) RR as compared to those in the highest quartile (1.0). A similarly-evaluated comparison of consumption of dairy products was not found to be related to the risk of lung cancer. It is not surprising that the nutritional factors influence predominantly the cancers of the gastrointestinal tract. In a case-control study on stomach cancer in Cracow, Poland, Jedrychowski et aL (1986) found that the consumption of vegetables and fruits affects the incidence of this disease profoundly. If the RR for people eating fruit daily was 1.0, for those consuming them rarely it rose to 3.24. If the RR of acquiring stomach cancer for people consuming high amounts of vegetables, salads and fruit combined was 1.0, for those with a low intake of the above foods it was found to be as high as 4.23. By "vegetables" the authors mean cabbage, sauerkraut, carrots or beets, consumed cooked, whereas as by "salads" they mean cabbage, lettuce or other vegetables consumed raw. In another study Jedrychowski and Popiela (1986) found even higher odds of incidence of stomach cancer in persons consuming low amounts of fruits and vegetables. If the odds ratio for this disease in persons consuming high amounts of fruit (once a day or more) was 1.0, then in persons consuming them at low level (once a week or less) it was 10.73. The same values for the consumption of vegetables were even higher: 1.0 for high level of consumption, but as high as 21.93 in persons consuming low levels (once a week or less) of vegetables. The increased incidence of stomach neoplasms in Poland the authors explain by the high consumption of cereals (RR 1.0 for persons consuming cereals once a day or less, but an estimated RR of 3.44 was found for those eating them twice a day or more often). A similar case-control study on the incidence of colorectal cancer carried out in France (MacquartMoulin et aL, 1986) found a lower consumption of vegetables and vegetable fiber in patients with these types of cancer as compared to matched healthy controls. The decreasing incidence of stomach cancer in the USA in the past fifty years is attributed to the increased consumption of fresh vegetables and fruits (Cordle, 1986). These food items became available due to rapid transportation and refrigeration all over the country and could, therefore, contribute to the prevention of neoplasms. From these and a number of other epidemiological studies it follows that the relative risk of overall cancers, but mainly the cancers of the gastrointestinal tract, seems to be directly related to the amount of meat or protein consumed, but related inversely to the consumption of green and yellow vegetables. In short, the more fresh vegetables and fruits a person consumes, the lower is their chance of acquiring GI cancer.

Prevention of cancer INDOLE DERIVATIVES

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tumors, 1.6 __.0.43 per mouse). When the dose of the BaP was lowered but the duration of the carcinogen Assuming, from the above epidemiologic studies, insult prolonged (0.3rag BaP, three times a week the preventive influence of vegetables and fruits upon for 8 weeks), more neoplasms developed in control cancer incidence and mortality, the researchers animals (100% incidence, 7.1 +0.64 tumors per sought to identify those chemical compounds which mouse) but the preventive effect of IN appeared to be may be responsible for this effect. A number of indole more pronounced (44% incidence of forestomach compounds present in edible cruciferous vegetables tumors, 1.1 _+ 0.33 tumors per mouse) (Wattenberg were identified as capable of lowering or preventing and Loub, 1978). the incidence of chemically-induced neoplasma. Since On the other hand Birt et al. (1986) found little these compounds need to be present in the organism or no modifying effect of IC upon the mutagenic at least a short time before the carcinogen insult activities of MNNG, MNU, BaP or 2-AA assayed by in order to protect the body against its harmful Ames' test on Salmonella typhimurium. On female effect, they are considered to be typically preventive SENCAR mice the action of a tumor-promoting substances. Three indole derivatives (indole-3- substance, phorbol acetate (TPA) applied to the skin carbinol, IC; 3,Y-diindolylmethane, DI; and indole- was even enhanced by IC, applied to the skin of 3-acetonitrile, IN) were identified in cruciferous the animals 30 min before the TPA. The influence of plants (cabbage, cauliflower, broccoli and Brussels TPA was measured by the induction of enzyme sprouts) which might exhibit such preventive proper- ornithine decarboxylase (ODC), an indicator of ties upon the development of cancer. tumor-promoting activity. Indole-3-carbinol (IC) Seven-week-old female Sprague-Dawley rats were both alone and in combination with TPA enhanced given 12 mg DMBA in olive oil by p.o. intubation. the ODC induction, indicating a possible enhanceTwenty hours prior to this treatment the animals ment of tumor promotion in mice treated with IC received, in a similar way, one single dose of either (Birt et al., 1986). 1 ml of solvent DMSO (controls) or one of the tested Male F 344 rats were treated with the colon indole derivatives in the same solvent. While 91% of carcinogen D M H (injected i.p. at a dose of 10 mg/kg the control animals developed mammary tumors at body weight, weekly for 16 weeks). Indole-3the average of 21 weeks (the average number of carbinole was administered to the animals tumors per rat being 1.45 + 0.28), among the rats continuously as 0.1% of their diet from 3 weeks receiving 0.10 mmol IC prior to the carcinogen treat- before the beginning of carcinogen administration. ment, only 21% ( P < 0 . 0 1 ) developed tumors Although control animals fed IC without any (0.29 + 0.16 tumors per rat). In the group of rats carcinogenic insult did not develop tumors, Pence receiving 0.05 mmol DI 27% acquired mammary et al. (1986) consider IC, together with wheat bran, tumors (0.36 _+ 0.46 tumors per rat) (Wattenberg and beef tallow and cholesterol to be one of the factors Loub, 1978). actually enhancing the DMH-induced colon tumor. In a similar experiment the rats were given two Perhaps the low concentration of IC in the food, its indole derivatives (IC and IN) in a purified labora- administration during and after the carcinogenic tory diet (0.014mmol IC/g of diet, or 0.030mmol insult as well as the possible interaction with other IN/g of diet), eight days before the carcinogen insult dietary components (beef tallow) could account for (12 mg of DMBA as above) and remained on this diet the above results. However, according to the view of with the added indoles for the duration of the experi- the authors (Pence et al., 1986) IC is a colon tumor ment, i.e. till the rats'were 28 weeks old. In the enhancer, acting during the promotional stage of control animals not receiving any indole derivative carcinogenesis. 73% acquired mammary tumors (1.20 + 0.30 tumors A number of other experiments yielded results per rat); in animals receiving IN 54% developed which were ambiguous at best. Swiss mice kept on neoplasms (1.00 + 0.39 tumors per rat), but among cabbage-supplemented (13%) diet showed 17 weeks those receiving IC only 20% developed cancer after repeated injections of D M H no significant (0.33 + 0.19 tumors per rat) (Wattenberg and Loub, differences in colon tumor incidence (Temple and 1978). E1-Khatib, 1987). The preventive effect of the above indole derivaThe juice of vegetables (washed, minced, centritives was tested on 8-week-old ICR/Ha mice, too. fuged and the supernatant used) lowered in the The animals were fed purified laboratory diets con- presence of $9 mix (liver microsomes containing taining indole derivatives and after four days they enzymes metabolizing carcinogens) the mutagenic received 1 mg of BaP by p.o. intubation, in corn oil, activity of beef extract on Salmonella typhimurium twice-weekly for four weeks. They were kept on the TA 1538 assay. The beef extract and $9 mix (without diet containing the indole derivative till three days any vegetable juice) gave rise to 1050 revertants in after the last dose of BaP and the experiment was this assay indicating its comparatively high mutaterminated when the animals were 31 weeks old. genicity. The addition of vegetable juice to the test Among the control mice not receiving indole deriva- system reduced this number of revertants strongly tives 93% developed tumors of the forestomach (beans, to 720; paprika, 700; red cabbage, 358; kohl (5.0 + 0.54 tumors per mouse) but in those receiving rabi, 240; white cabbage, 182; Brussels sprouts, 142) 0.03 mmol IC per g of diet only 80% acquired tumors (Miinzer, 1986). It could be concluded that the influence of indole (1.9 +_0.35 tumors per mouse). The DI (0.02 mmol/g) proved to be less effective (94% of mice with tumors, derivatives from vegetables upon the prevention of 3.2 +0.51 per mouse) but IN (0.03mmole/g) pro- carcinogenesis is rather complicated. The preventive tected the animals more effectively (68% of mice with action of the mentioned compounds (IC, DI, IN)

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seems to depend upon the carcinogen insult (preventing the influence of BaP or DMBA, ineffective for DMH), its dose, the target issue (protecting the mammary or forestomach but- not the colon), the kind of experimental animals used, the time of the treatment (before or after the carcinogen insult), as well as its duration (IC seems to prevent the carcinogen insult if present in the organism at least some time before the carcinogen, but probably enhances its effect when given concurrently or after the carcinogen), the concentration of indole derivative in the food, and, perhaps, on the influence of many other factors, too. The influence of indole derivatives upon the organism of mammals does strongly depend upon other compounds already present in the body or reaching it together with the edible vegetables. In Chinese cabbage which is a usual item of the Japanese diet Wakabayashi et al. (1986) established the presence of two harmless, non-mutagenic indole derivatives (4-methoxyindole-3-acetonitrile and 4-methoxyindole-3-aldehyde). These compounds, when treated with nitrites (NO{) under conditions resembling those in the human stomach (37°C; pH 3) convert to strongly mutagenic derivatives. Nitrites arise from nitrates (NO3-) even by means of enzymes in saliva in human mouth, and nitrates are usually present in vegetables treated with large amounts of fertilizers. Therefore, the consumption of overfertilized vegetables could do more harm to the organism than its possible benefits. Moreover, fresh cabbage, radish root and spinach were found to produce compounds mutagenic in the Salmonella (TA 100) assay upon treatment with nitrite, too. Similarly, some tyramine derivatives found in soy sauce could produce strongly mutagenic compounds after treatment with even low concentrations of nitrite (Nagao et al., 1986). The mechanism by which the indole derivatives exert their influence upon the organism is not yet fully understood. Indole-3-carbinol (0.1% in food for 16 weeks) was found to increase the activity of AHH in the liver of rats to almost twice its basal value (Pence et al., 1986). This enzyme converts the potentially carcinogenic polycyclic aromatic hydrocarbons (PAH's) to their respective phenol metabolites. Therefore AHH may play a key role either in detoxifying the aromatic carcinogens or in converting them to their ultimate carcinogenic derivatives. All three indole derivatives (IC, DI, IN) are inducers of this enzyme and may therefore affect the metabolism of PAH's profoundly, altering thus their carcinogenic potency in the organism. Feeding cabbage to experimental animals may also induce other enzyme systems involved in the detoxification of foreign compounds (glutathione-Stransferase, glutathione reductase), increase the concentration of detoxifying compounds (glutathione) and prevent the damage of DNA (Stohs et al., 1986). Indole-3-carbinol reduces the DNA-binding of carcinogenic aflatoxins and increases substantially their excretion as aflatoxin-glucuronides through the bile, thus reducing their hepatocarcinogenic action (Goeger et al., 1986). Eighteen-month-old female mice kept for two weeks on chow containing 20% freeze-dried cabbage

had lower body weights exhibited higher activities of glutathione reductase and glutathione-S-transferase and higher concentrations of glutathione in the liver as compared to controls without cabbage in their food. Moreover, the animals given cabbage showed lowered malondialdehyde content (a measure of lipid peroxidation) and significantly lower (by about 50% P < 0.05) hepatic single-strand DNA breaks (Stohs et al., 1986). The higher activities of detoxifying enzymes as well as the lowered number of DNA single-strand breaks seem to indicate the possible mechanisms of the prevention: enhanced detoxification of possibly carcinogenic foreign compounds and protection of the DNA against potential DNA damage. Trout (Salmo gairdneri) fed for 12 weeks on a semipurified (dextrose and casein-gelatin) diet either containing 0.2% IC or without this compound, were given aflatoxin B~ (AFB~) administered by i.p. injection. In another experiment liver cells of trout were incubated with AFBI. The fish receiving IC had lower body weights but somewhat enlarged livers. The experiments indicated lowered binding of AFB~ (by about 50%) to hepatic DNA, and enhanced (approx 7-times) elimination of glucuronide-aflatoxin M~ derivative by bile in the animals receiving IC (Goeger et al., 1986). It seems, therefore, that the indole-3-carbinol (IC) could prevent or lower the hepatocarcinogenic insult of aflatoxin in trout. The mechanism of this action could be attributed either to enhanced detoxification of this compound by bile which may involve the induction of detoxifying enzymes, or reducing the binding of the ultimate carcinogen to nuclear DNA. It could be assumed that both mechanisms may contribute to the prevention of carcinogenesis by indole derivatives in higher mammals, too. The influence of vegetables upon carcinogenesis is rather complex. Therefore, to achieve the desired preventive effect upon carcinogenesis, the whole vegetable should be present in the organism, that is, not only the isolated indole derivatives or other compounds alone. To this protective effect a contributory fact is that consuming more vegetables means consequently consuming less meat, fats or sugars, too. On the other hand, vegetables are to be consumed devoid of excess nitrates or pesticide residues, which, at least in some cases, may convert the harmless indole derivatives into mutagenic or even carcinogenic ones. PLANT PHENOLS

Another group of compounds originating in plants which are supposed to influence the onset of cancer in mammals are plant phenols, mainly flavones. Their action, as in indole compounds, is ambiguous. Some of them have been reported to decrease the mutagenic properties of some carcinogens thus preventing their possible harmful effects. (For review see Newmark, 1987.) Others seem to have negligible influence upon carcinogenesis or are indifferent. A few substances belonging to this group of compounds have been reported to have mutagenic and carcinogenic properties. The following examples should illustrate the preventive properties of plant phenols.

Prevention of cancer The mutagenicity of M N N G (5 nmol/plate) and MNU (400nmol/plate) was tested on Salmonella typhimurium TA 100. The addition of apigenin (4,5,7-trihydroxyflavone) to t h e assay system (20/~g/plate) decreased the mutagenity of MNNG by 170 and that of MNU by 13%. The mutagenity of BaP (30nmol/plate) and 2-AA (13 nmol/plate) tested in Salmonella TA 98 was decreased by the same amount of apigenin by 43% (for BaP) and 62% (for 2-AA). In a similar test with the same concentrations the addition of robinetin (3,Y,4',5',7-pentahydroxyflavone) decreased the mutagenicity of MNNG by 6 0 o and that of MNU by 11% in the TA 100 strain, and the mutagenicity of BaP by 24% and that of 2-AA by 87% in the strain TA 98 (Birt et al., 1986). It has been estimated that humans ingest with their daily food approximately 1 g of plant phenols. These may contribute to the overall chemopreventive effect of the vegetable diet in humans. Although the vegetarian diet may effectively lower the incidence of cancer in mammals, it is not able to abolish the carcinogenic effect altogether. Even herbivorous animals have not been found to be free of cancer. Some researchers reported remarkable lowering of the mutagenic and carcinogenic effect of PAH's by means of plant phenols. This influence is attributed to inhibition of the activity of carcinogen-metabolizing enzymes and to hindrance of the interaction of carcinogenic metabolites with cellular DNA. Microsomes of the skin of six-week-old female SENCAR mice were incubated for 30min with ~4C-BaP either in the presence or absence of various plant phenols. The formation of BaP metabolites in these systems was determined by HPLC. Tannic acid, quercetin, myricetin and anthraflavic acid inhibited the formation of BaP-diols and BaP-phenols by approx 30-60%, the tannic acid being the most effective. The microsomal fraction of the skin of control SENCAR mice treated with solvent (acetone) only, exhibited an AHH enzyme activity of 3.3 (pmol of metabolite/min.mg protein) while in those with the addition (240/~mol) of various plant phenols to the microsomes it was considerably less (tannic acid, 0.55, 83% inhibition; quercetin, 0.41, 88%; myricetin, 0.28, 92%; anthraflavic acid, 1.11, 63%). If the skin of mice was pretreated with MCA (50 mg/kg body weight), the AHH activity rose considerably (38.86) but the addition of 240/~mol of plant phenols decreased this activity markedly (tannic acid, 2.81, 92% inhibition; quercetin, 4.83, 88%; myricetin, 4.00, 90%; and anthraflavic acid 15.83, 57%). (Das et al., 1987a). If the above plant phenols were applied topically to the skin of the animals, they inhibited the activities of several carcinogen-metabolizing enzymes in a dosedependent manner (for example, 400/~mol/kg body weight of plant phenol inhibited the AHH activity in the skin of mice: tannic acid by 53%, quercetin by 650, myricetin by 590, while anthraflavic acid exhibited no such effect (Das et al., 1987a). In a similar experiment the skin of six-weekold female SENCAR mice was treated with a single topical application of a given plant phenol (400 #mol/kg body weight) and after 1 hr the skin

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was treated with 3H-BaP (5 nmol). After 24 hr the animals were killed, the DNA from their lungs isolated and the covalent binding of BaP to DNA determined (pmol/mg DNA). In control animals treated with solvent only (acetone), the value of the BaP binding was 1.38 whereas the treatment with plant phenols decreased this binding considerably (tannic acid, 0.49, 56% inhibition; quercetin, 0.68, 51%; myricetin, 0.58, 58% anthraflavic acid, 0.56, 59%) (Das et aL, 1987 B). The natural phenolic compounds caffeic acid and chlorogenic acid reduced the mutagenic activity of MNNG in the Salmonella TA 1535, when added to the assay system concurrently (Chan et al., 1986). Ellagic acid, a common plant polyphenol, inhibits the conversion of various PAWs in mouse keratinocytes to their ultimately carcinogenic derivatives in a dosedependent manner. This compound was found to protect some experimental animals against the influence of such strong carcinogens as BaP or DMBA. This chemopreventive effect is probably achieved by inhibition of the activity of xenobiotictransforming cytochrome P-450-dependent monooxygenases AHH and ECD which convert these carcinogens to their ultimate metabolites (Mukhtar et al., 1984). While these and other researchers ascribe powerful cancer-preventing properties to plant phenols, others have been unable to demonstrate this unequivocally beneficial effect. Although a number of plant phenols decrease the mutagenic properties of many known carcinogens in vitro, mainly in bacterial assays with Salmonella, the in vivo prevention of carcinogenesis in mammals often did not exhibit the expected results. This seems to be due to low uptake and rapid removal of plant phenols from the organism of mammals. While these compounds are retained in the in vitro systems, they are rapidly removed from the living organism, and, hence, they are probably not able to perform their preventive action. The strong mutagenic influence of the ultimate carcinogen BaP-diol-expoxide in the Salmonella TA 100 assay was intensely lowered not only by ellagic acid, but with two of its synthetic derivatives (3-O-decylellagic acid and 3,3'-di-O-methylellagic acid) as well (Smart et aL, 1986a). However, these compounds given concurrently with BaP, or shortly (30 min) before did not inhibit, even in rather high concentrations, the formation of BaP-DNA adducts in mouse lungs. These compounds did not lower the rate of MCA-induced skin tumorigenesis in mice, either (Smart et al., 1986b). This effect is probably due to poor absorption and rapid elimination of ellagic acid and its derivatives in the organism. The estimated half-life of ellagic acid in plasma and lung is about 5 min, in liver about 10 min. The biochemical influence by which plant phenols exert their antimutagenic and chemopreventive properties against chemical carcinogenic factors probably involves their phenolic hydroxyl groups (Das et al., 1987b). The ability of flavones to inhibit the BaP-DNA adduct formation seems to depend essentially upon the hydroxyl groups in positions 5,7 of the A ring, 3', 4', and 5' of the B ring and in position 3 of the C ring. Methylation or glycosylation of these hydroxyl groups, as well as the saturation of

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the 2,3 double bond, renders them less active (Shah and Bhattacharya, 1986). The mechanisms of the preventive action of plant phenols upon the carcinogenesis may include: (1) induction of carcinogen-metabolizing enzymes such as AHH and the following enhancement of the metabolism of foreign compounds; (2) inhibition of the binding of carcinogenic compounds to cellular DNA; (3) exhibition of the antioxidant properties of those phenols which could inactivate the electrophilic radical forms of carcinogens; (4) some plant phenols (ellagic acid) may form a covalent ether adduct with the ultimate carcinogen (BaP-diol-epoxide). The adduct effectively removes the BaP derivative from aqueous environment thus preventing the carcinogen from exerting its effect. This part of the review is concerned mainly with the chemopreventive properties of phenolic plant constituents. However, much research was devoted to the examination of mutagenic and carcinogenic properties of this group of compounds. In general, plant phenols seem to possess negligible, if any, carcinogenic activity, with a few possible exceptions (for reviews on this topic see, for example, Knudsen, 1982; Aeschbacher, 1982; Stavric, 1984; Hatch, 1986; Rueff, 1986, etc). The purpose of the above few examples is to illustrate the ambiguity of the influence of plant phenols upon experimental chemical carcinogenesis. While some phenolic compounds in certain experimental animals may defend the organism against the influence of given carcinogens, others may, under the same, or different experimental conditions be indifferent or even harmful. Moreover, the in vitro and in vivo influences of the same compound may differ, too. Nevertheless, the plant phenols do influence the metabolism of carcinogens, above all that of the ubiquitous PAWs in the organism, rather intensely and therefore their possible chemopreventive properties against carcinogenesis deserves further attention. EDIBLE SEAWEEDS

Epidemiologic studies revealed that the rate of incidence of breast cancer in premenopausal women in Japan is about three times lower, and in postmenopausal women even nine times lower than in the USA (see Teas et al., 1984). In the search for the explanation of this fact one of the differing variables (besides the higher body weight of US women which may be a risk factor) was the much higher consumption of dietary seaweed (Laminaria) in Japan. This notion prompted a study of the influence of possible preventive action of dietary seaweed upon carcinogenesis. Even traditional Chinese medicine accepted hot water extracts of certain marine algae in the treatment of cancer. The renewed interest of Japanese scientists led to the isolation of a number of compounds, probably polysaccharides from edible seaweeds which slow down the development of various induced cancers thus prolonging the life of the experimental animals.

The edible marine algae were dried, extracted in Boiling water for 4 hr, concentrated by freeze-drying and dialyzed. The concentrated non-dialyzable fraction was injected i.p. into CDF] male mice inoculated 24hr previously with a suspension of L-1210 leukemia cells. The experimental animals received 400 mg/kg day of an extract of Ecklonia cava for six successive days, the control mice receiving water instead of the extract. The life-span of treated mice was 37% longer than that of control animals. Similarly, extracts from Laminaria angustata and Laminaria japonica prolonged the life of treated animals by 25 and 39%, resPectively (Yamamoto et al., 1982). Extract of edible brown seaweed, Laminaria religiosa prolonged the life of mice infected with L-1210 leukemia by 33% (Maruyama and Yamamoto, 1984), and the extract of Sargassum kjellmanianum inhibited the growth of Sarcoma 180 by 92% (Yamamoto et al., 1981). A preparation of polysaccharide (sodium alginate, mol.wt 33400) isolated by Sepharose 4B chromatography from an extract of Sargassumfulvellum was found to be active against Sarcoma 180, too (Fujihara et al., 1984). The effect of antitumor polysaccharides from edible seaweeds seems to be host-mediated, probably by enhancing the immunological defense of the organism of the experimental animal against the carcinogen insult (Yamamoto et al., 1986). On the other hand, these extracts seem to inhibit the mutagenity of overt carcinogens in bacterial in vitro assays as well. In in vitro experiments extracts from Laminaria angustata inhibited in a dose-dependent manner the mutagenicity of both DMBA and DMAB in the Salmonella test (TA 98 and TA 100) by 80-90% (Reddy et al., 1984). Both direct feeding of seaweed and enrichment of laboratory food of experimental animals with powdered algae or hot-water extracts from edible algae or its non-dialyzable residue resulted in slower growth of tumors. It is possible that for the chemopreventive action of edible seaweed-containing diets the greatly increased time-lag between the initiation and the overt manifestation of the disease could be responsible. That is, consumption of edible seaweed could delay the onset of the disease for a long time. In male ddY mice, aged 4 or 5 weeks, sarcoma 180 tumors were implanted. Starting 24 hr after the carcinogen insult, the control animals were fed a basic diet while the experimental ones received the same diet containing either powdered seaweed or its hot-water extract, or its non-dialyzable fraction. Extracts from both Laminaria angustata and Laminaria angustata, vat. longissima inhibited

strongly (by 70-76% and 83.6%) the growth of tumor, the powdered weed-also being effective (Yamamoto et al., 1986). Twenty-one-day-old female Spraguc-Dawley rats were fed sun-dried Laminaria anguatata (brown kelp) constituting 5 % of a standard semipurified laboratory diet. The control animals received the same amount of non-nutritive cellulose fiber (alphacel).At 55 days of age both groups of rats were given 5 m g of D M B A by gavage. The median time of appearance of palpable m a m m a r y tumors was 77 days in control rats, whereas it was 139 days in seaweed-consuming animals. The control rats had a

Prevention of cancer significantly higher number of adenoearcinomas per sick rat (3.3) as had the seaweed-consuming animals (2.2, P = 0.04), although at the end of the experiment both groups o f rats developed similar numbers of adenocarcinomas (Teas et al., 1984). The estimated per capita intake of seaweed in Japan ranges from 4.9 to Z3 g daily (see Teas et aL, 1984). The seaweed-enriched food does not seem to be harmful either to people or to dairy cows and may contribute to the low incidence of mammary, and, perhaps, some other cancers in its consumers. The purpose of this example is to show that even unusual constituents of vegetable diet may possess preventive properties against certain cancers. OTHER COMPOUNDS

A number of compounds originating from plants or animals could be relative to carcinogenesis. For example, feeding 33% bracken fern (Pteridium aquilinum) to rats increased their incidence of intestinal (95%) and urinary bladder (60%) tumors. In the control animals not receiving bracken fern no such tumors were found (0%) (Pamukcu et al., 1980). The carcinogenic and mutagenic properties of bracken fern are ascribed to its alkaloid, ptaquiloside (Hirono, 1986). Its product of decomposition, a conjugated dienone exhibited mutagenieity in the Salmonella typhimurium (TA 98 and TA 100) tests (Matoba et al., 1987). Young fronds of this plant are customarily eaten in Japan and elsewhere as foodstuff. However, their previous treatment with wooden ash is reported to diminish their carcinogenicity. Feeding dried cabbage (approx 10% in the food) to rats increased the incidence of pancreatic and gall bladder cancers, induced by BOP (40 mg/kg body weight, s.c.) (Birt et al., 1987). Even commonlyconsumed fruits (grapes, peaches, raspberries, strawberries and raisins, as well as onions) were found to possess mutagenic activities attributed to the presence of flavones quercetin and kaempferol in them, in the Salmonella assay (Stoltz et al., 1984). However, this activity may, at least in part, be ascribed to residues of mutagenic agricultural chemicals, and the plant variety, its degree of maturity and growing conditions may affect the results of mutagenicity testing. On the other hand, compounds having anticarcinogenic properties were found in a great many plants and aquatic animals, too. Two alkaloids from Ochrosia elliptica (ellipticine, 5,1 l-dimethyl-6H-pyrido/4,3b/carbazole and 9-methoxyellipticine) exhibit distinct anticancer activities and may prevent the carcinogenic injury to the cell (Lesca et al., 1978; Kouadio et al., 1984). Since these compounds bind strongly to liver microsomal cytochrome P-450, they affect the activities of enzymes metabolizing foreign compounds depending upon the above cytochrome. Ellipticines were found (1) to inhibit the formation of water-soluble metabolites of PAH's such as BaP, (2) to inhibit the covalent binding of these metabolites to cellular DNA, (3) to lower the mutagenic activity of some carcinogenic compounds in the Salmonella assay (Lesca et al., 1978).

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Exposure of Chinese hamster ovary tumor cells/n vitro to 6#/ml of ellipticine for 2hr inhibited the formation of cell colonies by 50% (Traganos et al., 1980a). Exposure of Friend and L 1210 leukemia cells for 24 hr to 1 #g of ellipticine in vitro slowed down the proliferation of these cells considerably (Traganos e t al., 1980b). Some derivatives of ellipticines (e.g. N2-methyl-9-hydroxyellipticine) seem to have even more pronounced anticarcinogenic properties than the parent compounds (Dugue et al., 1984). On the other hand it seems that pretreatment of experimental animals with ellipticines (e.g. 9-hydroxyellipticine) renders them more susceptible to carcinogenesis by PAH's, probably due to the enhanced metabolism of these compounds (Cresteil a n d Lesca, 1983). Other alkaloids (for example, alstonine and serpentine from Rauwolfia plants and sempervirine) seem to possess anticancer activities, too. They are supposedly able to distinguish between DNA's from healthy and cancerous cells and by becoming bound to the initiation sites of destabilized cancer DNA's should be able to prevent cancer cell D N A synthesis. Thus the above-mentioned alkaloids (100#g/ml for 48 hr in vitro) destroyed the proliferative capacity of various cancer cells (KB, HeLa, Hep II) while they remained inactive against normal eucaryotic cells. If mice were injected with cancer cells previously treated with alkaloids, tumors did not appear (Beljanski and Beljanski, 1984). When d-limonene, a constituent of citrus oil was fed (as 10% of a usual diet) to rats in which mammary carcinogenesis was initiated by DMBA, a highly significant regression of the tumor was observed. Moreover, d-limonene inhibited the formation of subsequent tumors in such initiated rats (Elegbede et al., 1986). Some phenolic amides from peppers (Piperacae) were able to inactivate mutagens in food. Moreover, they exhibited antioxidative and antimicrobial activities as well (Nakatani et al., 1986). Soybean phosphatidylinositol showed selective cytotoxicity towards tumor cells (Myher and Kuksis, 1984). Diallyl sulfide, a component of garlic oil inhibits the induction of nuclear aberrations in the colons, urinary bladders and hair follicles of mice treated with carcinogens (DMH or cyclophosphamide). The defense mechanism probably involves the conjugation of carcinogenic metabolites with diallylsulfide which renders them inactive (Goldberg and Josephy, 1987). Water-soluble components of some edible clams (Mercenaria mercenaria), probably glycopeptides, prolonged the life of mice with P 388 leukemia and B 16 melanoma (Schmeer, 1979). Aqueous extracts of marine animals, some of them edible, contain compounds with antitumor activity which inhibits the growth of sarcoma 180 in mice. The activity of extracts from 25 different edible marine animals (shells, scallops, clams, mussels, oysters) were tested in mice with implanted sarcomas. The tumor growth was inhibited in almost all cases, and in some experiments even complete tumor regression was observed (Sasaki et al., 1985). Polysaccharide fractions from various fungi (eg. Grifolafrondosa) were also effective in inhibiting the growth of sarcoma 180 (Suzuki et al., 1984). The

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antitumor manifestations of the above compounds are considered to be host-mediated, enhancing the activity of their immune systems. There are countless compounds isolated from both edible and non-edible plants as well as animals which may possess properties preventing cancer or acting against its manifestations. However, this anticancer activity is usually low. Therefore these compounds may serve more for the elucidation of the mechanisms of cancer-preventing or anticancer action, than as chemicals practically utilizable in prevention or treatment. The very few examples presented above are intended to illustrate the width of research encompassing every imaginable plant and animal in search for beneficial anticancer principles.

DIETARY FIBER

Scores of epidemiological studies indicate an indirect relationship between the fiber content of food and cancers in the digestive tract, but also other sites, too (see, eg. Miller, 1985; Willet and MacMahon, 1984 a,b; Jacobs, 1986, 1987; Talbot, 1981). For example, in 450 lung cancer cases compared to 902 controls Byers et al. (1987) found a higher incidence of the disease in persons consuming low amounts of fiber (amounts of fiber consumed divided into quartiles vs. relative risk of lung cancer, highest quartile, RR 1.0, lowest quartile, RR 1.6 in both males and females). The rate of colon cancer is much lower in developing countries where the consumption of vegetables, and hence fiber, is high, than in industrialized countries, where the consumption of meat and fats prevails (the incidence of colon cancer per 100,000 inhabitants in Dakar, Senegal: 0.6, males, M, and 0.7, females, F, but 32.3 M and 26.4 F in Connecticut, U.S.A., or 31.4 M and 26.3 F in New York State, U.S.A.) (Tollefson, 1986). In an Israeli kibbutz the consumption of dietary fiber averaged 23.6g daily, whereas in Tel Aviv only 18.7g per day; yet the colon cancer incidence in the Kibbutz was only a third of the frequency found in Tel Aviv (see Walker et al., 1986). In a case-control study Lubin et aL, (1986) found a tendency for decreasing risk of breast cancer with increasing fiber consumption. A change in the nutritional habits, e.g. in migrant workers from developing countries to industrialized ones showed, too, that a shift from the fiber-rich, predominantly vegetable-orientated food to a diet consisting mainly of meat and fat, low in fiber, is accompanied by a rise in the incidence of cancer of the gastrointestinal tract. These studies indicated that the above types of cancer could not be directly attributed to hereditary factors but that they depend mostly on dietary habits. The protective role of dietary fiber against cancer in general, and the cancer of the gastrointestinal tract in particular, is strongly indicated but not unequivocally proven. Some studies indicate a less pronounced connection between cancer and dietary fiber. Macquart-Moulin et aL (1987) in a case-control study of coiorectal cancer and diet found only a slightly statisticallyinsignificantly lower intake of fiber in sick persons. In Japan, where the incidence of colon

cancer is pronouncedly lower than in Britain or Scandinavia (in Osaka, 7,8 M and 6.3 F, compared to 23.8 M and 21.1 F in North Scotland, and 14.3 M, 14.5 F in Norway) (Tollefson, 1986), the researchers did not find higher fiber consumption. The consumption of non-starch polysaccharides in Japan was found to be around 13 g/day, a value similar to that in Scandinavia and Britain (Kuratsune et al., 1986). Dietary fiber could be defined as the endogeneous components of plant material which is resistant to digestion by enzymes produced in humans. They are predominantly non-starch polysaccharides and lignin associated with other substances (see Jacobs, 1986). Therefore the relationship: the more fiber in the diet, the lower cancer incidence or mortality rate, could not be considered that simple. Since the "fiber" constitutes a rather complex chemical entity, supposedly there are fibers (probably the less soluble and less fermentable, such as wheat bran and cellulose) which inhibit tumor development. However, other kinds of the same entity with different properties (more soluble and fermentable fiber which increases fecal acid and bile excretion, such as corn or oat bran, pectine and agar) may promote or enhance tumorigenesis in the large bowel (Jacobs, 1986). The views upon the mechanisms by which the fiber exerts its supposed protective role on carcinogenesis differ a great deal and probably no single processs could be held responsible for its cancer-preventing effect. Since the fiber content increases the stool bulk mainly by increasing its water content,, the carcinogens in the bowel are more diluted and therefore probably less capable of exerting their action (see Jacobs, 1986, 1987). Moreover, increased fecal bulk decreases its transit time enabling the organism to get rid faster of the carcinogens and mutagens bound to fiber in the stool (Jacobson et al., 1984). The chemically stable material of fiber may adsorb certain types of mutagens, predominantly pyrolysate products of proteins from heat-processed food (Kada et al., 1984). Fiber prepared from edible vegetables (cabbage, carrots, spinach etc.) by boiling in water and dehydrated with 99% alcohol was added to aqueous solutions of mutagenic tryptophan pyrolysates (TrpP-I, Trp-P-2) and maintained at room temperature overnight. The mutagenicity of fiber-treated pyrolysates was then tested in Ames' assay on Salmonella typhimurium TA 98. The established percentage of Trp-P-1 mutagenity against a control with no added fiber, showed a marked decrease: no fiber, 100% edible burdock, 6.4%; cabbage 17.5% spinach 17.7% carrot 13.1%. Fiber from edible burdock reduced the Trp-P-1 mutagenity almost to zero in one hour, with Trp-P-2 the mutagenic activity disappeared completely within six hours. Since no recovery of mutagenic activity was found after the extraction of treated fiber with water and only a partial recovery was achieved with extraction by means of ethyl acetate, the authors conclude that the binding of mutagens to fiber is rather firm (Kada et al. 1984). The above finding might indicate one of the ways how the organism renders some mutagens harmless by binding them to fiber and subsequently getting rid of them via the intestine.

Prevention of cancer Supplementation of human diet with additional 11 g of fiber per day for four weeks (in form of whole grain bread) reduced the fecal concentration of secondary bile acids, a substance which is supposed to increase the colon cancer. Moreover, the above fiber supplementation directly inhibited the fecal mutagenic activity in Ames' test with both TA 98 and TA 1000 Salmonella typhimurium, with or without an addition of $9 mix (Reddy et al., 1987). Hamsters fed 100 #g of benzo(a)pyrene (BaP) in a gram of commercial food for 10 days excreted in feces only 3.8% of the induced carcinogen. Enrichment of their food with 5% wheat bran resulted in an increase of the amount of excreted BaP to 6% (Mirvish et al., 1981). According to the authors the increased excretion of BaP in animals receiving a fiber-rich diet may result from increased fecal weight while the BaP concentration remained constant, or by adsorption of BaP by the fecal (fiber) material. A strong statistical correlation indicates a direct relationship between the amount of fat in the diet and the incidence of cancer, above all, that of colon and breast. The supposed mechanism of this process probably includes the bile acids involved in the digestion of the lipids. The intestinal bacteria probably metabolize the bile acids to carcinogenic products. Dietary fiber seems to bind lipids, the bile acids, and, perhaps, their harmful products, too, dilute them and increase their excretion (Story, 1985; Willett and MacMahon, 1984 a,b). In a similar way the dietary fiber could increase the fecal excretion of cholesterol in humans (Kaneko et al., 1986). Enrichment of food with water-soluble, mainly oat or bean products consumed for 21 days resulted in about 20% reduction of serum cholesterol concentration in men (Anderson et al., 1984). Similarly, fiber in the form of cellulose or pectin reduced the plasma cholesterol content in rabbits (Beynen et al., 1987). However, it should be noted that the lowering of plasma cholesterol by means of its adsorption on fiber and subsequent excretion may mean a relative increase of this steroid in the intestine, thus relatively increasing the risk of cancer in the gut or colon. Moreover, different kinds of fiber may respond to lipid and bile acids in the gastrointestinal tract in different ways. Therefore the relationship between fats and bile acids, cholesterol and dietary fiber, gut microflora and carcinogenesis are extraordinarily complex. The effect of fiber ingestion on the whole organism is by no means simple, either. Enrichment of food with dietary fiber (15 g daily for 4 weeks) initiated a tendency to weight loss in obese subjects (Groppel and Acosta, 1987), on the other hand the adsorptive properties of massive doses of fiber could deprive the organism of valuable vitamins or other compounds adsorbed to them (e.g. Vitamin E and lipids, Mongeau et al., 1986) or iron (Groper and Acosta, 1987). However, moderate doses of fiber could not be considered harmful in this respect. Since the dietary fiber intake influences plasma cholesterol and its excretion as well as obesity, it may, as so many factors related to carcinogenesis, play a role in heart and vascular diseases, too. A 12-year study of a cohort of over 800 men and women in California indicated lower relative risk of ischemic

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heart disease mortality in persons consuming at least 16g dietary fiber daily (RR 0.33 in men and 0.37 in women, against those with a lesser intake of fiber). A daily 6 g increment of dietary fiber intake was associated with a 25% reduction in ischemic heart disease mortality (Khaw and Barrett-Connor, 1987). Dietary fiber may influence the intestinal bacterial microflora. Since some of these bacteria, represented by around 400 different species (Salycrs et al., 1985) can metabolize compounds present in the colon to carcinogens, they may play an important role in the complex interplay of dietary fiber, carcinogens and the host organism. It is supposed that about a half of cellulose and hemicellulose are metabolically changed by these microflora during their passage through the colon. Colon bacteria ferment the polysaccharides of dietary fiber but metabolize steroids and bile acids as well. Some of their products appeared to be mutagenie in Ames' assay and thus might be associated with cancer of the colon. It is supposed that these compounds are produced mainly by the species Bacteroides (see Salyers et al., 1985). The fiber content of food reduces the activities of most digestive enzymes (amylase, lipase, trypsin, chymotrypsin) (see Schneeman and Gallaher, 1985). In this respect cellulose and xylan appeared to be the most effective. These reductions could be due to nonspecific binding of enzymes to fiber constituents, or, perhaps, to the possible presence of enzyme inhibitors in non-purified fiber. In contrast, the presence of pectin raised the activities of the above enzymes (Schneeman and Gallahcr, 1985). Therefore the activities of enzymes metabolizing compounds involved in carcinogenesis, e.g. bile acids or lipids in the gut could be changed by dietary fiber, too. It appears that they dietary fiber in relation to its involvement in the carcinogenesis process could be approximately divided into two groups. The first, which was found to inhibit tumorigenesis is characterized by low fermentability (cellulose, wheat bran, lignin). The second, which, under certain conditions may even enhance tumorigenesis shows pronounced fermentability (pectin, agar, oat bran, corn bran, carrageenan, guar, metamucil). The presence of fiber belonging to this second group was found to increase fecal bile acid concentration, too (Jacobs, 1986). In the process of fermentation of dietary fiber the content of the intestines is acidified and short-chain fatty acids are produced, which might stimulate cell growth in the colon (see Jacobs, 1986). On the other hand, Walker et al. (1986) found low fecal pH (6.12 to 6.15 in black inhabitants of South Africa) associated with very low incidence of colon cancer, while higher pH (6.88 in white population) was connected with higher incidence of this type of cancer. Jacobs and Lupton (1986) found that fiber added to the diet of rats acidified their large bowel content but did not inhibit the colon carcinogenesis evoked by the carcinogen 1,2-dimethylhydrazine. These controversial results stress even more the complex nature of the fiber-carcinogen interaction in the gastrointestinal tract of mammals. Moreover, while increased fiber consumption seems to lower the incidence of colon cancer rates, on the other hand it appears at the same time to raise the incidence of stomach cancer (see Jacobs, 1987). It could be concluded that there are

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justified indications that the increased fiber content of food, above all that of the non-fermentable, chemically-resistant cellulose could prevent the appearance of cancer, mainly in the colon.

(Ames, 1982, 1986 a,b; Miller, 1985; Newraark, 1987; Wattenberg, 1980, 1982; Willett and MacMahon, 1984 a,b). REFERENCES

A WORD OF WARNING Besides all the beneficial properties, some vegetables or plant constituents may represent a threat to human health. They could be of natural origin, e.g. mutagenic activities were found in some edible vegetables, in onions, grapes, raisins, peaches, raspberries, strawberries (Stoltz et al., 1984), or in plants not used for human consumption such as bracken fern (Pamukcu et al., 1980). The overconsumption of natural spices should be also avoided. Some natural compounds, eg. pyrrolizidine alkaloids or ptaquiloside from bracken fern (Hirono, 1986), and, perhaps, some flavonoides may be harmful or carcinogenic to humans (Stavric et al., 1984). Spoiled or mouldy vegetables may contain highly toxic mycotoxins, which might be carcinogenic as well. Fruits and vegetables may contain harmful compounds introduced into them by humans. The unrestricted and often improper use of pesticides or herbicides may result in the presence of their residues in the vegetables. Overfertilization with nitrates may increase their presence in agricultural products. The nitrates reduced in the saliva or stomach of mammals to nitrites could react with primary amines which are present in many food items, e.g. in cheese. Some of the products of their reaction, nitrosamines, are highly carcinogenic to humans. The fact that the actual carcinogens are synthesized from their noncarcinogenic precursors within the body of mammals only increases the danger constituted by these compounds. CONCLUSIONS Results of epidemiological research suggest that a human diet rich in vegetables and fruits may lower the incidence of cancers, predominantly in the gastrointestinal tract. This effect could be ascribed to (I) lowered intake of energy (joules) in food containing vegetables (see Hocman, 1988a), (2) the presence of vitamins, above all vitamin A and carotene and vitamins C and E in green and yellow vegetables; (3) the increased fiber content of vegetable-containing diet, (4) the presence of specific compounds in the vegetables (indoles, plant phenols, etc.) having chemopreventive properties against the action of some carcinogens, (5) the increased consumption of vegetables is usually associated with decreased use of meat and fats, which in itself may contribute to lowering the incidence of cancers. However, the fruits and vegetables consumed should be devoid of excess nitrogen fertilizers and pesticide residues which are by no means healthy. One should avoid the consumption of some plants containing natural carcinogens as well as excess spices and that of spoiled vegetables contaminated with mycotoxins. Both the carcinogenic and anticarcinogenic properties of vegetables and plants have in the past years been critically evaluated in a few excellent reviews

Aeschbacher H. U. (1982) The significance of mutagcns in food. In Mutagens in Our Environment, pp. 349-362. Alan R. Liss, New York. Ames B. N. (1982) Carcinogens and anti-carcinogens. In Mutagens in Our Environment, pp. 3-19. Alan R. Liss, New York. Ames B. N. (1986a) Food constituents as a source of mutagens, carcinogens and anticarcinogens. In Genetic Toxicology of the Diet. pp. 3-32. Alan R. Liss, New York. Ames B. N. (1986b) Dietary carcinogens and anticarcinogens. In Diet and Prevention of Coronary Heart Disease and Cancer (Edited by HaUgren B. et al.), pp. 25-45. Raven Press, New York. Anderson J. W., Story L., Seiling B., Chen W. J. L., Petro M. S. and Story J. (1984) Hypocholestrolemic effects of oat-bran or bean intake for hypercholesterolemic men, Am. J. clin. Nutr. 40, 1146-1155. Beljanski M. and Boljanski M. S. (1984) Three alkaloids as selective destroyers of the proliferative capacity of cancer cells. IRCS Med. Sci. 12, 587-588. Beynen A. C., Versluis A., Katan M. B. and Van Zutphen L. F. M. (1987) Interaction of dietary cholesterol with the type of fat and fiber with regard to plasma cholesterol response in rabbits. Nutr. Rep. Int. 35, 355--360. Birt D. F., Walker B., Tibbcls M. G. and Bresnick E. (1986) Anti-mutagenesis and anti-promotion by apigenin, robinetin and indole-3-carbinol. Carcinogeneais 7, 959-963. Birt D. F., PeUig J. C., Pour P. M., Tibbels M. G., Schweickert L. and Bresnick E. (1987) Enhanced pancreatic and skin tumorigenesis in cabbage-fed hamsters and mice. Carcinogenesis 8, 913-917. Byers T. E., Graham S., Hanghey B. P., Marshall J. R. and Swanson M. K. (1987) Diet and lung cancer risk: findings from the western New York diet study, Am. J. Epidemiol. 125, 351-363. Chan R. I. M., San R. H. C. and Stich H. F. (1986) Mechanism of inhibition of N-methyl-N'-nitro-N-nitrosoguanidine-induced mutagenesis by phenolic compounds. Cancer Lett. 31, 27-34. Cordle F. (1986) The use of epidemiology, scientific data and regulatory authority to determine risk factors in cancers of some organs of the digestive system--5. Stomach cancer. Regul. Toxicol. Pharmac. 6, 171-180. Cresteil T. and Lesca P. (1983) Enhancement of DNAbinding, mutagenicity and carcinogenicity of polycyclic aromatic hydrocarbons after induction of cytochrome P-450 by ellipticines in rats and mice. Chem-biol. Interact. 47, 145-156. Das M., Mukhtar H., Bik D. P. and Bickers D. R. (1987a) Inhibition of epidermal xenobiotic metabolism in SENCAR mice by naturally occurring plant phenols. Cancer Res. 47, 760-766. Das M., Khan W. A., Asokan P., Bickers D. R. and Mukhtar H. (1987B) Inhibition of polycyclic aromatic hydrocarbon-DNA adduct formation in epidermis and lungs of SENCAR mice by naturally-occurring plant phenols. Cancer Res. 47, 767-773. Dugne B., Paoletti O. and Meunier B. (1984) Covalent binding of the antitumor agent N2-methyl-9-hydroxyellipticinum acetate (NSC-264137) on RNA and polyA in vitro. Biochem. biophys. Res. Commun. 124, 416-422. Elegbcde J. A., Elson Ch.E., Tanner M. A., Qureshi A. and Gould M. N. (1986) Regression of rat primary mammary tumors following dietary d-limonene, J. natn. Cancer Inst. 76, 323-325. Fujihara M., Iizima N., Yamamoto I. and Nagumo T. (1984) Purification and chemical and physical character-

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