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Mechanisms of Action of Antioxidants as Exemplified in Vegetables, Tomatoes and Tea1
Food and Chemical Toxicology 37 (1999) 943±948
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Mechanisms of Action of Antioxidants as Exempli®ed in Vegetables, Tomatoes and Tea* J. H. WEISBURGER American Health Foundation, Valhalla, NY 10585, USA SummaryÐMost chronic diseases, including coronary heart disease and many types of cancer depend on the in vivo conversion of cellular macromolecules or of carcinogens to speci®c reactive, oxidized forms. For that reason, health promoting nutrition involves the daily intake of ®ve to 10 vegetables and fruits, fruit juices, red wine and tea that are rich sources of micronutrients with antioxidant properties, including the antioxidant vitamins C, E and b-carotene. Tomatoes contain lycopene, a stable, active antioxidant. Many vegetables contain quercetin and related polyphenolic compounds. Tea is a source of epigallocatechin gallate, in green tea, and thea¯avin and the associated thearubigins, in black tea. Red wine contains resveratrol. The diverse antioxidants in foods, red wine and tea provide the necessary antioxidant resources for the body to control oxidation reactions in the body with possible adverse consequences. For example, the oxidation of low density lipoprotein (LDL) cholesterol yields a product that damages the vascular system. Thus, a lower intake of saturated fats to decrease the levels of LDL cholesterol, together with an adequate intake of antioxidants, is the optimal approach to lower heart disease risk. Cancer of the stomach involves the consumption of salted, pickled foods yielding direct-acting carcinogens, and their formation is inhibited by vitamins C and E. Cancer in the colon, breast, prostate and pancreas may be caused by a new class of carcinogens, the heterocyclic amines, formed during the broiling or frying of creatinine-containing foods, including ®sh and meats. Their formation and action can be inhibited by antioxidants such as those in soy, tea, vitamin C and also by the synthetic antioxidants BHA or BHT. The growth, cell proliferation and development of abnormal preneoplastic and neoplastic cells also involves oxidation reactions, including the formation of active oxygen or peroxy compounds. Such reactions can be inhibited by antioxidants, such as those in tea, tomatoes or vegetables. Even ageing and longevity in good health would be favoured by the availability of adequate amounts of varied antioxidants. Prevention of the formation and of action of reactive products by antioxidants as present in fruits, vegetables, tomatoes, red wine and tea is of great public health importance in decreasing the risk of major diseases. Prevention is the optimal approach to disease control, and also as an eective route to lower costs of medical care. # 1999 Elsevier Science Ltd. All rights reserved Keywords: tea; tomato; lycopene; polyphenol; antioxidant; cancer and heart disease prevention. Abbreviations: HAAs = heterocyclic arylamines; LDL = low density lipoprotein.
Introduction Medical research has uncovered the causes and modi®ers of the major chronic diseases aecting humans worldwide. In particular, in the Western world, cardiovascular diseases with emphasis on coronary heart disease is a major problem with high mortality (Byers et al., 1998; Diaz et al., 1997; Lifshitz, 1997; Pryor, 1993). In addition, there are many types of cancer and fortunately the key aetio*Presented at the Antioxidant and Health Symposium of the CNRA/CNERNA in Bordeaux, 18±20 March 1998; Chairman of Committee, Professor G. Pascal, Secretary, Dr M. Suschetet.
logical factors have been identi®ed (Adamson et al., 1995; Hill et al., 1994; Micozzi and Moon, 1992; Weisburger and Williams, 1995; Wynder and Homann, 1994). In virtually all of these diseases, the underlying mechanisms depend on oxidative processes leading to products that display reactive properties, aecting speci®c molecular targets in the vascular system, and cellular DNA as regards many kinds of cancer. Speci®cally, high titres of LDLcholesterol represent one risk factor for heart disease. This macromolecule undergoes oxidation to the corresponding oxy derivative, in the absence of protective antioxidants (Cader et al., 1997; Ishikawa et al., 1997; Knekt et al., 1996). However, not all
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investigators reported such an oxidative process (Katan, 1997; van het Hof et al., 1997). The majority of human cancers are caused by DNA-reactive genotoxic carcinogens, modifying the DNA of normal cells. Speci®c DNA adducts of many kinds of carcinogens have been identi®ed (Adamson et al., 1995; Micozzi and Moon, 1992; Weisburger and Williams, 1995; Wynder and Homann, 1993). However, since the discovery by Kasai and Nishimura (1984) that DNA also can undergo oxidative modi®cation, particularly the formation of 8oxydG, it has been found that most chemical carcinogens not only form the conventional DNA adducts but also generate 8-oxydG (Fiala et al., 1996; Poulsen et al., 1998; Xu et al., 1992). In addition, promotion is involved in the growth and development of many forms of cancer. In turn, these processes involve active oxygen and peroxy compounds (Fujiki et al., 1996; Klaunig et al., 1995). Epidemiological research has revealed a lower incidence of many chronic diseases in areas and populations that are regular consumers of vegetables, fruits, red wine and tea (Block et al., 1992; Frankel et al., 1993; Goldberg, 1995; Renaud and de Lorgeril, 1992; Steinmetz and Potter, 1991). With vegetables and fruits, the obvious conclusion drawn would be that these foods are sources of vitamins with antioxidant properties, such as vitamins A and C. However, detailed analysis of the protective foods and drinks show that they all contain speci®c antioxidants. A major component in vegetables is quercetin, a polyphenol. Tomatoes are rich in lycopene, an interesting antioxidant because it is fairly stable to storage and cooking and thus is present in the cooked tomatoes that are consumed frequently in the Mediterranean region, and account in part for the lower heart disease and cancer risk in that area (Weisburger, 1998b). Research for the last 20 years has indicated that one-third of the weight of dry green or black tea is represented by polyphenols that display exquisite antioxidant protection (Clydesdale, 1996; Fiala et al., 1996; Fujiki et al., 1996; Lu et al., 1997; Weisburger, 1997; Xu et al., 1992; Yang et al., 1996). In addition, some foods such as garlic and onions contain antioxidants that provide additional nutritional elements in areas where such foods are consumed frequently, such as Eastern Europe, the Mediterranean region, and in parts of the Western world. Red wine contains a triphenolic compound, resveratrol, as a conjugate that releases the active compound in the intestinal tract. Green tea is rich in epigallocatechin gallate with powerful antioxidant potential. During the oxidation, sometimes improperly called fermentation, of green tea, polyphenols are produced to others characteristic of black tea, thea¯avin and thearubigins, by the action of the enzyme polyphenol oxidase present in the tea leaf. Detailed research shows that the polyphenols
in green and in black tea have quite similar bene®cial eects in inhibiting oxidation reactions in vivo (Apostolides et al., 1997; Clydesdale, 1996; Hara, 1994; Hernaez and Dashwood, 1997; Tanaka et al., 1997; Weisburger, 1997, 1998a). The tea polyphenols also induce phase I and II enzymes (BuAbbas et al., 1996; Weisburger, 1998a).
Causes of heart disease Before detailed medical research provided rational answers, the public perception was that stress was a the leading cause of heart disease. It was discovered, however, that heart disease occurred to a much greater extent in the Western world than in parts of Asia or Africa. Yet the stresses aecting people could be construed to be quite similar. Key leads to the aetiological factors were obtained by the observation stemming from geographic pathology that pinpointed dietary and lifestyle dierences are associated with disease risk. Thus, it was found that populations at high risk were consumers of meats and of some dairy products, sources of elevated levels of saturated fats. Milk also contains lactose with atherogenic potential. In turn, these nutritional customs gave rise to elevated serum cholesterol levels and in particular, higher levels of LDL cholesterol, that can undergo oxidation to reactive products, that attack the vascular mucosa as well as the cellular components in the heart. The modi®ed vascular components, in turn, not only contain the bulky cholesterol residue, but also attract other elements such as macrophages in the circulating blood that is the eventual causative element in the formation of atheromas. Individuals who also smoke have an exaggerated tendency for these reactions with adverse potential. In addition, the blood of smokers has a reduced level of free haemoglobin with oxygen carrying capacity, since the carbon monoxide in tobacco smoke leads to the formation of carboxyhaemoglobin, a stable form that cannot carry oxygen. Antioxidants from foods can largely prevent the occurrence of the sequence of steps leading to atheroma formation, and therefore decrease the risk of heart disease. This appears to be true for most of the antioxidants oered by nutrition, whether they would be of a polyphenol nature such as in vegetables or tea, or of slightly dierent chemical structure such as in red wine, or lycopene from tomatoes, or the structurally closely related b-carotene, or vitamins C and E.
Causes of cancer About 35% of cancers in the Western world, and probably a higher percentage in Africa and a lower percentage in Asia, are associated with tobacco use
Antioxidants and chronic disease prevention
and in particular, cigarette smoking. In the Indian subcontinent, tobacco chewing or snu dipping are causes of cancer in the oral cavity and the oesophagus, again in populations with a lower intake of vegetables and fruits. One good reason why some tobacco users seems to have a lower risk of the tobacco-related cancers in various parts of the world is that they do consume protective foods or drinks. For example, in the Mediterranean region, and speci®cally in southern Italy and Greece, smokers have a lower incidence of the tobacco-related disease, because dietary traditions call for a regular intake of vegetables, including cooked tomato products and fruits. In parts of Japan that were so evaluated, it has been shown that the lower incidence of lung cancer can be attributed in part to the fact that the Japanese populations are fond of drinking tea, in this instance green tea, with demonstrated inhibiting action in lung cancer. Much is known about the mechanism whereby vegetables, tomatoes and tea have an inhibiting eect in cancer causation and development. One aspect deals with the inhibiting eect of the active ingredients in these foods on the formation of cancer causing agents. The major constituent of protective foods are antioxidants of varied chemical structures. In vegetables, the main antioxidants have polyphenolic structure, such as quercetin. Tea has very speci®c polyphenols such as epigallocatechin gallate (EGCG), typical of green tea, and thea¯avin and thearubigens, typical of black tea. Despite slight dierences in chemical structure, all these compounds are excellent antioxidants (see Clydesdale, 1997; Weisburger, 1997, 1998a). The antioxidant in raw and cooked tomatoes is not a polyphenol, but a hydrocarbon-like chemical, lycopene. This chemical is bound to the skin and ®bre in fresh tomatoes and is not available as much from uncooked tomatoes than from cooked tomatoes. Also, being a hydrocarbon-like structure, it is more fat soluble than water soluble. For that reason, the Mediterranean way of consuming cooked tomatoes is together with some olive oil, which favours maximal absorption of this antioxidant (see Weisburger, 1998b). An important class of carcinogens for the Western type of cancers, namely those in the colon, breast, prostate, pancreas and certain of the endocrine associated cancers are caused by a novel class of chemical carcinogens, heterocyclic aromatic amines (HAAs) (Felton and Gentile, 1997). These chemicals are in the human food environment of all non-vegetarians and meat eaters, since they are formed during the cooking of meats. These chemicals were discovered originally since they displayed powerful mutagenic activity which was present at the surface of browned meat. Epidemiological data now show that individuals regularly consuming browned meat have a higher risk of cancer of the breast and colon. Animal data indicate that the
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pancreas and the prostate may be additional target organs. The mode of action of HCAs indicate that they require metabolic activation through the cytochrome P450 system, and speci®cally the family of the 1A2 enzymes. Research now shows that the level of these enzymes, and also of glucuronyl transferase, are changed appreciably through the intake of vegetables, tomatoes and black or green tea, in the sense of higher detoxi®cation of HCAs. Therefore, the conclusion can be drawn that the regular intake of protective foods increases the detoxi®cation of these carcinogens, and thus provides a lower risk situation in meat eaters. During the development and growth of many types of cancer there are distinct oxidation reactions that have been documented to play a key role during that phase of carcinogenesis. Indeed, these speci®c reactions involve the generation of active oxygen such as OH radicals and hydrogen peroxide that in turn control the cell duplication rates, and may aect apoptotic reactions that would lead to elimination of abnormal cells. Antioxidants, such as those present in fruits and vegetables as well as tea, antagonize powerfully the developmental aspects of carcinogenesis. In fact, new information indicates that this reaction may actually decrease the already existing benign and malignant tumour cells (Hirose et al., 1997; Katiyar et al., 1997; Kha®f et al., 1998; Liao et al., 1995; Lin et al., 1996; Lu et al., 1997; Nakachi et al., 1998; Yang et al., 1996; Zhao et al., 1997). In addition, the metastic spread of cancer cells may also be aected by antioxidants, although better documentation in this critical ®eld bearing on reduced cancer mortality is needed. Perhaps an eect on cell dierentiation and apoptosis, and not regulation of cell duplication need mechanistic consideration (Chang et al., 1998).
Discussion The detailed research has provided background information of the mechanisms whereby vegetables, tomatoes, fruits and tea are protective in heart disease and many types of cancer. The basic important element is the fact that these foods and drink are a source of potent antioxidants with speci®c chemical structures. The public often believes that one reason the intake of such foods are bene®cial is that they provide essential vitamins and minerals. This is clearly true. A number of vitamins such as C, A, E as well as carotene are excellent antioxidants, but they also contribute to good health through other mechanisms such as being co-factors for certain enzymes, involved in oxidation-reduction reactions in the body. However, as we demonstrate in this paper, these foods contain speci®c antioxidants, polyphenolic chemical compounds for most vegetables and for tea, and the chemical lycopene in tomatoes, that exists in a number of stereochemical
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Fig. 1. Fluid intake pyramid, suggesting desirable types and amounts of ¯uids consumed by adults per day.
versions, with dierential antioxidant potential in vivo. In the USA, research in nutrition and health has led to the conclusion that people should consume ®ve to nine vegetables and fruits a day, and this concept is part of the Federal Government recommendation for health maintenance. In addition, research for the last 20 years has extended knowledge not only on what people should eat, but what they might consume as a beverage. Thus tea, an excellent source of speci®c antioxidants, is part of a healthy nutritional scheme. In fact, we suggest that adults should consume 2±3 litres of ¯uids a day, as part of their cooked foods, and also as beverages. Water clearly is an important portion of the daily ¯uid intake, but research suggests that ®ve to seven cups of tea a day might well be part of the daily ¯uid consumption (Fig. 1). A major goal of contemporary medical research focuses on prevention of major chronic diseases not only as an ethical way of keeping people healthy, but this recommendation may also bear on a re-
duction of the current high cost of medical care (Greenwald and Sondik, 1986). Healthy people require only low-cost health testing at regular intervals, but they avoid high-cost disease care and hospital use. Thus, anywhere in the world, dietary traditions should involve daily intake of fruits, vegetables and tea, with adequate soluble and insoluble ®bres from wholegrain cereals and bran bread, and preferably olive oil at no more than 22±25% of calories. Regular exercise is part of a healthy lifestyle, as is maintenance of proper body weight. Healthpromoting traditions should start in childhood. A Greek adage says ``it is the function of medicine to help people to die young as late as possible'' (Wynder, 1972). AcknowledgementsÐResearch in my laboratory was funded by US National Cancer Institute Contract CN35569, and grants from the American Cancer Society CN157, the Tea Trade Health Research Association, and gifts from Texaco Foundation, and the Friends Against Cancer Team. The expert administrative assistance of Beth-Alayne McKinney is much appreciated.
Antioxidants and chronic disease prevention REFERENCES
Adamson R. H., Gustafsson J. A., Ito N., Nagao M., Sugimura T., Wakabayshi K. and Yamazoe Yeds. 1995. Heterocyclic amines in cooked foods. Proceedings of the 23rd International Symposium of the Princess Takamatsu Cancer Research Fund. pp. 306. Princeton Scienti®c Publishing, Princeton, NJ. Apostolides Z., Balentine D. A., Harbowy M. E., Hara Y. and Weisburger J. H. (1997) Inhibition of PhIP mutagenicity by catechins, and by thea¯avins and gallate esters. Mutation Research 389, 167±172. Block G., Patterson B. and Subar A. (1992) Fruit, vegetables and cancer prevention: a review of the epidemiological evidence. Nutrition and Cancer 18, 1±29. Bu-Abbas A., Cliord M. N., Ioannides C. and Walker R. (1995) Stimulation of rat hepatic UDP-glucuronosyl transferase activity following treatment with green tea. Food and Chemical Toxicology 33, 27±30. Byers T., Anda R., McQueen D., Williamson D., Mokdad A., Casper M., Ford E. and Marks J. (1998) The correspondence between coronary heart disease mortality and risk factor prevalence states in the United States, 1991±1992. Preventive Medicine 27, 311±316. Cader A. A., Steinberg F. M., Mazzone T. and Chait A. (1997) Mechanisms of enhanced macrophage apoE secretion by oxidized LDL. Journal of Lipid Research 38, 981±991. Chang W-C.L., Chapkin R. S. and Lupton J. R. (1998) Fish oil blocks azoxymethane-induced rat colon tumorigenesis by increasing cell dierentiation and apoptosis rather than decreasing cell proliferation. Journal of Nutrition 128, 491±497. Clydesdale F. M. (1997) Tea and health. Critical Reviews in Food Science and Nutrition 37, 691±785. Diaz M. N., Frei B., Vita J. A. and Keaney J. F. (1997) Antioxidants and atherosclerotic heart disease. New England Journal of Medicine 337, 408±416. Felton J. S., Gentile J. M. (Eds) (1997) Mutagenic/ carcinogenic N-substituted aryl compounds. Mutation Research 376, 1±276. Fiala E. S., Sodum R. S., Bhattacharya M. and Li H. (1996) (-)-Epigallocaatechin gallate, a polyphenolic tea antioxidant, inhibits perooxynitrite mediated formation of 8-oxodeoxyguanosine and 3-nitrotyrosine. Experientia 52, 922±926. Frankel E. N., Kanner J., German J. B., Parks E. and Kinsella J. E. (1993) Inhibition of oxidation of human low-density lipoprotein by phenolic substances in red wine. Lancet 341, 454±457. Fujiki H., Suganuma M., Okabe S., Komori, A, Sueoka E., Sueoka N., Kozu T. and Sakai Y. (1996) Japanese green tea as a cancer preventive in humans. Nutrition Reviews 54, 67±70. Goldberg D. (1995) Does wine work? Clinical Chemistry 41, 14±16. Greenwald P., Sondik E. J. (1986) Cancer control objectives for the nation: 1985±2000. National Cancer Institute Monograph 2, 3±74. Hara Y. (1997) Prophylactic functions of antioxidant tea polyphenols. In Food Factors for Cancer Prevention, ed. H. Ohigashi, T. Osawa, J. Terao, S. Watanabe and T. Yoshikawa, pp. 147±151. Springer Verlag, Tokyo. Hernaez J., Xu M. and Dashwood R. (1997) Eects of tea and chlorophyllin on the mutagenicity of N-hydroxyIQ: Studies of enzyme inhibition, molecular complex formation, and degradation/scavenging of the active metabolites. Environmental and Molecular Mutagenesis 30, 468±474. Hill M. J., Giacosa A. and Caygill, C. P. J. (1994) Epidemiology of Diet and Cancer, pp. 411. Ellis Horwood, Chichester.
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Hirose M., Mizoguchi Y., Yaono M., Tanaka H., Yamaguchi T. and Shirai T. (1997) Eects of green tea catechins on the progression or late promotion stage of mammary gland carcinogenesis in female SpragueDawley rats pretreated with 7,12-dimethylbenz(a)anthracene. Cancer Letters 112, 141±147. Ishikawa T., Suzukawa M., Ito T., Yoshida H., Ayaori M., Nishiwaki M., Yonemura A., Hara Y. and Nakamura H. (1997) Eect of tea ¯avonoid supplementation on the susceptibility of low-density lipoprotein to oxidative modi®cation. American Journal of Clinical Nutrition 66, 261±266. Kasai H. and Nishimura S. (1984) Hydroxylation of deoxyguanosine at C-8 position by ascorbic acid and other reducing agents. Nucleic Acids Research 12, 2137±2145. Katan M. B. (1997) Flavonoids and heart disease. American Journal of Clinical Nutrition 65, 1542±1543. Katiyar S. K., Mohan R. R., Agarwal R. and Mukhtar H. (1997) Protection against induction of mouse skin papillomas with low and high risk of conversion to malignancy by green tea polyphenols. Carcinogenesis 18, 497±502. Kha®f A., Schantz S. P., Chou T-C., Edelstein D. and Sacks P. G. (1998) Quantitation of chemopreventive synergism between (-)-epigallocatechin-3-gallate and curcumin in normal, premalignant and malignant human oral epithelial cells. Carcinogenesis 19, 419±424. Klaunig J. E., Xu Y., Bachowski S., Ketcham C. A., Isenberg J. S., Kolaja K. L., Baker T. K., Walborg E. F., Jr and Stevenson D. E. (1995) Oxidative stress in nongenotoxic carcinogenesis. Toxicology Letters 82, 683±691. Knekt P., Jarvinen R., Reunanen A. and Maatela J. (1996) Flavonoid intake and coronary mortality in Finland: a cohort study. British Medical Journal 312, 478±481. La Vecchia C. and Tavani A. (1998) Fruit and vegetables, and human cancer. European Journal of Cancer Prevention 7, 3±8. Leger A. S., Cochrane A. L. and Moore F. (1979) Factors associated with cardiac mortality in developed countries with particular reference to the consumption of wine. Lancet i, 1017±1020. Liao S., Umekita Y., Guo J., Kokontis J. M. and Hiipakka R. A. (1995) Growth inhibition and regression of human prostate and breast tumor in athymic mice by tea epigallocatechin gallate. Cancer Letters 96, 239±243. Lifshitz F. (1997) Dietary guidelines with particular emphasis on nutritional intake of fat and cholesterol. Journal of the American College of Nutrition 16, 507± 509. Lin Y-L., Juan I-M., Chen Y-L., Liang Y-C. and Lin JK. (1996) Composition of polyphenols in fresh tea leaves and associations of their oxygen-radical-absorbing capacity with antiproliferative actions in ®broblast cells. Journal of Agricultural and Food Chemistry 44, 1387±1394. Lu Y-P., Lou Y-R., Xie J-G., Yen P., Huang M.-T. and Conney A. H. (1997) Inhibitory eect of black tea on the growth of established skin tumors in mice: eects on tumor size, apoptosis, mitosis and bromodeoxyuridine incorporation into DNA. Carcinogenesis 18, 2163±2169. Micozzi M. S., Moon T. E. (1992) Macronutrients: Investigating their Role in Cancer, pp. 477. Marcel Dekker, New York. Nakachi K., Suemasu K., Suga K., Takeo T., Imai K. and Higashi Y. (1998) In¯uence of drinking green tea on breast cancer malignancy among Japanese patients. Japanese Journal of Cancer Research 89, 254±261. Poulsen H. E., Prieme H. and Loft S. (1998) Role of oxidative DNA damage in cancer initiation and promotion. European Journal of Cancer Prevention 7, 9±16.
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Pryor W. A. (1993) Measurement of oxidative stress status in humans. Cancer Epidemiology, Biomarkers and Prevention 2, 289±292. Renaud S. and De Lorgeril M. (1992) Wine, alcohol, platelets, and the French paradox for coronary heart disease. Lancet 339, 1523±1526. Sies H. (1997) Antioxidants in Disease Mechanisms and Therapy Advances in Pharmacology, Vol. 38, pp. 707. Academic Press, San Diego. Steinmetz K. A. and Potter J. D. (1991) Vegetables, fruit and cancer. II Mechanisms. Cancer Causes and Control 2, 427±442. Tanaka H., Hirose M., Kawabe M., Sano M., Takesada Y., Hagiwara A. and Shirai T. (1997) Post-initiation inhibitory eects of green tea catechins on 7,12-dimethylbenz[a]anthracene-induced mammary gland carcinogenesis in female Sprague-Dawley rats. Cancer Letters 116, 47±52. van het Hof K.H., de Boer H. S. M., Wiseman S. A., Lien N., Weststrate J. A. and Tijburg L. B. M. (1997) Consumption of green or black tea does not increase resistance of low-density lipoprotein to oxidation in humans. American Journal of Clinical Nutrition 66, 1125±1132. Weisburger J. H. (1997) Tea and health: A historical perspective. Cancer Letters 114, 315±317. Weisburger J. H. (1998) International symposium on lycopene and tomato products in disease prevention. Proceedings of the Society for Experimental Biology and Medicine 218, 93±143.
Weisburger J. H. (1998) Chemoprevention of cancer by tea. In Cancer and Nutrition, ed. K. N. Prasad and W. C. Cole, pp. 167±171. IOS Press, Amsterdam. Weisburger J. H. and Williams G. (1995) Causes of cancer. In American Cancer Society Textbook on Clinical Oncology, ed. G. Murphy, W. Lawrence and R. E. Lenhard, pp. 10±39. American Cancer Society, Atlanta, GA. Wynder E. L. (1972) Preventive Medicine Introduction. Preventive Medicine 1, 1±5. Wynder E. L. and Homann D. (1994) Smoking and lung cancer: Scienti®c challenges and opportunities. Cancer Research 54, 5284±5295. Xu M., Bailey A. C., Hernaez J. F., Taoka C. R., Schut H. A. J. and Dashwood R. H. (1996) Protection by green tea, black tea, and indole-3-carbinol against 2amino-3-methylimidazo[4,5-f]quinoline-induced DNA adducts and colonic aberrant crypts in the F344 rat. Carcinogenesis 17, 1429±1434. Xu Y., Ho C-T., Amin S. G., Han C. and Chung FL. (1992) Inhibition of tobacco-speci®c nitrosamine induced lung tumorigenesis in A/J mice by green tea and its major polyphenols as antioxidants. Cancer Research 52, 3875±3879. Yang C. S., Chen L., Lee M. J. and Landau J. M. (1996) Eects of tea on carcinogenesis in animal models and humans. Advances in Experimental Medicine and Biology 401, 51±61. Zhao Y., Cao J., Ma H. and Liu J. (1997) Apoptosis induced by tea polyphenols in HL-60 cells. Cancer Letters 121, 163±167.
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Mechanisms of Action of Antioxidants as Exempli®ed in Vegetables, Tomatoes and Tea* J. H. WEISBURGER American Health Foundation, Valhalla, NY 10585, USA SummaryÐMost chronic diseases, including coronary heart disease and many types of cancer depend on the in vivo conversion of cellular macromolecules or of carcinogens to speci®c reactive, oxidized forms. For that reason, health promoting nutrition involves the daily intake of ®ve to 10 vegetables and fruits, fruit juices, red wine and tea that are rich sources of micronutrients with antioxidant properties, including the antioxidant vitamins C, E and b-carotene. Tomatoes contain lycopene, a stable, active antioxidant. Many vegetables contain quercetin and related polyphenolic compounds. Tea is a source of epigallocatechin gallate, in green tea, and thea¯avin and the associated thearubigins, in black tea. Red wine contains resveratrol. The diverse antioxidants in foods, red wine and tea provide the necessary antioxidant resources for the body to control oxidation reactions in the body with possible adverse consequences. For example, the oxidation of low density lipoprotein (LDL) cholesterol yields a product that damages the vascular system. Thus, a lower intake of saturated fats to decrease the levels of LDL cholesterol, together with an adequate intake of antioxidants, is the optimal approach to lower heart disease risk. Cancer of the stomach involves the consumption of salted, pickled foods yielding direct-acting carcinogens, and their formation is inhibited by vitamins C and E. Cancer in the colon, breast, prostate and pancreas may be caused by a new class of carcinogens, the heterocyclic amines, formed during the broiling or frying of creatinine-containing foods, including ®sh and meats. Their formation and action can be inhibited by antioxidants such as those in soy, tea, vitamin C and also by the synthetic antioxidants BHA or BHT. The growth, cell proliferation and development of abnormal preneoplastic and neoplastic cells also involves oxidation reactions, including the formation of active oxygen or peroxy compounds. Such reactions can be inhibited by antioxidants, such as those in tea, tomatoes or vegetables. Even ageing and longevity in good health would be favoured by the availability of adequate amounts of varied antioxidants. Prevention of the formation and of action of reactive products by antioxidants as present in fruits, vegetables, tomatoes, red wine and tea is of great public health importance in decreasing the risk of major diseases. Prevention is the optimal approach to disease control, and also as an eective route to lower costs of medical care. # 1999 Elsevier Science Ltd. All rights reserved Keywords: tea; tomato; lycopene; polyphenol; antioxidant; cancer and heart disease prevention. Abbreviations: HAAs = heterocyclic arylamines; LDL = low density lipoprotein.
Introduction Medical research has uncovered the causes and modi®ers of the major chronic diseases aecting humans worldwide. In particular, in the Western world, cardiovascular diseases with emphasis on coronary heart disease is a major problem with high mortality (Byers et al., 1998; Diaz et al., 1997; Lifshitz, 1997; Pryor, 1993). In addition, there are many types of cancer and fortunately the key aetio*Presented at the Antioxidant and Health Symposium of the CNRA/CNERNA in Bordeaux, 18±20 March 1998; Chairman of Committee, Professor G. Pascal, Secretary, Dr M. Suschetet.
logical factors have been identi®ed (Adamson et al., 1995; Hill et al., 1994; Micozzi and Moon, 1992; Weisburger and Williams, 1995; Wynder and Homann, 1994). In virtually all of these diseases, the underlying mechanisms depend on oxidative processes leading to products that display reactive properties, aecting speci®c molecular targets in the vascular system, and cellular DNA as regards many kinds of cancer. Speci®cally, high titres of LDLcholesterol represent one risk factor for heart disease. This macromolecule undergoes oxidation to the corresponding oxy derivative, in the absence of protective antioxidants (Cader et al., 1997; Ishikawa et al., 1997; Knekt et al., 1996). However, not all
0278-6915/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII S0278-6915(99)00086-1
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J. H. Weisburger
investigators reported such an oxidative process (Katan, 1997; van het Hof et al., 1997). The majority of human cancers are caused by DNA-reactive genotoxic carcinogens, modifying the DNA of normal cells. Speci®c DNA adducts of many kinds of carcinogens have been identi®ed (Adamson et al., 1995; Micozzi and Moon, 1992; Weisburger and Williams, 1995; Wynder and Homann, 1993). However, since the discovery by Kasai and Nishimura (1984) that DNA also can undergo oxidative modi®cation, particularly the formation of 8oxydG, it has been found that most chemical carcinogens not only form the conventional DNA adducts but also generate 8-oxydG (Fiala et al., 1996; Poulsen et al., 1998; Xu et al., 1992). In addition, promotion is involved in the growth and development of many forms of cancer. In turn, these processes involve active oxygen and peroxy compounds (Fujiki et al., 1996; Klaunig et al., 1995). Epidemiological research has revealed a lower incidence of many chronic diseases in areas and populations that are regular consumers of vegetables, fruits, red wine and tea (Block et al., 1992; Frankel et al., 1993; Goldberg, 1995; Renaud and de Lorgeril, 1992; Steinmetz and Potter, 1991). With vegetables and fruits, the obvious conclusion drawn would be that these foods are sources of vitamins with antioxidant properties, such as vitamins A and C. However, detailed analysis of the protective foods and drinks show that they all contain speci®c antioxidants. A major component in vegetables is quercetin, a polyphenol. Tomatoes are rich in lycopene, an interesting antioxidant because it is fairly stable to storage and cooking and thus is present in the cooked tomatoes that are consumed frequently in the Mediterranean region, and account in part for the lower heart disease and cancer risk in that area (Weisburger, 1998b). Research for the last 20 years has indicated that one-third of the weight of dry green or black tea is represented by polyphenols that display exquisite antioxidant protection (Clydesdale, 1996; Fiala et al., 1996; Fujiki et al., 1996; Lu et al., 1997; Weisburger, 1997; Xu et al., 1992; Yang et al., 1996). In addition, some foods such as garlic and onions contain antioxidants that provide additional nutritional elements in areas where such foods are consumed frequently, such as Eastern Europe, the Mediterranean region, and in parts of the Western world. Red wine contains a triphenolic compound, resveratrol, as a conjugate that releases the active compound in the intestinal tract. Green tea is rich in epigallocatechin gallate with powerful antioxidant potential. During the oxidation, sometimes improperly called fermentation, of green tea, polyphenols are produced to others characteristic of black tea, thea¯avin and thearubigins, by the action of the enzyme polyphenol oxidase present in the tea leaf. Detailed research shows that the polyphenols
in green and in black tea have quite similar bene®cial eects in inhibiting oxidation reactions in vivo (Apostolides et al., 1997; Clydesdale, 1996; Hara, 1994; Hernaez and Dashwood, 1997; Tanaka et al., 1997; Weisburger, 1997, 1998a). The tea polyphenols also induce phase I and II enzymes (BuAbbas et al., 1996; Weisburger, 1998a).
Causes of heart disease Before detailed medical research provided rational answers, the public perception was that stress was a the leading cause of heart disease. It was discovered, however, that heart disease occurred to a much greater extent in the Western world than in parts of Asia or Africa. Yet the stresses aecting people could be construed to be quite similar. Key leads to the aetiological factors were obtained by the observation stemming from geographic pathology that pinpointed dietary and lifestyle dierences are associated with disease risk. Thus, it was found that populations at high risk were consumers of meats and of some dairy products, sources of elevated levels of saturated fats. Milk also contains lactose with atherogenic potential. In turn, these nutritional customs gave rise to elevated serum cholesterol levels and in particular, higher levels of LDL cholesterol, that can undergo oxidation to reactive products, that attack the vascular mucosa as well as the cellular components in the heart. The modi®ed vascular components, in turn, not only contain the bulky cholesterol residue, but also attract other elements such as macrophages in the circulating blood that is the eventual causative element in the formation of atheromas. Individuals who also smoke have an exaggerated tendency for these reactions with adverse potential. In addition, the blood of smokers has a reduced level of free haemoglobin with oxygen carrying capacity, since the carbon monoxide in tobacco smoke leads to the formation of carboxyhaemoglobin, a stable form that cannot carry oxygen. Antioxidants from foods can largely prevent the occurrence of the sequence of steps leading to atheroma formation, and therefore decrease the risk of heart disease. This appears to be true for most of the antioxidants oered by nutrition, whether they would be of a polyphenol nature such as in vegetables or tea, or of slightly dierent chemical structure such as in red wine, or lycopene from tomatoes, or the structurally closely related b-carotene, or vitamins C and E.
Causes of cancer About 35% of cancers in the Western world, and probably a higher percentage in Africa and a lower percentage in Asia, are associated with tobacco use
Antioxidants and chronic disease prevention
and in particular, cigarette smoking. In the Indian subcontinent, tobacco chewing or snu dipping are causes of cancer in the oral cavity and the oesophagus, again in populations with a lower intake of vegetables and fruits. One good reason why some tobacco users seems to have a lower risk of the tobacco-related cancers in various parts of the world is that they do consume protective foods or drinks. For example, in the Mediterranean region, and speci®cally in southern Italy and Greece, smokers have a lower incidence of the tobacco-related disease, because dietary traditions call for a regular intake of vegetables, including cooked tomato products and fruits. In parts of Japan that were so evaluated, it has been shown that the lower incidence of lung cancer can be attributed in part to the fact that the Japanese populations are fond of drinking tea, in this instance green tea, with demonstrated inhibiting action in lung cancer. Much is known about the mechanism whereby vegetables, tomatoes and tea have an inhibiting eect in cancer causation and development. One aspect deals with the inhibiting eect of the active ingredients in these foods on the formation of cancer causing agents. The major constituent of protective foods are antioxidants of varied chemical structures. In vegetables, the main antioxidants have polyphenolic structure, such as quercetin. Tea has very speci®c polyphenols such as epigallocatechin gallate (EGCG), typical of green tea, and thea¯avin and thearubigens, typical of black tea. Despite slight dierences in chemical structure, all these compounds are excellent antioxidants (see Clydesdale, 1997; Weisburger, 1997, 1998a). The antioxidant in raw and cooked tomatoes is not a polyphenol, but a hydrocarbon-like chemical, lycopene. This chemical is bound to the skin and ®bre in fresh tomatoes and is not available as much from uncooked tomatoes than from cooked tomatoes. Also, being a hydrocarbon-like structure, it is more fat soluble than water soluble. For that reason, the Mediterranean way of consuming cooked tomatoes is together with some olive oil, which favours maximal absorption of this antioxidant (see Weisburger, 1998b). An important class of carcinogens for the Western type of cancers, namely those in the colon, breast, prostate, pancreas and certain of the endocrine associated cancers are caused by a novel class of chemical carcinogens, heterocyclic aromatic amines (HAAs) (Felton and Gentile, 1997). These chemicals are in the human food environment of all non-vegetarians and meat eaters, since they are formed during the cooking of meats. These chemicals were discovered originally since they displayed powerful mutagenic activity which was present at the surface of browned meat. Epidemiological data now show that individuals regularly consuming browned meat have a higher risk of cancer of the breast and colon. Animal data indicate that the
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pancreas and the prostate may be additional target organs. The mode of action of HCAs indicate that they require metabolic activation through the cytochrome P450 system, and speci®cally the family of the 1A2 enzymes. Research now shows that the level of these enzymes, and also of glucuronyl transferase, are changed appreciably through the intake of vegetables, tomatoes and black or green tea, in the sense of higher detoxi®cation of HCAs. Therefore, the conclusion can be drawn that the regular intake of protective foods increases the detoxi®cation of these carcinogens, and thus provides a lower risk situation in meat eaters. During the development and growth of many types of cancer there are distinct oxidation reactions that have been documented to play a key role during that phase of carcinogenesis. Indeed, these speci®c reactions involve the generation of active oxygen such as OH radicals and hydrogen peroxide that in turn control the cell duplication rates, and may aect apoptotic reactions that would lead to elimination of abnormal cells. Antioxidants, such as those present in fruits and vegetables as well as tea, antagonize powerfully the developmental aspects of carcinogenesis. In fact, new information indicates that this reaction may actually decrease the already existing benign and malignant tumour cells (Hirose et al., 1997; Katiyar et al., 1997; Kha®f et al., 1998; Liao et al., 1995; Lin et al., 1996; Lu et al., 1997; Nakachi et al., 1998; Yang et al., 1996; Zhao et al., 1997). In addition, the metastic spread of cancer cells may also be aected by antioxidants, although better documentation in this critical ®eld bearing on reduced cancer mortality is needed. Perhaps an eect on cell dierentiation and apoptosis, and not regulation of cell duplication need mechanistic consideration (Chang et al., 1998).
Discussion The detailed research has provided background information of the mechanisms whereby vegetables, tomatoes, fruits and tea are protective in heart disease and many types of cancer. The basic important element is the fact that these foods and drink are a source of potent antioxidants with speci®c chemical structures. The public often believes that one reason the intake of such foods are bene®cial is that they provide essential vitamins and minerals. This is clearly true. A number of vitamins such as C, A, E as well as carotene are excellent antioxidants, but they also contribute to good health through other mechanisms such as being co-factors for certain enzymes, involved in oxidation-reduction reactions in the body. However, as we demonstrate in this paper, these foods contain speci®c antioxidants, polyphenolic chemical compounds for most vegetables and for tea, and the chemical lycopene in tomatoes, that exists in a number of stereochemical
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Fig. 1. Fluid intake pyramid, suggesting desirable types and amounts of ¯uids consumed by adults per day.
versions, with dierential antioxidant potential in vivo. In the USA, research in nutrition and health has led to the conclusion that people should consume ®ve to nine vegetables and fruits a day, and this concept is part of the Federal Government recommendation for health maintenance. In addition, research for the last 20 years has extended knowledge not only on what people should eat, but what they might consume as a beverage. Thus tea, an excellent source of speci®c antioxidants, is part of a healthy nutritional scheme. In fact, we suggest that adults should consume 2±3 litres of ¯uids a day, as part of their cooked foods, and also as beverages. Water clearly is an important portion of the daily ¯uid intake, but research suggests that ®ve to seven cups of tea a day might well be part of the daily ¯uid consumption (Fig. 1). A major goal of contemporary medical research focuses on prevention of major chronic diseases not only as an ethical way of keeping people healthy, but this recommendation may also bear on a re-
duction of the current high cost of medical care (Greenwald and Sondik, 1986). Healthy people require only low-cost health testing at regular intervals, but they avoid high-cost disease care and hospital use. Thus, anywhere in the world, dietary traditions should involve daily intake of fruits, vegetables and tea, with adequate soluble and insoluble ®bres from wholegrain cereals and bran bread, and preferably olive oil at no more than 22±25% of calories. Regular exercise is part of a healthy lifestyle, as is maintenance of proper body weight. Healthpromoting traditions should start in childhood. A Greek adage says ``it is the function of medicine to help people to die young as late as possible'' (Wynder, 1972). AcknowledgementsÐResearch in my laboratory was funded by US National Cancer Institute Contract CN35569, and grants from the American Cancer Society CN157, the Tea Trade Health Research Association, and gifts from Texaco Foundation, and the Friends Against Cancer Team. The expert administrative assistance of Beth-Alayne McKinney is much appreciated.
Antioxidants and chronic disease prevention REFERENCES
Adamson R. H., Gustafsson J. A., Ito N., Nagao M., Sugimura T., Wakabayshi K. and Yamazoe Yeds. 1995. Heterocyclic amines in cooked foods. Proceedings of the 23rd International Symposium of the Princess Takamatsu Cancer Research Fund. pp. 306. Princeton Scienti®c Publishing, Princeton, NJ. Apostolides Z., Balentine D. A., Harbowy M. E., Hara Y. and Weisburger J. H. (1997) Inhibition of PhIP mutagenicity by catechins, and by thea¯avins and gallate esters. Mutation Research 389, 167±172. Block G., Patterson B. and Subar A. (1992) Fruit, vegetables and cancer prevention: a review of the epidemiological evidence. Nutrition and Cancer 18, 1±29. Bu-Abbas A., Cliord M. N., Ioannides C. and Walker R. (1995) Stimulation of rat hepatic UDP-glucuronosyl transferase activity following treatment with green tea. Food and Chemical Toxicology 33, 27±30. Byers T., Anda R., McQueen D., Williamson D., Mokdad A., Casper M., Ford E. and Marks J. (1998) The correspondence between coronary heart disease mortality and risk factor prevalence states in the United States, 1991±1992. Preventive Medicine 27, 311±316. Cader A. A., Steinberg F. M., Mazzone T. and Chait A. (1997) Mechanisms of enhanced macrophage apoE secretion by oxidized LDL. Journal of Lipid Research 38, 981±991. Chang W-C.L., Chapkin R. S. and Lupton J. R. (1998) Fish oil blocks azoxymethane-induced rat colon tumorigenesis by increasing cell dierentiation and apoptosis rather than decreasing cell proliferation. Journal of Nutrition 128, 491±497. Clydesdale F. M. (1997) Tea and health. Critical Reviews in Food Science and Nutrition 37, 691±785. Diaz M. N., Frei B., Vita J. A. and Keaney J. F. (1997) Antioxidants and atherosclerotic heart disease. New England Journal of Medicine 337, 408±416. Felton J. S., Gentile J. M. (Eds) (1997) Mutagenic/ carcinogenic N-substituted aryl compounds. Mutation Research 376, 1±276. Fiala E. S., Sodum R. S., Bhattacharya M. and Li H. (1996) (-)-Epigallocaatechin gallate, a polyphenolic tea antioxidant, inhibits perooxynitrite mediated formation of 8-oxodeoxyguanosine and 3-nitrotyrosine. Experientia 52, 922±926. Frankel E. N., Kanner J., German J. B., Parks E. and Kinsella J. E. (1993) Inhibition of oxidation of human low-density lipoprotein by phenolic substances in red wine. Lancet 341, 454±457. Fujiki H., Suganuma M., Okabe S., Komori, A, Sueoka E., Sueoka N., Kozu T. and Sakai Y. (1996) Japanese green tea as a cancer preventive in humans. Nutrition Reviews 54, 67±70. Goldberg D. (1995) Does wine work? Clinical Chemistry 41, 14±16. Greenwald P., Sondik E. J. (1986) Cancer control objectives for the nation: 1985±2000. National Cancer Institute Monograph 2, 3±74. Hara Y. (1997) Prophylactic functions of antioxidant tea polyphenols. In Food Factors for Cancer Prevention, ed. H. Ohigashi, T. Osawa, J. Terao, S. Watanabe and T. Yoshikawa, pp. 147±151. Springer Verlag, Tokyo. Hernaez J., Xu M. and Dashwood R. (1997) Eects of tea and chlorophyllin on the mutagenicity of N-hydroxyIQ: Studies of enzyme inhibition, molecular complex formation, and degradation/scavenging of the active metabolites. Environmental and Molecular Mutagenesis 30, 468±474. Hill M. J., Giacosa A. and Caygill, C. P. J. (1994) Epidemiology of Diet and Cancer, pp. 411. Ellis Horwood, Chichester.
947
Hirose M., Mizoguchi Y., Yaono M., Tanaka H., Yamaguchi T. and Shirai T. (1997) Eects of green tea catechins on the progression or late promotion stage of mammary gland carcinogenesis in female SpragueDawley rats pretreated with 7,12-dimethylbenz(a)anthracene. Cancer Letters 112, 141±147. Ishikawa T., Suzukawa M., Ito T., Yoshida H., Ayaori M., Nishiwaki M., Yonemura A., Hara Y. and Nakamura H. (1997) Eect of tea ¯avonoid supplementation on the susceptibility of low-density lipoprotein to oxidative modi®cation. American Journal of Clinical Nutrition 66, 261±266. Kasai H. and Nishimura S. (1984) Hydroxylation of deoxyguanosine at C-8 position by ascorbic acid and other reducing agents. Nucleic Acids Research 12, 2137±2145. Katan M. B. (1997) Flavonoids and heart disease. American Journal of Clinical Nutrition 65, 1542±1543. Katiyar S. K., Mohan R. R., Agarwal R. and Mukhtar H. (1997) Protection against induction of mouse skin papillomas with low and high risk of conversion to malignancy by green tea polyphenols. Carcinogenesis 18, 497±502. Kha®f A., Schantz S. P., Chou T-C., Edelstein D. and Sacks P. G. (1998) Quantitation of chemopreventive synergism between (-)-epigallocatechin-3-gallate and curcumin in normal, premalignant and malignant human oral epithelial cells. Carcinogenesis 19, 419±424. Klaunig J. E., Xu Y., Bachowski S., Ketcham C. A., Isenberg J. S., Kolaja K. L., Baker T. K., Walborg E. F., Jr and Stevenson D. E. (1995) Oxidative stress in nongenotoxic carcinogenesis. Toxicology Letters 82, 683±691. Knekt P., Jarvinen R., Reunanen A. and Maatela J. (1996) Flavonoid intake and coronary mortality in Finland: a cohort study. British Medical Journal 312, 478±481. La Vecchia C. and Tavani A. (1998) Fruit and vegetables, and human cancer. European Journal of Cancer Prevention 7, 3±8. Leger A. S., Cochrane A. L. and Moore F. (1979) Factors associated with cardiac mortality in developed countries with particular reference to the consumption of wine. Lancet i, 1017±1020. Liao S., Umekita Y., Guo J., Kokontis J. M. and Hiipakka R. A. (1995) Growth inhibition and regression of human prostate and breast tumor in athymic mice by tea epigallocatechin gallate. Cancer Letters 96, 239±243. Lifshitz F. (1997) Dietary guidelines with particular emphasis on nutritional intake of fat and cholesterol. Journal of the American College of Nutrition 16, 507± 509. Lin Y-L., Juan I-M., Chen Y-L., Liang Y-C. and Lin JK. (1996) Composition of polyphenols in fresh tea leaves and associations of their oxygen-radical-absorbing capacity with antiproliferative actions in ®broblast cells. Journal of Agricultural and Food Chemistry 44, 1387±1394. Lu Y-P., Lou Y-R., Xie J-G., Yen P., Huang M.-T. and Conney A. H. (1997) Inhibitory eect of black tea on the growth of established skin tumors in mice: eects on tumor size, apoptosis, mitosis and bromodeoxyuridine incorporation into DNA. Carcinogenesis 18, 2163±2169. Micozzi M. S., Moon T. E. (1992) Macronutrients: Investigating their Role in Cancer, pp. 477. Marcel Dekker, New York. Nakachi K., Suemasu K., Suga K., Takeo T., Imai K. and Higashi Y. (1998) In¯uence of drinking green tea on breast cancer malignancy among Japanese patients. Japanese Journal of Cancer Research 89, 254±261. Poulsen H. E., Prieme H. and Loft S. (1998) Role of oxidative DNA damage in cancer initiation and promotion. European Journal of Cancer Prevention 7, 9±16.
948
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Pryor W. A. (1993) Measurement of oxidative stress status in humans. Cancer Epidemiology, Biomarkers and Prevention 2, 289±292. Renaud S. and De Lorgeril M. (1992) Wine, alcohol, platelets, and the French paradox for coronary heart disease. Lancet 339, 1523±1526. Sies H. (1997) Antioxidants in Disease Mechanisms and Therapy Advances in Pharmacology, Vol. 38, pp. 707. Academic Press, San Diego. Steinmetz K. A. and Potter J. D. (1991) Vegetables, fruit and cancer. II Mechanisms. Cancer Causes and Control 2, 427±442. Tanaka H., Hirose M., Kawabe M., Sano M., Takesada Y., Hagiwara A. and Shirai T. (1997) Post-initiation inhibitory eects of green tea catechins on 7,12-dimethylbenz[a]anthracene-induced mammary gland carcinogenesis in female Sprague-Dawley rats. Cancer Letters 116, 47±52. van het Hof K.H., de Boer H. S. M., Wiseman S. A., Lien N., Weststrate J. A. and Tijburg L. B. M. (1997) Consumption of green or black tea does not increase resistance of low-density lipoprotein to oxidation in humans. American Journal of Clinical Nutrition 66, 1125±1132. Weisburger J. H. (1997) Tea and health: A historical perspective. Cancer Letters 114, 315±317. Weisburger J. H. (1998) International symposium on lycopene and tomato products in disease prevention. Proceedings of the Society for Experimental Biology and Medicine 218, 93±143.
Weisburger J. H. (1998) Chemoprevention of cancer by tea. In Cancer and Nutrition, ed. K. N. Prasad and W. C. Cole, pp. 167±171. IOS Press, Amsterdam. Weisburger J. H. and Williams G. (1995) Causes of cancer. In American Cancer Society Textbook on Clinical Oncology, ed. G. Murphy, W. Lawrence and R. E. Lenhard, pp. 10±39. American Cancer Society, Atlanta, GA. Wynder E. L. (1972) Preventive Medicine Introduction. Preventive Medicine 1, 1±5. Wynder E. L. and Homann D. (1994) Smoking and lung cancer: Scienti®c challenges and opportunities. Cancer Research 54, 5284±5295. Xu M., Bailey A. C., Hernaez J. F., Taoka C. R., Schut H. A. J. and Dashwood R. H. (1996) Protection by green tea, black tea, and indole-3-carbinol against 2amino-3-methylimidazo[4,5-f]quinoline-induced DNA adducts and colonic aberrant crypts in the F344 rat. Carcinogenesis 17, 1429±1434. Xu Y., Ho C-T., Amin S. G., Han C. and Chung FL. (1992) Inhibition of tobacco-speci®c nitrosamine induced lung tumorigenesis in A/J mice by green tea and its major polyphenols as antioxidants. Cancer Research 52, 3875±3879. Yang C. S., Chen L., Lee M. J. and Landau J. M. (1996) Eects of tea on carcinogenesis in animal models and humans. Advances in Experimental Medicine and Biology 401, 51±61. Zhao Y., Cao J., Ma H. and Liu J. (1997) Apoptosis induced by tea polyphenols in HL-60 cells. Cancer Letters 121, 163±167.