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Green Tea as an Antioxidant
Virtually everybody reading this page will have heard the term "antioxidant" by now (probably more 
times than you care to count). As a very broad generalization, the term "antioxidant" refers to the activity possessed by numerous vitamins, minerals and other phytochemicals to serve as protection against the damaging effects of highly reactive molecules known as free radicals. Free radicals have the ability to chemically react with, and damage, many structures in the body. Particularly susceptible to oxidative damage are the cell membranes of virtually all cells and the very source of our genetic material - DNA. Free radical reactions and oxidative damage have been linked to many of the diseases of aging such as heart disease and cancer.
The free radical theory of aging (and disease promotion) holds that through a gradual accumulation of microscopic damage to our cell membranes, DNA, tissue structures and enzyme systems, we begin to lose function and are predisposed to disease. Literally thousands of scientific studies have clearly documented the beneficial effects of dozens of antioxidant nutrients. There is certainly no shortage of nutrients and phytochemicals that possess significant antioxidant activity in the test tube - in fact, it seems as if every nutraceutical on the market possesses some degree of antioxidant activity. It is very well established that an increased dietary intake of antioxidant phytonutrients is linked to a reduced rate of oxidative damage as well as reduced incidence of chronic diseases such as heart disease and cancer. Perhaps the best specific data regarding antioxidant activity and the potential for real health benefits exists for green tea extract.
Aside form being the second-most consumed beverage in the world (water is the first), green tea has been used medicinally for centuries in India and China. The active constituents in green tea are a family of polyphenols (catechins) and flavonols which possess potent antioxidant activity. Large polyphenol molecules called tannins form the bulk of the active compounds in green tea, with catechins comprising nearly 90%. Several catechins are present in significant quantities; epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG) and epigallocatechin gallate (EGCG). EGCG makes up about 10-50% of the total catechin content and appears to be the most powerful of the catechins - with antioxidant activity about 25-100 times more potent than vitamins C and E. A cup of green tea may provide 10-40mg of polyphenols and has antioxidant activity greater than a serving of broccoli, spinach, carrots or strawberries. A number of commercial green tea extracts are standardized to total polyphenol content and/or EGCG content (but many are not).
Several epidemiological studies show an association between consumption of total flavonoids in the diet and the risk for cancer and heart disease. Men with the highest consumption of flavonoids (from fruits and vegetables) have approximately half the risk of heart disease and cancer compared with those with the lowest intake. The primary catechin in green tea, EGCG, appears to inhibit the growth of cancer cells as well as play a role in stimulating apoptosis (programmed cell death), both of which are crucial aspects for cancer prevention. In terms of heart disease protection, the potent antioxidant properties of polyphenols would be expected to reduce free radical damage to cells and prevent the oxidation of LDL cholesterol - both of which would be expected to inhibit the formation of atherosclerotic plaques.
This review concerns the specific actions of green tea extract as a powerful antioxidant (versus its use as a weight loss agent which has also been reviewed here). In this regard, green tea may be used by consumers looking for a broad-spectrum antioxidant or as specific treatment/prevention for certain cancers. For either of these uses, as a general antioxidant or as an "anti-cancer" agent, SupplementWatch urges you to look for the highest polyphenol content with the lowest caffeine content at the best price. Aside from the clear benefits of green tea as an antioxidant, however, recent studies have suggested a role catechins in promoting weight loss - and another review (using a different set of criteria) evaluates the value of specific green tea extracts as adjuncts to a weight loss regimen.
References
1. Benzie IF, Szeto YT, Strain JJ, Tomlinson B. Consumption of green tea causes rapid increase in plasma antioxidant power in humans. Nutr Cancer. 1999;34(1):83-7.
2. Dulloo AG, Duret C, Rohrer D, Girardier L, Mensi N, Fathi M, Chantre P, Vandermander J. Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans. Am J Clin Nutr. 1999 Dec;70(6):1040-5.
3. Dulloo AG, Seydoux J, Girardier L, Chantre P, Vandermander J. Green tea and thermogenesis: interactions between catechin-polyphenols, caffeine and sympathetic activity. Int J Obes Relat Metab Disord. 2000 Feb;24(2):252-8.
4. Gupta S, Ahmad N, Mohan RR, Husain MM, Mukhtar H. Prostate cancer chemoprevention by green tea: in vitro and in vivo inhibition of testosterone-mediated induction of ornithine decarboxylase. Cancer Res. 1999 May 1;59(9):2115-20.
5. Hasegawa R, Chujo T, Sai-Kato K, Umemura T, Tanimura A, Kurokawa Y. Preventive effects of green tea against liver oxidative DNA damage and hepatotoxicity in rats treated with 2-nitropropane. Food Chem Toxicol. 1995 Nov;33(11):961-70.
6. Hirose M, Hoshiya T, Akagi K, Futakuchi M, Ito N. Inhibition of mammary gland carcinogenesis by green tea catechins and other naturally occurring antioxidants in female Sprague-Dawley rats pretreated with 7,12-dimethylbenz[alpha]anthracene. Cancer Lett. 1994 Aug 15;83(1-2):149-56.
7. Kao YH, Hiipakka RA, Liao S. Modulation of endocrine systems and food intake by green tea epigallocatechin gallate. Endocrinology. 2000 Mar;141(3):980-7.
8. Lin JK, Liang YC, Lin-Shiau SY. Cancer chemoprevention by tea polyphenols through mitotic signal transduction blockade. Biochem Pharmacol. 1999 Sep 15;58(6):911-5.
9. Sato D. Inhibition of urinary bladder tumors induced by N-butyl-N-(4-hydroxybutyl)-nitrosamine in rats by green tea. Int J Urol. 1999 Feb;6(2):93-9.
10. Tanaka H, Hirose M, Kawabe M, Sano M, Takesada Y, Hagiwara A, Shirai T. Post-initiation inhibitory effects of green tea catechins on 7,12-dimethylbenz[a]anthracene-induced mammary gland carcinogenesis in female Sprague-Dawley rats. Cancer Lett. 1997 Jun 3;116(1):47-52.
11. Wang ZY, Huang MT, Ho CT, Chang R, Ma W, Ferraro T, Reuhl KR, Yang CS, Conney AH. Inhibitory effect of green tea on the growth of established skin papillomas in mice. Cancer Res. 1992 Dec 1;52(23):6657-65.
12. Weisburger JH, Rivenson A, Aliaga C, Reinhardt J, Kelloff GJ, Boone CW, Steele VE, Balentine DA, Pittman B, Zang E. Effect of tea extracts, polyphenols, and epigallocatechin gallate on azoxymethane-induced colon cancer. Proc Soc Exp Biol Med. 1998 Jan;217(1):104-8.
13. Xu Y, Ho CT, Amin SG, Han C, Chung FL. Inhibition of tobacco-specific nitrosamine-induced lung tumorigenesis in A/J mice by green tea and its major polyphenol as antioxidants. Cancer Res. 1992 Jul 15;52(14):3875-9.
14. Zhu M, Gong Y, Ge G. Effects of green tea on growth inhibition and immune regulation of Lewis lung cancer in mice. Chung Hua Yu Fang I Hsueh Tsa Chih. 1997 Nov;31(6):325-9.
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