Green Tea: It’s role in cancer and heart disease prevention
Tea is grown in about 30 countries and, next to water, is the most widely consumed beverage in the world. Although there is only one plant (camellia sinensis), tea is manufactured as black (78%), green (20%), or oolong (2%). Black tea is more popular in western countries and green tea is primarily consumed in Asian countries, North Africa and the Middle East. Leaves intended for green tea are picked by the same method as those picked for black tea. Black tea is fermented; green tea is not.
Fermentation alters the chemical structure of the tea leaf, permitting principal flavor attributes to emerge. However, green tea contains a higher concentration of polyphenol antioxidants. Green tea contains 35-52% (measured in weight percent of extract of solids) catechins and flavonols, which provide antioxidant protection to the body. Epigallocatechin-3-gallate (EGCG) is the primary antioxidant accounting for 40% of the total polyphenol content. Other polyphenols include epicatechin, epicatechin-3-gallate, and epigallocatechin.
Much of the research documenting the health benefits of green tea is based on the amount of green tea typically consumed in Asian countries—about 3 cups (750 ml) per day (providing 240–320 mg of polyphenols). However, other research suggests as much as 10 cups (2,500 ml) per day is necessary to obtain noticeable benefits from green tea ingestion. Tablets and capsules containing standardized extracts of polyphenols, particularly EGCG, are available. Some provide up to 97% polyphenol content—which is equivalent to drinking 4 cups (1,000 ml) of tea. Many of these standardized products are decaffeinated.
Green Tea May Prevent Cancer
Animal cancer research using green tea is very compelling. In animal studies green tea has been shown to increase the activities of antioxidant and detoxification enzymes (glutathione reductase, glutathione peroxidase, glutathione S-transferase, catalase, and quinone reductase) in the lungs, liver, and small intestine. .
Laboratory studies have suggested that green tea polyphenols may inhibit cancer by blocking the formation of cancer-causing nitrosamines and suppressing the activation of carcinogens in lung, breast, colon, and melanoma cancer cells. Green tea extracts also block estrogen from attaching to estrogen receptors on breast cells, a function associated with decreasing breast cancer risk.
With respect to human studies Bushman reviewed the worldwide evidence on green tea and cancer in the Journal of Nutrition and Cancer in 1998. Her review included thirty-one human studies and four pre-existing reviews of the scientific literature. The best evidence exists for the prevention of stomach cancer and cancer of the esophagus.
There is some evidence that green tea consumption may reduce the risk of pancreatic cancer, colon cancer and bladder cancer, however, more research is clearly required before any definitive statements can be made in this regard. One final suggestion by Gensler and fellow researchers is that the low Japanese lung cancer rate, despite high smoking rates, may be attributed to green tea consumption. This remains speculation at this point in time, but green tea polyphenols have been shown to increase detoxification of lung enzymes in animal studies as previously mentioned.
In general, it is difficult to measure the impact of green tea on the prevention of human cancers, although the biological plausibility of a preventive effect is strong and consistent across the available experimental, animal research studies, as well as human observation (epidemiological) studies.
Green Tea May Reduce Heart Disease
Studies reveal that green tea may have mild effect on lowering the bad LDL-cholesterol, while raising the good HDL-cholesterol in the bloodstream. Green tea may also have a mild effect on reducing the stickiness of blood platelets and protecting LDL-cholesterol from free radical damage. All of these effects are associated with reduced risk of heart attack and stroke.
My present view is that most of us would do well to substitute more green tea for coffee, black tea and other herbal teas, as a daily beverage. To avoid the stimulant effects of the caffeine, decaffeinated green tea products are widely available.
Much of the research documenting the health effects on humans provided by green tea is based on the amount of green tea typically consumed in Asian countries, which is about 3 cups (750 ml) per day. This provides 240-320 mg of polyphenol antioxidants, which is a generous amount of antioxidant protection.
The only precautionary note is to disclose the fact that ingesting scalding hot beverages of any kind increases the risk of stomach and upper gastrointestinal tract (i.e. esophagus) cancers. Therefore, be sure to allow some cooling before you ingest green tea or any other hot beverage. This is also true for very hot foods, so pay heed.
Selected References On Green Tea
Benzie IF, Szeto YT, Strain JJ, Tomlinson B. Consumption of green tea causes rapid increase in plasma antioxidant power in humans. Nutr Cancer 1999;34:83–7
Blot WJ, Chow WH, and McLaughlin, JK: Tea and cancer: a review of the epidemiological evidence. Eur J Cancer Prev 1996;5:425-438
Bushman JL. Green Tea and Cancer in Humans: A Review of the Literature.. Nutrition and Cancer 1998;31(3):151-159
Chung FL, Morse MA, Eklind KI, Xu Y. Inhibition of the tobacco-specific nitrosamine-induced lung tumorigenesis by compounds derived from cruciferous vegetables and green tea. Ann NY Acad Sci 1993;686:186-202
Fujiki H, Suganuma M, Okabe S, Komori A, Sueoka E, et al. Japanese green tea as a cancer preventive in humans. Nutr Rev 1996;54:S67-S70
Gensler HL, Timmermann BN, Valcic S, Wachter GA, Dorr R, et al. Prevention of photocarcinogenesis by topical administration of pure epigallocatechin gallate isolated from green tea. Nutr Cancer 1996;26:325-335
Graham HN. Green tea composition, consumption, and polyphenol chemistry. Prev Med 1992;21:334–50
International Agency for Research on Cancer. Coffee, Tea, Matè, Methylxanthines, and Methylgloxyal. 1990 Lyon , France: Int Agency Res Cancer 1991:207-271. (IARC Monogr 51)
Khan SG, Katiyar SK, Agarwal R, Mukhtar H. Enhancement of antioxidant and phase ll enzymes by oral feeding of green tea polyphenols in drinking water to SKH-1 hairless mice: possible role in cancer chemoprevention. Cancer Res 1992;52:4050-4052
Komori A, Yatsunami J, Okabe S, Abe S, Hara K, et al. Anticarcinogenic activity of green tea polyphenols. Jpn J Clin Oncol 1993;23:186-190
Kono S, Shinchi K, Ikeda N, et al. Green tea consumption and serum lipid profiles: A cross-sectional study in Northern Kyushu, Japan. Prev Med 1992;21:526–31
La Vecchia C, Negri E, D'Avanzo B, Franceschi S: Food temperature gastric cancer. Int J Cancer 46, 432-434
Mukhtar H, Katiyar SK, Agarwal R. Green tea and skin – anticarcinogenic effects. J Invest Dermatol 1994;102:3-7
Sagesaka-Mitane Y, Milwa M, Okada S. Platelet aggregation inhibitors in hot water extract of green tea. Chem Pharm Bull 1990;38:790–3
Sasazuki S, Komdama H, Yoshimasu K, et al. Relation between green tea consumption and severity of coronary atherosclerosis among Japanese men and women. Ann Epidemiol 2000;10:401–8
Serafini M, Ghiselli A, Ferro-Luzzi A. In vivo antioxidant effect of green tea in man. Eur J Clin Nutr 1996;50:28–32
Stensvold I, Tverdal A, Solvoll K, et al. Tea consumption. Relationship to cholesterol, blood pressure, and coronary and total mortality. Prev Med 1992;21:546–53
Stoner G, Mukhtar H. Polyphenols as cancer chemopreventive agents. J Cell Biochem 1995; 22:169-180
Tsubono Y, Tsugane S. Green tea intake in relation to serum lipid levels in middle-aged Japanese men and women. Ann Epidemiol 1997;7:280–4
Valcic S, Timmermann BN, Alberts DS, Wachter GA, Krutzsch M, et al. Inhibitory effect of six green tea catechins and caffeine on the growth of four selected human tumor cell lines. Anticancer Drugs 1996;7:461-468
Yamaguchi Y, Hayashi M, Yamazoe H, et al. Preventive effects of green tea extract on lipid abnormalities in serum, liver and aorta of mice fed an atherogenic diet. Nip Yak Zas 1991;97:329–37
Yu G, Hsieh C, Wang L, Yu S, Liu X, et al. Green tea consumption and risk of stomach cancer: a population-based case-control study in Shanghai, China. Cancer Causes Control 1995;6:532-538