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Green Tea and Cancer


[How Green Tea Works | Green Tea or Black?]
[Current Research on Green Tea and Cancer]
[Green Tea Safety | How Much Green Tea? | References]

In December, 2002, the National Cancer Institute published results on green tea studies, which state:

  • The antioxidants found in green tea--called catechins--may selectively inhibit the growth of cancer.

  • In laboratory studies using animals, green tea catechins scavenged oxidants before cell damage occurred, reduced the number and size of tumors, and inhibited the growth of cancer cells.

  • However, human green tea cancer studies have proven more contradictory, perhaps due to such factors as variances in diet, environments, and populations.

  • NCI researchers are investigating the therapeutic and preventive use of green tea catechins against a variety of cancers.

Early in the thirteenth century, a Zen priest named Eisai extolled the virtues of green tea for maintaining good health and enhancing longevity.  Until the twentieth century, there was little scientific evidence to support his intuitive wisdom about green tea.  But today, a growing body of green tea research suggests that the legendary health benefits of green tea are solidly rooted in fact.  The green tea good news has attracted a whole new generation of green tea devotees – health-conscious consumers seeking extra protection from major diseases, including cancer and heart disease. 

One of the world’s oldest and best-loved beverages, green tea serves as a perfect example of a “functional food” – a food or beverage that confers health benefits above and beyond basic nutrition.  Green tea’s best-studied benefits lie in the areas of antioxidant activity, cancer prevention, and protection against heart disease.  Much of the evidence comes from large population studies that looked at the dietary habits and health status of thousands of people.  One particularly interesting green tea study, conducted in 1992, showed that 3,380 Japanese women who were regular practitioners of chanoyu (the Japanese tea ceremony) had surprisingly low mortality rates, compared with other Japanese women in their age group.  While other lifestyle factors beyond the green tea consumption also may have played a role, the investigators attributed the women’s longevity at least in part to their consumption of green tea.   

Population studies are not always reliable, but results like these attracted attention from researchers.  Scientists now believe that many of green tea’s health effects are related to green tea’s antioxidant power, which may rival that of well-known antioxidant vitamins C and E.  Population studies have suggested that regular consumption of green tea may help prevent high cholesterol, stroke, and cancers of the mouth, esophagus, stomach, and pancreas.  These results are supported by green tea animal studies, in which green tea and isolated tea compounds

  • helped protect against the development of various cancers, and

  • green tea inhibited the growth of existing tumors. 

Other research indicates that

  • green tea may protect the liver against damage from toxins,

  • green tea may stimulate the function of the immune system,

  • green tea may improve cancer prognosis, and

  • green tea may enhance healthy blood flow. 

Scientists currently are investigating the potential of green tea for myriad other uses, including

  • green tea for weight loss,

  • green tea for dental health,

  • green tea for infectious disease and

  • green tea for skin care (including protection against skin cancer). 

Not all green tea studies have demonstrated positive results, and researchers agree that green tea data are not yet conclusive.  Nonetheless, based on the available evidence, green tea beverages and supplements have soared in popularity.  Green tea dietary supplements – products claiming to pack the benefits of many cups of tea into a single green tea capsule – were the seventh best sellers in U.S. health food stores in 2000, up from 26th place in 1999. 

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How Green Tea Works

Green tea – a minimally processed product of the same plant that gives us black and oolong teas – is rich in powerful antioxidant compounds called polyphenols.  Green tea polyphenols, more specifically known as catechins, protect cells and DNA by counteracting the damaging effects of free radicals, toxic compounds that play a role in aging and the development of many diseases.  One of the most important and best studied green tea compounds is epigallocatechin gallate (EGCG), which researchers believe may be responsible for many of green tea’s cancer-protective effects. 

A number of different mechanisms have been proposed to explain green tea’s anticancer activity.  Green tea polyphenols may inhibit the activity of liver enzymes that activate carcinogens, and these green tea polyphenols may prevent the formation of some carcinogens.  Green tea polyphenols also may enhance the antioxidant capacity of body tissues and/or green tea polyphenols may increase the activity of protective antioxidant enzymes that occur naturally in the body, creating a less hospitable environment for tumor formation. 

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Current Findings on Green Tea and Cancer

The National Cancer Institute reports that in 2002, a number of green tea research studies were published which further validate the anti-cancer effects of green tea.  Years of research show that green tea inhibits the development of many types of cancer and suppresses the progression of cancer (including the metastatic process). 

Green tea has been found to work by itself and in combination with chemotherapy or with natural compounds such as selenium or curcumin.  The following are highlights of 2002 green tea and cancer studies. 

  • Epigallocatechin gallate, the active ingredient of green tea, was shown to disrupt the cell cycle in human colon cancer cells 1

  • The anti-cancer effectiveness of green tea was increased when used in combination with 3 other ingredients.  In one study, the anti-cancer effectiveness of green tea was increased with the consumption of selenium.  In another study, the combination of green tea extract and curcumin was shown to be effective in decreasing the proliferation index in squamous cell carcinoma.  In the third study, the combination of sulindac, an anti-inflammatory drug, synergistically increased apoptosis in colon cancer cells.  2,3,4

  • Some notable effects were found in studies specifically for women.  Women who regularly consume green tea were found to experience a dramatic decrease in the risk of ovarian cancer.  In studies in mice, green tea extract at .1% concentration greatly delayed the development of mammary tumors.  Mice given a 1% green tea extract did not develop any tumors during this study.  5,6

  • Two studies showed that green tea extract inhibited the formation of deadly heterocylic amines and reduced the size of tumors if cancer developed.  Heterocyclic amines are potent carcinogens formed primarily by overcooking meat.  To a lesser extent, they are also formed through normal metabolic processes in the body.  7,8

  • Green tea appears to increase the activity of ornithine decarboxylase.  This drug has long been used in cancer therapy.  Ornithine decarboxylase is an enzyme that limits the rate of production of polyamines needed for tumor proliferation. 9

  • A common tumor growth factor is a protein called kinase C.  Green tea extract inhibited the activity of kinase C.  Tumors require a constant supply of new blood vessels to sustain their rapid growth.  A major cancer growth factor is called vascular endothelial growth factor (VEGF).   In this study, green tea extract significantly decreased levels of VEGF, with larger green tea dosages yielding larger decreases in VEGF. 10

  • One of the ways that carcinogens initiate cancer is to disrupt what is referred to as the gap junction.  The gap junction regulates the flow of signals between cells.  Green tea extract in an in-vitro study counteracted carcinogen-induced damage to the gap junction.  11

  • Green tea extract was shown to significantly suppress the growth and invasive ability of three different lines of pancreatei cancer cells.  The larger the dose of green tea, the more suppression. 12

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Green Tea or Black?

Tea harvest and preparation methods determine both the type and quality of the final product.  All non-herbal tea – including green tea, black, oolong, and white tea – comes from the same plant, Camellia sinensis, a member of the botanical family Theaceae.  The difference lies in the processing.  Black tea gets its characteristic color and flavor from a fermentation process in which the leaves are bruised to release enzymes, changing the color and chemistry of the leaves.  Oolong tea, which is typically yellow or red, is semi-fermented.  Green tea is only minimally processed, giving it a different flavor and a much higher polyphenol content.  (White tea, a delicately flavored specialty variety from China, is processed still less than green tea, and preliminary research suggests it contains even higher levels of polyphenols.)

Some of the polyphenolic compounds in black tea are lost during the fermentation process.  However, growing evidence shows that black tea has health benefits of its own.  While it is clear that green tea is a much stronger antioxidant, studies show that black tea also has considerable antioxidant power.  This is good news for Westerners, who traditionally have consumed more black tea than green tea. 

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Green Tea Safety

Green tea is a nontoxic, safe food herb.  However, green tea does contain caffeine, making green tea an inappropriate choice for infants, and possibly for people with high blood pressure, insomnia, or anxiety disorders.  On the other hand, green tea is relatively low in caffeine compared with other beverages.  Green tea contains about 15 to 25 mg of caffeine per cup, compared with 35 to 50 mg for black tea and 85 to 100 mg for coffee.  If you are concerned about caffeine, try a decaffeinated green tea, as decaffeination apparently has no effect on green tea’s polyphenol content.  The astringent tannins in green tea may irritate sensitive stomachs. 

How Much Green Tea?

An average “dose” of green tea is between 2 and 3 cups a day, although some in the population studies drank up to 10 cups of green tea per day. 

Brewed green tea: 2 to 5 cups green tea/day

Standardized green tea extract: 250 to 400 mg/day of green tea extract standardized to 90 percent polyphenols. 

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References

  1. Br J Cancer 2002; 86(10:1645-51.

  2. Biol Trace Elem Res 2002;86(2):177-91.

  3. Cancer Lett 2002; 177(1):49-56.

  4. Carcinogenesis 2002:1307-13.

  5. Int J Oncol 2002 Jun;20(6):1233-9.

  6. Cancer Epidemiol Biomarkers Prev 2002; 11(8):713-8.

  7. Food Chem Toxicol 2002; 40: 1145-54.

  8. Environ Mol Mutagen 2002; 39(2-3):271-8.

  9. Prostaglandins Leukot Essent Fatty Acids 2002; 66: 519-24.

  10. J Nutr 2002; 132(8):2307-11.

  11. Biochem Pharmacol 2002; 63:2145-9.

  12. Pancreas 2002; 25:45-8.

Article (with the exception of the footnoted section on 2002 cancer studies provided by the National Cancer Institute) was reproduced with the exclusive permission from the Herb Research Foundation.

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