Four different forms of tea (white, green, oolong, and black) come from the same source: the leaves of the Camellia sinensis plant. (â€śHerbalâ€ť tea or tisane is not considered true tea, because it contains no Camellia sinensis leaves and is instead derived from the dried flowers, stems, leaves, or berries of numerous other plants.)
What distinguishes the four types of true tea is how the leaves are processed between the time they are picked and the time they are packaged:
White tea is the produced chiefly from tea leaf buds. Because it is minimally processed, it may exhibit potent disease-fighting potential. To date, however, there is comparatively little research on its health effects.
To make green tea, the leaves are picked and preserved (usually by steaming or baking) to keep them from undergoing the process of fermentation (or oxidation.)
To make oolong and black tea, the leaves are picked and exposed to the air for a period of time. During this period, the leaves ferment. Oolong tea is exposed to the sun and allowed to partially ferment; black tea is fermented completely.
The process of fermentation slightly changes the essential chemical makeup of tea. The longer the leaves are allowed to ferment, the weaker the teaâ€™s natural roster of cancer-fighting compounds becomes, while the caffeine content of the tea leaves steadily increases.
Generally, green tea has one-half to one-third the caffeine of black tea.
Green tea contains several substances collectively called polyphenols that have displayed potent antioxidant effects and other cancer-combating properties. Approximately 90 percent of the polyphenols found in green tea are called catechins (KAT-uh-kins). Green tea contains approximately three times the quantity of catechins found in black tea. The chief catechins found in green tea are:
- epicatechin gallate
- epigallocatechin gallate (also known as EGCG).
EGCG is the most active component in green tea, and is a stronger antioxidant than either vitamin C or E. For this reason, it is the most widely studied green tea compound.
Green Tea Data: A Sampling of the Scientific Literature
Throughout China and Japan, green tea is a staple of the diet, particularly among the older generation. Epidemiological studies (mostly conducted in Asian populations) have consistently associated green tea consumption with lower incidence of many different cancers.
- A 1989 study in the Japanese Journal of Nutrition reported that in tea-producing regions of Japan (where residents consume green tea in several forms, including gum, candy and desserts) stomach cancer mortality rates are lower than in other regions of Japan.
- One of the first studies to suggest a protective effect of green tea appeared in the journal Cancer Research in 1994. The studyâ€™s authors found that rates of lung cancer among Japanese smokers were half that of American smokers, and postulated that this difference was likely due to differences in tea consumption between the two populations.
- In a case-control study conducted at the Shanghai Cancer Institute in China that appeared in the Journal of the National Cancer Institute in 1994, green tea drinkers had a 50 percent reduction in risk for esophageal cancer.
- Green tea seemed to disrupt the process of stomach cancer in another Chinese case-control study. A 50 percent reduction in risk for stomach cancer was associated with consumption of green tea, regardless of the age at which green tea drinking began, according to a study that appeared in the journal Cancer Causes Control in 1995.
- A 1997 case-control study among residents of Shanghai found regular consumption of green tea to be associated with significant (between 12 and 53 percent) reductions in risk for cancers of the colon, rectum and pancreas. The study appeared in the International Journal of Cancer.
- A landmark Japanese cohort study with 8,552 subjects that was published in the journal Preventive Medicine in 1997 found that green tea had a protective effect against cancer in all organs including the stomach, lung, colorectum and liver. This effect was most pronounced among females drinking 10 cups of green tea per day. (Note: A typical Japanese teacup holds 4 fluid ounces or 120 milliliters of tea; a typical American teacup holds 6 fluid ounces or approximately 180 milliliters of tea.)
- According to a 1998 Japanese study appearing in Cancer Causes Control, consumption of seven or more cups of green tea per day decreased the risk of stomach cancer 31 percent.
- A study published in 1998 involving Stage I and Stage II breast cancer patients in Japan showed that subjects who drank more than five cups of green tea a day had a lower recurrence rate and longer disease-free period than subjects who drank four or less cups per day. This study, in which it was also shown that green tea consumption did not affect recurrence among Stage III breast cancer patients, was published in the Japanese Journal of Cancer Research.
- A 1999 Phase I/Phase II study found that persons who drank green tea had short-term reductions in levels of specific lipid compounds (prostaglandin E2) that are believed to be an important factor in the development of cancer in rectal tissue. This study appeared in the journal Cancer Epidemiology, Biomarkers and Prevention.
- A study involving 1,160 breast cancer patients found that consumption of three or more cups of green tea daily was associated with significantly reduced recurrence of stage I breast cancer after seven years. The effect of tea consumption of stage II recurrence was smaller. This study appeared in the journal Cancer Letters in 2001.
- The Iowa Womenâ€™s Health Study cohort examined 34,651 postmenopausal women for 12 years and concluded that intake of tea catechins was associated with lower risk for rectal cancer. This finding appeared in the journal Cancer Causes Control in 2002.
- A case-control study conducted among Asian-American women in Los Angeles County found that green tea drinkers showed a significantly reduced risk of breast cancer that was dose-dependent. Those who drank more than 85.7 milliliters (about 3 fluid ounces or Â˝ of an American teacup) per day showed 47 percent lower risk of breast cancer than non-green tea drinkers. This study was published in the International Journal of Cancer in 2003.
- A case-control study conducted in southeast China showed that prostate cancer risk declined steadily as the amount, frequency and duration of green tea consumption increased. This study was published in the International Journal of Cancer in 2004.
- A case-control study conducted in Taixing, China and published in the April 2005 issue of the International Journal of Cancer found that regular consumption of green tea was associated with a 41 percent reduction in risk for stomach cancer.
- In a study appearing in the May 2005 issue of Carcinogenesis, intake of green tea was associated with 13 percent lower levels of circulating estrogen among a small group of postmenopausal Chinese women. In the same group, intake of black tea was associated with circulating estrogen levels that were 19 percent higher than non-tea drinkers. (Increased blood levels of sex hormones like estrogen are thought to be linked to increased risk of certain cancers.)
- A meta-analysis published in June 2005 in the journal Integrative Cancer Therapies reviewed the epidemiological evidence on green tea and breast cancer and concluded that, to date, consumption of 5 or more cups of green tea per day shows a non-statistically significant trend toward prevention of breast cancer. The evidence also indicates that consumption of green tea may help prevent recurrence of Stage I and Stage II breast cancers.
- Currently, several NIH-funded clinical trials are underway involving green tea (and specific green tea extracts) testing safety and efficacy in humans. These trials involve primary prevention, cancer recurrence and integrative cancer therapy (combinations of green tea with cancer drugs.)
The laboratory/experimental evidence linking green tea in general (and EGCG in particular) to cancer prevention is considerable and continually growing. Numerous potential biological mechanisms are currently being investigated. A database search of the published scientific literature showed that over 100 separate scientific papers on EGCGâ€™s potential anti-cancer benefits have been published in peer-reviewed journals in 2005 alone.
Generally, researchers looking at EGCGâ€™s anti-cancer potential are investigating its ability to:
- Keep cancer from being triggered by specific carcinogens
- Interfere with the signals sent between cells that â€śturn onâ€ť cancer genes
- Directly inhibit the expression of COX-2 and other enzymes involved in cancer
- Help regulate a cellâ€™s natural growth cycle
- Induce spontaneous â€ścell deathâ€ť in cancerous cells
- Keep cancerous cells from growing the blood vessels they need to feed themselves
At the AICR/WCRF International Research Conference on Food, Nutrition and Cancer held in Washington, D.C. on July 14 and 15, 2005, several papers will be presented containing new evidence on the link between green tea and cancer, including:
- EGCG targets and defuses a basic, cancer-sparking protein found throughout the body. (This paper is the subject of the AICR Press Conference held at 10:00 a.m. on July 14, 2005.)
- EGCG interacts with NfkB, an important regulator of the bodyâ€™s immune and inflammatory responses.
- EGCG and other green tea components prevent the proliferation of prostate cancer cells.
- Green tea slows the growth of breast tumors.
- Green tea suppresses the metastasis of breast cancer cells.
- Green tea can induce â€ścell deathâ€ť in lymphoma cells.
How Should it be Prepared?
Research on the compounds found in green tea suggests that the best way to take advantage of green teaâ€™s health benefits is to drink it freshly brewed, after steeping the leaves for one to five minutes.
In Asian cultures, green tea is found in many forms, including gum, candy, ice cream, bread and other foods. In America, bottled, flavored green tea â€śdrinksâ€ť and instant green teas are increasingly common. Although these preparations offer alternative dietary sources of green tea compounds and add variety to the diet, they contain fewer, and less bioavailable, catechins than fresh-brewed green tea.
AICR believes that those concerned about lowering risk for cancer should consider adding green tea to a diet that is rich in a variety of plant foods and low in fat and salt.
Clinical trials are now underway to determine the precise amount of green tea needed for maximum anti-cancer activity. Until those results come in, researchers who have studied green tea in Asian populations note that drinking between three to four cups (roughly equivalent to 2 to 3 U.S. cups) of green tea per day has been associated with cancer protection in many studies.
Who Should, and Who Shouldnâ€™t?
Green tea contains caffeine. Although green teaâ€™s caffeine content is less than that of black tea or coffee â€“ in fact, three or four cups of green tea contains approximately the same amount of caffeine found in a single cup of coffee — it is important to be aware that caffeine has deleterious effects in large doses.
For this reason, people who suffer from heart or kidney problems, stomach ulcers or anxiety should avoid green tea. Pregnant and breastfeeding women should not drink green tea. Caffeine has also been shown to interact with various prescription drugs, including heart and blood pressure medications, sedatives, oral contraceptives, and drugs used to treat depression.
In large doses, green tea has been shown to interact with drugs that affect blood clotting, such as aspirin and warfarin (Coumadin). People on any of the above medications should check with a health professional before adding green tea to the diet.
Reprinted with permission from the American Institute for Cancer Research