Green Tea: What It Is and Why It's Studied
Four different forms of tea (white, green, oolong, and black) come from the same source: the leaves of the Camellia sinensis plant. ("Herbal" 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 slightlychanges the essential chemical makeup of tea. The longer the leaves are allowed to ferment, the weaker the tea'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 thecaffeine 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 study'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 Women'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