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Other common
name(s): green tea extract, Chinese tea
Scientific/medical
name(s): Camellia
sinesis
Description
Green tea is a drink made from the steamed and dried leaves of
the Camellia sinesis
plant, a shrub native to Asia. Black tea is also made from this plant,
but unlike green tea, black tea is made from leaves that have been
fermented. Fermentation may reduce the levels of some compounds, such
as antioxidants, in the tea.
Overview
Some researchers believe green tea may protect against certain
types of cancer because it contains antioxidants. However, results from
human studies have been mixed. More research is needed to determine the
role of green tea in cancer prevention.
How is it promoted for use?
Green tea is widely consumed in Japan, China, and other Asian
nations and is becoming more popular in Western nations. Some reports
indicate green tea may have the ability to help prevent cancers of the
skin, esophagus, stomach, colon, pancreas, lung, bladder, prostate, and
breast.
Green tea contains chemicals known as polyphenols, which have
antioxidant properties. The major group of polyphenols in green tea are
called catechins,
and the most important catechin seems to be epigallocatechin-3-gallate
(EGCG). EGCG may cause cancer cells to die in much the same way that
normal cells do. This effect is important because cancer cells are
different from normal cells in that they do not die when they
should—they continue to grow and spread.
Herbalists use green tea and extracts of its leaves for
stomach problems, vomiting, and diarrhea and to reduce tooth decay,
blood pressure, cholesterol levels, and blockages of the blood vessels
in the heart that can lead to heart attacks. Green tea is also promoted
as preventing certain bacterial infections. In recent years, some
researchers have suggested that black tea may also be effective in
cancer prevention. These claims are currently being studied.
What does it involve?
The typical amount of green tea consumed varies widely, and it
is not clear how much might be needed for beneficial effects. Three
cups a day or more is the amount typically taken in Asian countries.
Green tea is usually brewed using 1 to 2 teaspoons of the dried tea in
a cup of boiling water or is steeped for 3 to 15 minutes. Bottles and
cans of prepared green tea are sold under a variety of brand names,
often with additional ingredients.
Green tea extract is also available in capsule form. Three
capsules of green tea extract a day is a common recommended dosage, but
this dosage and its effects remain uncertain. There is wide variation
in the contents of these extracts.
What is the history behind it?
The Chinese have been drinking green tea for at least 3,000
years, and this beverage has been popular in some other Asian countries
for at least 1,000 years. In recent years, scientists have begun to
study its health effects more closely in laboratory and animal studies
and in observational human studies.
What is the evidence?
Many laboratory studies in cell cultures and animals have
shown green tea acts against cancer cells. Test tube studies have
suggested that compounds in the tea may help stop new blood vessels
from forming, thereby cutting off the supply of blood to cancer cells.
It is tempting to assume that it may therefore help prevent some
cancers, but studies in humans have been mixed. Most human studies have
been epidemiologic studies in East Asia, in which researchers compared
tea drinkers with non–tea drinkers while trying to account
for other lifestyle differences. These types of studies are complex,
and it is often hard to draw firm conclusions from them.
Large population-based studies in East Asia generally have not
found that green tea drinkers have a lower risk of breast, stomach, or
colon cancer than non–tea drinkers. One study found that
Asian-American women who drink green tea regularly have a lower risk of
breast cancer than those who do not. A Chinese study found that green
tea drinking was linked to fewer cancers of the esophagus for people
who did not smoke. On the other hand, a 2006 Japanese study showed that
those with cancer of the esophagus were more likely to be green tea
drinkers than those who did not have the cancer. Other studies of green
tea’s ability to prevent or treat lung, prostate, bladder, or
other types of cancer have yielded similarly mixed results.
While the results of laboratory studies have been promising,
at this time the available scientific evidence does not support claims
that green tea can help prevent or treat any specific type of cancer in
humans. Controlled, randomized clinical trials are needed to determine
its effectiveness. Several studies are currently under way.
Are there any possible problems or
complications?
This product is sold as a
dietary supplement in the United States. Unlike companies that produce
drugs (which must provide the FDA with results of detailed testing
showing their product is safe and effective before the drug is approved
for sale), the companies that make supplements do not have to show
evidence of safety or health benefits to the FDA before selling their
products. Supplement products without any reliable scientific evidence
of health benefits may still be sold as long as the companies selling
them do not claim the supplements can prevent, treat, or cure any
specific disease. Some such products may not contain the amount of the
herb or substance that is written on the label, and some may include
other substances (contaminants). Though the FDA has written new rules
to improve the quality of manufacturing processes for dietary
supplements and the accurate listing of supplement ingredients, these
rules do not take full effect until 2010. And, the new rules do not
address the safety of supplement ingredients or their effects on health
when proper manufacturing techniques are used.
Most such
supplements have not been tested to find out if they interact with
medicines, foods, or other herbs and supplements. Even though some
reports of interactions and harmful effects may be published, full
studies of interactions and effects are not often available. Because of
these limitations, any information on ill effects and interactions
below should be considered incomplete.
Moderate intake of green tea is generally considered safe.
Asians have consumed this tea for thousands of years with few dangerous
side effects. However, some people may have allergic reactions and
should stop drinking it should reactions occur. Drinking large amounts
of tea may cause nutritional and other problems because of the caffeine
content and the strong binding activities of the polyphenols, which can
make it harder for the body to absorb certain medicines and iron
supplements. Always tell your doctor and pharmacist about any herbs and
supplements you are taking.
Possible effects from too much caffeine are the major concern
with green tea. Too much caffeine can lead to nausea, trouble sleeping,
and frequent urination. Because caffeine acts as a stimulant, people
with irregular heartbeats or who have anxiety attacks should be
cautious in its use.
Women who are pregnant or breast-feeding should not drink
green tea in large amounts. Caffeine can cross the placenta and affect
the fetus and can also be passed along in breast milk. Relying on this
type of treatment alone and avoiding or delaying conventional medical
care for cancer may have serious health consequences.
Additional Resources
More information from your American Cancer
Society
The following information on complementary and alternative
therapies may also be helpful to you. These materials may be found on
our Web site (www.cancer.org)
or ordered from our toll-free number (1-800-ACS-2345).
References
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Reduced risk of esophageal cancer associated with green tea
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http://www.mskcc.org/mskcc/html/69247.cfm. Updated August 14, 2007.
Accessed June 5, 2008.
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Note: This information may not cover
all possible claims,
uses, actions, precautions, side effects or interactions. It is not
intended as medical advice, and should not be relied upon as a
substitute for consultation with your doctor, who is familiar with your
medical situation.
Last Medical Review: 11/01/2008
Last Revised: 11/01/2008
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