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Tea
And Cancer Prevention
 Tea drinking is an
ancient tradition dating back 5,000 years in China and India. Long
regarded in those cultures as an aid to good health, researchers now
are studying tea for possible use in the prevention and treatment of
a variety of cancers. Investigators are especially interested in the
antioxidants-called catechins-found in tea. Despite promising early
research in the laboratory, however, studies involving humans so far
have been inconclusive.
1. What are antioxidants?
The human body constantly produces unstable molecules called
oxidants, also commonly referred to as free radicals. To become
stable, oxidants steal electrons from other molecules and, in the
process, damage cell proteins and genetic material. This damage may
leave the cell vulnerable to cancer. Antioxidants are substances
that allow the human body to scavenge and seize oxidants. Like other
antioxidants, the catechins found in tea selectively inhibit
specific enzyme activities that lead to cancer. They may also target
and repair DNA aberrations caused by oxidants.
2. What is the level of antioxidants found
in tea?
All varieties of tea come from the leaves of a single evergreen
plant, Camellia sinensis. All tea leaves are picked, rolled, dried,
and heated. With the additional process of allowing the leaves to
ferment and oxidize, black tea is produced. Possibly because it is
less processed, green tea contains higher levels of antioxidants
than black tea.
Although tea is consumed in a variety of ways and varies in its
chemical makeup, one study showed steeping either green or black tea
for about five minutes released over 80 percent of its catechins.
Instant iced tea, on the other hand, contains negligible amounts of
catechins.
3. What are the laboratory findings?
In the laboratory, studies have shown tea catechins act as powerful
inhibitors of cancer growth in several ways: They scavenge oxidants
before cell injuries occur, reduce the incidence and size of
chemically induced tumors, and inhibit the growth of tumor cells. In
studies of liver, skin and stomach cancer, chemically induced tumors
were shown to decrease in size in mice that were fed green and black
tea.
4. What are the results of human studies?
Although tea has long been identified as an antioxidant in the
laboratory, study results involving humans have been contradictory.
Some epidemiological studies comparing tea drinkers to non-tea
drinkers support the claim that drinking tea prevents cancer; others
do not. Dietary, environmental, and population differences may
account for these inconsistencies.
Two studies in China, where green tea is a mainstay of the diet,
resulted in promising findings. One study involving over 18,000 men
found tea drinkers were about half as likely to develop stomach or
esophageal cancer as men who drank little tea, even after adjusting
for smoking and other health and diet factors. A second study at the
Beijing Dental Hospital found consuming 3 grams of tea a day, or
about 2 cups, along with the application of a tea extract reduced
the size and proliferation of leukoplakia, a precancerous oral
plaque.
However, a study in the Netherlands did not support these findings.
It investigated the link between black tea consumption and the
subsequent risk of stomach, colorectal, lung, and breast cancers
among 58,279 men and 62,573 women ages 55 to 69. The study took into
account such factors as smoking and overall diet. It found no link
between tea consumption and protection against cancer.
5. Is NCI evaluating tea?
National Cancer Institute (NCI) researchers are also investigating
the therapeutic use of green tea. One recently completed but
unpublished NCI trial studied the antitumor effect of green tea
among prostate cancer patients. The 42 patients drank 6 grams of
green tea, or about 4 cups, daily for four months. However, only one
patient experienced a short-lived improvement, and nearly 70 percent
of the group experienced unpleasant side effects such as nausea and
diarrhea. The study concluded drinking green tea has limited
antitumor benefit for prostate cancer patients.
Other ongoing NCI studies are testing green tea as a preventive
agent against skin cancer. For example, one is investigating the
protective effects of a pill form of green tea against sun-induced
skin damage while another explores the topical application of green
tea in shrinking precancerous skin changes. For more information
about NCI-sponsored studies on green tea, go to
http://www.cancer.gov/clinical_trials/ .
Source: National Cancer Institute |
