Drinking
Tea for 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.
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 (1).
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 (1).
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 (1, 2).
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 (3).
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 (1).
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
(4).
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 (5).
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/.
References:
(1) Dufresne CJ, Farnworth ER. A review
of latest research findings on the health promotion properties
of tea. J. Nutri Biochem 2001; 12 (7): 404-421.
(2) Hakim IA, Harris RB. Joint effects
of citrus peel use and black tea intake on risk of squamous cell
carcinoma of the skin. BMC Derm 2001; 1 (3).
(3) Sun CL, Yuan JM, Lee MJ, Yang CS, Gao
YT, Ross RK, Yu MC. Urinary tea polyphenols in relation to gastric
and esophageal cancers: a prospective study of men in shanghai,
china. Carcin 2002; 23 (9): 1497-1503.
(4) Goldbohm RA, Hertog MG, Brants HA,
van Poppel G, van den Brandt PA. Consumption of black tea and
cancer risk: a prospective cohort study. JNCI 1996; 88 2): 93-100.
(5) Phase II Study of Green Tea Extract
in Patients with Androgen-Independent Metastic Prostate Cancer.
Protocol Ids: NCCTG-N9951. NCI Clinical Trials http://www.cancer.gov/clinical_trials/.
