National Institutes of Health/National Cancer Institute -- December 5,
2002 -- 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 (1).
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 (1).
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 (1, 2).
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 (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).
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 (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/. |
