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 for tea and cancer prevention?
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 the NCI (National Cancer Institute) 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.
The above information has been
provided with the kind permission of the National Cancer Institute, National
Institutes of Health (www.nci.nih.gov/).
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