The British, Chinese and Japanese have been
sipping tea for centuries for pleasure and
medicinal purposes, and now a growing
number of health-conscious people in this
country are jumping on the tea cart.
They're downing cups of tea every day with the hope of keeping disease at
bay.
Some signs of the times: Tea sales have more than doubled since 1990, and
tea producers are crowding grocery store shelves with new varieties of
brews.
What's the new appeal of this ancient drink?
Based on a growing body of research, both green and black teas are being
touted as miracle elixirs that might prevent everything from cancer and heart
disease to arthritis.
It's too early to put so much faith in tea, many
scientists say. Most of the studies have been done
on animals and in the test tube, and full-scale clinical
studies on humans are needed to find out whether
tea can deliver on those preliminary promises.
Right now, early-stage clinical studies are under way at several centers,
including the University of Texas M.D. Anderson Cancer Center in
Houston, the University of Arizona in Tucson and Boston University.
Scientists are trying to determine whether tea has beneficial effects on the
cardiovascular system and which cancers it might protect against. They're
also trying to figure out how much tea you need to drink to get any
presumed health benefits.
Green tea, oolong and black tea, which have been around for 4,000 years,
come from the same plant, Camellia sinensis.
Green tea is most popular in Japan and China, while black tea is favored in
the USA, Great Britain and India. Many herbal teas do not come from the
same plant and have not been studied in the same way.
Much of the early research was done by the Japanese and Chinese on green
tea because that is what they drink, says John Weisburger of the American
Health Foundation, a non-profit research center. But a growing body of
research is accumulating on black tea, and Weisburger believes that when it
comes to possible cancer prevention, green tea and black tea are similar.
So far, scientists think that tea's potential health benefits are tied to its
polyphenols. They are antioxidants that, like the good-guy substances found
in fruits and vegetables, destroy cell-damaging molecules known as free
radicals.
Free radicals are formed as a byproduct of normal metabolism and
exposure to the sun, chemicals and environmental pollutants, including
cigarette smoke. They are believed to contribute to aging and a variety of
diseases.
Besides neutralizing free radicals, tea polyphenols may affect certain enzyme
activity and slow the conversion of normal cells into tumor cells, says C.S.
Yang, a researcher who has studied tea at Rutgers University in Piscataway,
N.J.
Yang says his studies of mice and rats show that green tea and black tea
inhibit cancer formation, but that doesn't necessarily mean they work in a
similar way in blocking cancer formation in humans.
"I think it's a healthy beverage, but more research is needed before we can
conclude that it prevents certain types of cancer," he says.
The caffeine in tea also may play a role. Studies on mouse skin indicate that
regular tea has more cancer preventive activities than decaffeinated, says
Allan Conney, director of the Laboratory for Cancer Research at Rutgers
University's College of Pharmacy.
Yang and Conney believe that, based on animal and human studies, tea's
impact on preventing cancer of the esophagus, mouth and stomach looks the
most promising.
One study in China showed that tea drinking clearly had a protective effect
against cancer of the esophagus in people who didn't smoke or drink, Yang
says. But he cautions against drinking tea that's too hot, because it could
hurt the esophagus.
Weisburger thinks tea holds promise for many other types of cancer,
including breast and prostate.
Results from human dietary assessments of tea's potential effects on cancer
and heart disease are mixed, says Jeffrey Blumberg, a professor of nutrition
at Tufts University in Boston. Some show that tea drinkers have a reduced
risk of cancer, heart disease, arteriosclerosis and cataracts. Some show the
opposite. And some show nothing.
Barbara Howard, vice chairwoman of the nutrition committee for the
American Heart Association, says that although some studies have shown
tea to be beneficial for heart disease, "there are no long-term, large,
randomized trials on humans to prove it's effective."
"We think tea can be a part of a healthy diet," she says, "but there's no
conclusive evidence that it has beneficial effects on heart disease. We don't
want people to think that by drinking large amounts of tea they can ignore
what's proven to be beneficial for heart disease prevention: lowering
cholesterol, lowering blood pressure, stopping smoking, keeping their
weight under control and exercising."
Doug Balentine, manager of tea research at Lipton in Englecliffs, N.J., says
tea may have a lot of different actions in its potential to protect against heart
disease and cancer.
Balentine agrees that the human trials under way are crucial to determining
tea's potential benefits. Researchers are studying tea's effects on the
cardiovascular system, particularly the impact it may have on blood platelets
and healthy blood vessels.
Conney says human studies also need to be done on tea's possible link to
reducing symptoms of arthritis. Tea has been shown to have some
anti-inflammatory properties in animal and cell studies.
There is some real promise for tea, Blumberg says. "I think tea will prove to
be a healthful beverage choice, but it's inappropriate to think that tea or any
other food or beverage is some sort of magic bullet."
Adds Colleen Doyle, director of nutrition and physical activity for the
American Cancer Society: "The science isn't there yet. It looks really
promising, but in terms of human data, the jury is still out.
"I usually tell people, 'If you like tea, drink it, but keep eating your fruits and
vegetables.' We know for sure that fruits and vegetables are protective
against cancer. We can't say that at this time about tea."
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