Steeped in Lore, Green Tea Is an Unknown Quantity to Researchers
Tea is the world’s second most popular drink, after water. But is it more?
Green tea is increasingly touted--by natural-food stores, Web sites and health books--as a natural hedge against chronic ills ranging from cancer, heart disease and arthritis to gum disease and cavities. And as a “restorative.†Even as a force for enlightenment.
“High in the Himalayas, monks spend days chanting and meditating in hopes of reaching complete enlightenment,†reads the wrapper on a bag of Tazo’s Zen green tea. “Periodically, they stop for a cup of tea quite like this.â€
“Scientific research suggests that a diet rich in plant food such as fruits, vegetables and tea may help reduce the risk of some cancers and heart disease,†tea giant Lipton says on its Tea & Health Information Center Web site.
So where’s the proof?
“We have thousands of years of history with green tea and very little clinical research,†said Frank L. Meyskens Jr., director of the cancer center at UC Irvine. “We haven’t proven how it works--or if it works.â€
Research on the health effects of foods such as green tea falls into three categories: epidemiology studies, which compare disease patterns in large populations; laboratory studies in mice and test tubes; and clinical trials in people. With green tea, the epidemiological evidence is mixed, the laboratory studies tantalizing and the clinical trials virtually nonexistent.
Studies in Japan suggest that people who drink five to 10 cups of green tea a day may live longer and be less likely to have cancer or heart disease, Meyskens said. But such studies can’t answer whether tea drinkers tend to be healthier because they drink tea or because of other factors. Epidemiology is a powerful tool for picking up major health effects, such as from smoking.
“But the effect of tea is very small and much more likely to be confounded by other factors,†such as genetic, dietary and lifestyle differences, said Barbara Howard, president of the MedStar Research Institute at Washington Hospital Center and vice chairwoman of the American Heart Assn.’s nutrition committee.
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Health interest in green tea focuses on ingredients called antioxidants, which are also plentiful in fruits and vegetables. Antioxidants help neutralize unstable oxygen molecules called free radicals that are byproducts of metabolism and can damage cells. A particular antioxidant, epigallocatechin-3-gallate, or EGCG, is thought to be the most potent in green tea.
“It’s critical to put this into context,†said Jeffrey Blumberg, chief of the antioxidants research laboratory at the Department of Agriculture’s Human Nutrition Research Center on Aging at Tufts University in Massachusetts.
Studies in mice show that antioxidants in tea have a “cancer-fighting ability, particularly against cancers of the gastrointestinal tract--the colon, stomach and esophagus,†Blumberg said. And laboratory studies suggest that when the antioxidants in tea are added to blood, the blood cells become better able to fight off free radicals. But that doesn’t prove that green tea prevents chronic disease, he said.
And the effect appears modest. Even the most promising data suggest that the antioxidants in green tea might reduce some people’s chances of lung cancer by 20%. By contrast, a pack-a-day smoking habit increases a person’s risk by 1,500%.
To put that in perspective, Blumberg cited a hypothetical smoker who decides to drink a cup or two of green tea a day to offset the risk from smoking.
“I would tell that person: ‘You just moved your risk down from 15 times normal to 14.8 times normal. Maybe,’ †he said.
The three basic types of tea--black, green and oolong--all come from the leaves of the same evergreen plant, Camellia sinensis. The difference lies in the processing. Green tea is steamed and dried rapidly, to prevent fermentation. (Herbal tea is a different bag of leaves. It’s made from other plants and spices--and often contains no tea at all.)
Most tea consumed in the United States is black tea, although green is growing in popularity. Until recently, it was available mainly in the form of loose tea. Now several brands come in bag form, and green-tea extract is sold as a supplement in capsules.
Although the animal studies are promising for antioxidants in tea, the “human evidence just isn’t there,†said Bonnie Liebman, director of nutrition at the Center for Science in the Public Interest.
“It would be great, but that’s not the same as it is great,†Liebman said.
Nor does an effect on blood cells in a test tube prove that antioxidants in tea can prevent heart disease by blocking the buildup of fatty deposits along the insides of human artery walls, MedStar’s Howard said.
“The heart disease process is happening in the blood vessel wall. It’s not happening in the bloodstream,†she said.
“Experience with more famous antioxidants--vitamin C, vitamin E and beta carotene--is further cause for caution. All three showed promise for blocking cancer and heart disease in laboratory studies, but the most recent studies in humans have been disappointing.
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The gold-standard evidence sought by scientists is the controlled clinical trial, in which a drug or treatment is tested against an identical-looking but inactive placebo in comparable patients randomly divided into two groups. Unfortunately, Blumberg said, “we really don’t have much in the way of randomized clinical trials for tea.â€
Only two clinical trials of green tea in cancer patients are underway, according to the National Institutes of Health. Both are still recruiting patients. One, at Memorial Sloan-Kettering Cancer Center in New York, will compare two types of diet in men with prostate cancer. The other, at Sloan-Kettering and the University of Texas’ M.D. Anderson Cancer Center in Houston, will try to determine how much green-tea extract cancer patients can take before experiencing side effects from caffeine. Brewed green tea contains about half as much caffeine as coffee.
Green tea’s bottom line? It’s a safe, inexpensive, noncaloric drink that tastes good. And two cups of tea contain about as many antioxidants as a serving of fruit or vegetables, Blumberg said.
“But,†he said, “tea is not a magic bullet for preventing cancer or heart disease.â€
