Dec. 2, 1999 (Atlanta) -- Lately, green tea has been touted as an elixir for a myriad of illnesses, including heart disease and cancer. However, a researcher from Rutgers University says that while preliminary data point to tea's curative powers in animals, nothing has been proven in humans. In fact, writes Chung S. Yang, PhD, in an editorial column in the November/December issue of the journal Nutrition, "if such beneficial effects do exist in humans, they are likely to be mild."
Tea has been a popular beverage for centuries in countries like China, Japan, India, and England. In fact, tea consumption in the world ranks second only to water consumption. Recently, the United States has experienced a surge in tea drinking. But tea's unique flavor is not the only reason consumers are increasing their tea consumption. Recent research has shown that tea may protect against several types of cancer, it may reduce the risk of heart disease and stroke, and it has greater antioxidant power than many fruits and vegetables. That is, if you drink a lot of it.
The three basic tea types come from the evergreen shrub Camellia sinensis. Black, green, and oolong teas are produced from the tea plant by varying the processing conditions. Black tea is produced by fermenting the leaves, while green tea leaves are not fermented. Oolong teas go through a shorter fermentation period than black teas and are regarded as semi-fermented. All three kinds go through a heating process to halt fermentation.
Although most people drink black tea (made from fermented leaves), about a quarter of all tea consumed is green (unfermented). All teas from Camellia sinensis contain compounds known as polyphenols -- a class of bioflavonoids -- which are found in all plants. Polyphenols have anti-cancer, antioxidant, anti-bacterial, and anti-viral properties. Besides tea, these compounds are especially high in coffee, red grapes, kidney beans, raisins, prunes, and red wine.
It is believed that green tea guards against cancer by scavenging for free radicals, which are the by-products from all the chemical reactions that occur in the body -- like the exhaust from an automobile. Free radicals can damage the cells that block the action of cancer-causing agents, or carcinogens, and detoxify them. Tea polyphenols also may limit cell replication, the primary characteristic of cancer.
In one widely publicized study, researchers at the Case Western Reserve University School of Medicine, in Cleveland, reported that an ingredient in the polyphenols in green tea kills cancer cells while sparing healthy cells. In the study, reported in the Dec. 17, 1997, issue of the Journal of the National Cancer Institute, the researchers tested this ingredient, EGCG, on cancerous human and mouse cells of the skin, lymph system, and prostate and on normal human skin cells. They found that EGCG caused cell death in cancer cells while leaving healthy cells unharmed.
Purdue University (West Lafayette, Ind.) researchers Dorothy Morre and D. James Morre reported in December 1998 at the 38th annual meeting of the American Society for Cell Biology in San Francisco, that EGCG inhibits an enzyme called NOX. This enzyme helps carry out several cell functions and is required for growth in both normal and cancerous cells. The overactive and cancer-causing form of NOX is known as tNOX.
"Our research shows that green tea leaves are rich in this anti-cancer compound, with concentrations high enough to induce anti-cancer effects in the body," Dorothy Morre, professor of foods and nutrition in Purdue's School of Consumer and Family Sciences, tells WebMD. "Drinking more than four cups of green tea a day," she says, "could provide enough of the active compound to slow and prevent the growth of cancer cells. Granted, for most people that's a lot of tea."
No one is saying, yet, that drinking green tea cures cancer or heart disease in humans. In fact, in testing tea's effects, researchers have used a strong concentration -- about 100 times what the Lipton Company estimates is in one cup of tea. More laboratory research plus human studies are needed to see whether extracts of green tea can be effective as drugs to prevent cancer and stop prostate and other tumors from growing.
Yang says that if in fact tea does, people would need to slurp 3-10 cups a day for maximum protection from common forms of cancer. He says this in itself may be harmful. "Ingestion of large amounts of tea may cause nutritional and other problems because of the caffeine and the strong binding activities of tea polyphenols," notes Yang, who is with the Laboratory for Cancer Research at Rutgers University, in Piscataway, N.J.
Elizabeth Kaegi, MB, reporting in 1998 on behalf of the Task Force on Alternative Therapies of the Canadian Breast Cancer Research Initiative, says that while she believes moderate consumption of green tea appears safe, "because excess caffeine can cause nervousness, insomnia, and irregularities in heart rate, pregnant women, nursing mothers, and patients with cardiac problems are usually advised to limit their intake to two cups." Kaegi is the former director of Medical Affairs and Cancer Control at the National Cancer Institute of Canada and the Canadian Cancer Society.
- Although research has shown that tea may protect against diseases such as heart disease and cancer, nothing has been proven in humans.
- Tea contains polyphenols, which exhibit antioxidant, anti-cancer, anti-bacterial, and anti-viral properties, but a person would have to consume several cups per day to reap the benefits.
- Drinking large quantities of tea, however, may not be so healthy, because the caffeine content could cause nervousness, insomnia, and heart rate irregularities.