Saturday, November 04, 2006

How Tea Helps

One way the brew saved lives in the past is that boiling water kills disease-carrying bacteria. Now researchers are investigating what’s behind the other health-giving properties as well. Tea leaves come from a tropical and subtropical evergreen known to scientists as Camellia sinensis. All 3000 different varieties of tea consumed in the world come from this one plant and its hybrids. After the tea leaves are harvested, it’s the method of processing that turns them into one of three basic forms of tea—green, oolong and black. More than three-quarters of the tea harvested in the world becomes black tea, the form consumed by most Americans and Europeans. Leaves are crushed and exposed to the air to undergo chemical changes before they are dried. This turns the leaves brown and accounts for black tea’s distinctive taste. Green tea, the oldest form, is favored mainly in Japan and China. Here the leaves undergo less processing; only heated and dried, they retain their green color. Oolong is a form halfway between black and green. All three types have healthful effects and contain biochemical compounds called polyphenols, which include flavonoids. Also found in fruits and vegetables, flavonoids are antioxidants, which prevent the cell damage thought to contribute to more than 50 diseases.

Green tea has been found to contain the flavonoid EGCg, a potent antioxidant. Black tea has similar disease-preventing effects, reports biochemist Allan Conney of Rutgers University. Researchers have not yet determined whether decaffeination removes tea’s health benefits. In one Dutch study, men who drank between four and five cups of black tea a day had a nearly 70 percent reduced risk of stroke compared with those who drank two cups or less. Another 1993 study reported that higher black-tea consumption corresponded with fewer fatal heart attacks. “The key protective factor does appear to be the flavonoids,” says John Folts, director of the University of Wisconsin Medical School’s Coronary Artery Thrombosis Research and Prevention Center. He has found that black-tea flavonoids inhibit blood platelets from clumping, preventing the dangerous clots that lead to almost all heart attacks and most strokes. Other studies have found that some tea drinkers have lower cholesterol levels and lower blood pressure—although it’s unclear if tea is the actual cause.

More than 20 studies on animals have indicated that tea may prevent some cancers, including those of the digestive and respiratory tracts and the skin. Once again, polyphenols are thought to be the major disease-preventing ingredients. “Along with eating plenty of fruits and vegetables, drinking tea may turn out to be a cheap and practical way to reduce the risk of certain cancers,” asserts Weisburger.

Researchers at Case Western Reserve University in Cleveland found that applying green tea was up to 90 percent effective in preventing sunburns, which can lead to skin cancer. Says dermatologist Hasan Mukhtar, who headed the unpublished study, “In the future I expect it to be an ingredient in sunscreens.” Scientists caution, though, that tea may be protective against some cancers but not others because of the disease’s different causes.

Finally, since tea contains fluoride, it can strengthen tooth enamel and help prevent tooth decay. In laboratory studies, Japanese researchers found that tea also keeps dental plaque from forming and kills some oral bacteria that can cause gum disease.

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