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Special to msnbc.com
updated 11/10/2006 3:24:11 PM ET 2006-11-10T20:24:11

Headlines announcing a recent Journal of the American Medical Association study linked consumption of green tea with protection from stroke, but not cancer, caught many people by surprise. Tea provides a good example of why we need to consider both laboratory and population studies in choosing steps to lower cancer risk and promote overall health.

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Green tea contains relatively high amounts of a type of phytochemical called EGCG, proven in laboratory studies to be a powerful antioxidant. A variety of antioxidants from plant foods seem to stabilize “free radicals,” protecting DNA from damage that could allow cancer to develop, and protecting blood vessels from damage that could promote blockages. Studies show EGCG can also increase production of enzymes that protect us from cancer-causing substances, decrease levels of a substance key to promoting cancer development, and increase cancer cell self-destruction.

In most studies checking signs of green tea’s effects in people, antioxidant levels in the blood increase, showing we can absorb antioxidant compounds in tea. For example, in a study of 133 heavy smokers — who are exposed to DNA damage from smoking — four months of drinking four cups (8 ounces each) of decaffeinated green tea daily reduced the signs of DNA damage by 31 percent.

But large population-based studies of the impact of green tea show much less consistent results.

Conflicting - and confusing - results
Some link higher green tea consumption with lower risk of colon, stomach, breast, prostate and other cancers. Other, similar-type studies, show no effect. In the recent study of more than 40,000 Japanese adults, those who drank at least one, 3 ounce cup of green tea daily suffered 25 percent fewer deaths due to cardiovascular disease than those who did not drink it daily. But, deaths due to stomach, lung or colorectal cancer did not decrease, even with five or more 3 ounce cups daily.

It’s possible we might see cancer protection if we look at those drinking the equivalent of three or four 8 ounce cups daily, which some researchers suggest may be needed for cancer protection.

Do benefits of soy decrease protection of tea?
Benefits in the effects seen in tea drinkers may come from other healthy habits. Differences of study findings may also mean green tea offers more benefit for some people than others, depending on diet or exposure to risk-producing substances such as tobacco or pollution. One study, for example, suggests tea and soy may produce similar benefits, and consuming one may decrease the chances of benefit from the other. That might explain why cancer protection was not seen in the recent study of Japanese adults, almost all of whom ate soy foods daily.

Genetics may explain part of why population studies tend to show large variation in how study participants are affected by tea. The antioxidant benefits of tea, which could help reduce blockages in blood vessels, may not be strongly affected by genetics. However, research has shown genetic differences will influence how people respond to phytochemicals in tea that can lead to stopping cancer-causing substances. This may mean green tea offers more cancer protection for some people than others.

Green tea doesn't replace exercise
Simply drinking green tea can’t save us from the damages caused by overeating, a poorly-balanced diet, or a sedentary lifestyle. Today’s research identifies a plant-based diet, a healthy weight, physical activity, and avoiding tobacco as the most effective steps to lower cancer risk. For those who would like to do more to reduce their risk, replacing three or four cups of coffee or soda per day with green tea is still a reasonable step that may bring a variety of health benefits.

Nutrition Notes is provided by the American Institute for Cancer Researchin Washington, D.C.

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