根据下面资料，回答题
We have heard a lot about the health benefits of tea, especially green tea. It is high in poly-phenols, compounds with strong antioxidant activity that in test-tube and animal models show anti-cancer and heart-protective effects. Good clinical studies are few, however, and although physi-cians tell their patients to drink green tea, there hasn´ t been any definite proof of the value of that advice.
A team of Japanese researchers was able to link green tea consumption with decreased mortali-ty from many causes--including heart disease. The researchers tracked 40,530 healthy adults ages 40 to 79 in a region of northeastern Japan where most people drink green tea, following them for up to 11 years. Those who drank five or more cups of green tea a day had significantly lower mor-tality rates than those who drank less than one cup a day. There were also fewer deaths from cardiovascular disease.
But no such association was seen with deaths from cancer. Nor was consumption of oolong or black tea connected with any decrease in mortality. Those teas are easier to be combined with oxy-gen in processing, which not only darkens the color of the leaves and changes their flavor but also reduces their polyphenol content.
Coffee is more complicated. It has received both gold stars and black marks in medical litera-ture. It, too, contains antioxidants, although they are less well studied than tea polyphenols. Evi-dence for the health benefits of coffee is growing, however. A group of investigators from Finland,Italy and the Netherlands reports that coffee seems to protect against age-related decline in mental capacity. The scientists studied 676 healthy men born from 1900 to 1920 and followed them for 10 years, using standardized measures of brain function. Their conclusion: the men who consumed
coffee had significantly less decline in mental capacity than those who didn´ t. Three cups a day seemed to provide the most protection.
Population studies like those help us form assumptions about relationships between dietary hab-its and long-term health. We still have to test our suppositions in controlled conditions, and measure the effects of coffee and tea on various systems of the body.
Physicians often tell their patients to drink green tea because__________．