Reality Check continued...
"Scientists can always say something remains to be proven," says Lenore Arab, PhD, professor of epidemiology and nutrition at the University of North Carolina at Chapel Hill School of Public Health.
Studying the effects of tea is difficult because the pattern of consumption varies from country to country -- and even within countries. And understanding the long-term health effects of tea requires long-term studies, she says.
In spite of the difficulties, there is a "large and convincing body of evidence that tea is chemo-preventive," Arab tells WebMD. "What has impressed me most recently is the accumulating evidence of tea's protective role in prostate cancer."
With regard to heart disease, Arab says a recent analysis pooling the results of 12 studies looking at tea consumption in a quarter million people, found that people who drink more green tea than others have less incidence of heart attack. Intriguingly, that effect was greater in Europe than in the U.S., she says.
Tea advocates say most of the health effects derive from "polyphenols," which are the antioxidants in tea. Weisburger explains that it is the oxidized form of cholesterol, for instance, that damages the surfaces of veins and arteries, leading to heart disease. "It turns out that polyphenols in tea prevent oxidation," Weisburger tells WebMD.
Weisburger was keynote speaker at this year's International Scientific Symposium on Tea and Human Health, sponsored by the U.S. Tea Council in Washington. A similar symposium will be held again next year, he tells WebMD.
The American Cancer Society has weighed in with a cautious statement on tea as a cancer preventive. Some animal studies have been shown to reduce risk, "but beneficial effects in people are not proven," according to a 1996 ACS statement on the subject.
"It's only in the last 10 year that Western science has tried to look at antioxidant activities of tea," says Dave Ringer, PhD, scientific program director at the ACS. "It's a young science. Generally, it is felt that tea can inhibit the initiation of cancer and delay its progression in animal studies. But we don't really have large well-controlled epidemiological studies to look at this yet [in humans], because you need to correct for the effects of other dietary components."