So far, green tea has been shown to help prevent second heart attacks in people who have already had one, to reduce the infectivity of viruses and bacteria, and to help protect against prostate, breast, stomach and colon cancer.

In the last five years, the US government has funded more than 150 studies on green tea and its constituent chemicals, including an antioxidant, or catechin, called EGCG. Antioxidants can gobble up dangerous forms of oxygen called free radicals and can disrupt chemical pathways inside cells, especially cancer cells. (The concentration needed to kill cancer cells is lower than that which kills normal cells, for unclear reasons.)

Earlier this month, researchers from the University of Rochester
presented data at a conference on diet and cancer in Washington, D.C. suggesting that EGCG seems to targets a particular protein, called HSP90, that is present in higher levels in cancer cells than in normal cells.

At a meeting in April of the American Association for Cancer Research, Italian researchers showed that men at high risk of prostate cancer who took the equivalent of three to four cups of green tea a day as supplements were less likely to develop the cancer than similar men given a placebo.

Hasan Mukhtar, a biochemist and professor of cancer research at the University of Wisconsin School of Medicine has shown that green tea catechins block a substance called insulin-like growth factor-1, thus thwarting the ability of prostate cancer cells to grow.

Black tea, which has more complex antioxidants than green tea, also appears to have health benefits, especially for the heart, said Dr. Murray Mittleman, director of cardiovascular epidemiology at Beth Israel Deaconess Medical Center.