Both kinds of tea leaves are from the same plant, the evergreen shrub Camellia sinens. Black tea is made from the withered, fermented leaves. The leaves sold as green tea have been steamed after picking, and are not fermented. This makes a difference. Green tea, evidence suggests, is more potent in anti-cancer ingredients.
All plants contain compounds called polyphenols.
These are especially high in tea, coffee, red grapes, kidney beans, raisins, prunes, and red wine. In December 1997 researchers at the Case Western Reserve University School of Medicine reported that an ingredient in the polyphenols in green tea kills cancer cells while sparing healthy cells. The scientist in charge, Hasan Mukhtar, says the compound kills cancer cells by fragmenting their DNA - typical of apoptosis. Apoptosis happens all the time in a healthy body. The cells then decide that instead of being murdered, they will commit suicide.