The study took place in northeastern Japan, where 80 percent of the population drinks green tea. Researchers looked at more than 40,000 adults and compared those who drank less than one cup of tea a day to those who drank three to five cups a day. They found that over the span of 11 years, those who drank more tea were less likely to die of heart disease. Epidemiologist Shinichi Kuriyama from Tohoki University School of Medicine headed the study.

"We think our findings may explain different mortality profiles between Japan and the U.S," Kuriyama says. "Now, the Japanese have longest longevity in the world."

The study showed that women saw a greater decrease in heart disease than men. For women who drank five or more cups of green tea daily, there was a 31 percent lower risk of death from heart disease. For men, the risk was reduced by 22 percent. For both men and women the biggest decrease was in death due to stroke.

Green tea is made by a simpler process than black tea. Tea leaves are steamed immediately after picking. When black tea is made, tea leaves are fermented in a humidity-controlled room, then dried -- a longer process that may remove some antioxidants.

Joe Vinson is a biochemist at the University of Scranton in Pennsylvania who studies antioxidants and food. Vinson says the results of the study are important but not conclusive.

A rigorous long-term clinical trial would be needed, he says, to solidly establish cause and effect.