If you're going to drink green tea in hopes of preventing heart disease, you should start sipping before your arteries begin to harden.

A new animal study suggests that while an important antioxidant in green tea can help prevent the formation of plaques that can block blood flow, it has no effect on the fatty deposits once they have formed. Researchers at Cedars-Sinai Medical Center in Los Angeles report the finding in the May 25 issue of Circulation.

The study used the antioxidant epigallotcatechin-3-gallate (EGCG), the most powerful of the flavonoids that have been shown to provide protection against heart disease and cancer.

The EGCG, provided by Lipton Tea, was injected into mice that were genetically predisposed to rapid development of plaque whose arteries had been injured to spur that development. Other mice of the same strain with similar damage did not get the antioxidant.

Examination of the arteries after three and six weeks showed that the formation of new plaque in mice who got EGCG was reduced significantly, while plaques continued to form in the mice that did not get the antioxidant. However, the treatment had no effect on plaque that existed when the injections began.

"It appears that antioxidant therapy would have therapeutic benefits only if initiated during a critical window very early in the formation of plaque," said study author Dr. Kuang-Yuh Chyu, an assistant professor of medicine at the University of California, Los Angeles.

Antioxidants are believed to prevent atherosclerosis by protecting the delicate inner surface of the blood vessels. But while antioxidants have worked in laboratory tests and animal studies, results in human trials have been disappointing.

Most animal studies "are started when the animals are young, while randomized clinical trials typically enroll adult patients with varying stages of plaques," Chyu noted.

The study is "a small step toward understanding why the antioxidant story is very complex," said Dr. Robert A. Vogel, a professor of medicine at the University of Maryland School of Medicine who has done research in the field.

"We think antioxidants are good," Vogel said. "However, when you look at the many human trials that have been undertaken with antioxidants, the results have been disappointing."

There is always a difference between animals kept under carefully controlled conditions and "free-living human beings doing lots of good and bad things," Vogel said.

As for the timing of antioxidant use, "until a trial in humans shows that they reduce atherosclerosis, we don't know if they will be effective early, late or any time," he said.

There is no harm and some possible good in drinking green tea, Vogel said, but he advised against antioxidant supplements.

"Data on vitamin supplements to prevent heart disease is totally lacking," he said.

More information

An explanation of how antioxidants work is offered by the American Heart Association. The University of Nebraska Medical Center has more on the health benefits of green tea.

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