The polyphenols present in green tea plants or herbs could pose health risks to humans if extracted and packaged in highly concentrated doses, says a new University of Toronto study published in the current issue of Free Radical Biology and Medicine.

In small mammals, green and black tea phenolics -- a class of chemical compounds found in plants that include polyphenols -- have been proven to contain antioxidants that help reduce the risk of cancer and cardiovascular disease. Findings such as these have helped to make these teas popular choices among health-conscious tea drinkers around the world.

Working with a team of graduate students, Professor Peter O'Brien of the Leslie Dan Faculty of Pharmacy injected low and concentrated doses of polyphenols into mice. At low doses, "good" polyphenols protected the liver or isolated liver cells against oxygen radicals, while "bad" polyphenols caused liver toxicity at high concentrations.

"The low concentration is roughly equivalent to what people consume when they drink green or black tea," O'Brien says. "But the health benefits are not clear as only a small amount of the polyphenols in the teas seems to get absorbed across the intestine. We won't know how much is absorbed or metabolized without running large clinical trials involving humans."

O'Brien has no plans to stop drinking green or black tea anytime soon, but cautions those who might want to exploit the antioxidant and health promoting properties of tea polyphenols against consuming concentrated doses in pill form as this could create more health problems than it might fix.

"New drugs are subjected to exhaustive clinical trials," he says. "Our findings demonstrate that there simply isn't enough known at this time to substantiate green tea's health-promoting properties if taken in high concentrations."