"The most fascinating thing is, to my knowledge, there is no other natural product known that has such diversified effects," says Hasan Mukhtar, vice chair of dermatology at the University of Wisconsin-Madison.
Writing in the July issue of the journal Life Sciences. they note, scientific investigations of tea and the compounds found in it began less than 30 years ago, and most have been conducted in just the past five years.
Name the ailment, and research suggests tea might protect against it. Most of the studies are either population-based — for example, research shows that prostate cancer is less common in countries where people drink a lot of green tea — or in lab dishes or animals, none of which provide conclusive evidence for humans. But Mukhtar and other tea researchers point to tea's 5,000-year track record of safety and say at the very least, drinking tea can't hurt, and, most likely, it can help.
U.S. sales figures from the Tea Association suggest that, despite the dearth of human studies, many Americans already regard tea as a health drink. Last year was the 15th consecutive year that consumers bought more tea than ever, says the trade group. Retail supermarket sales neared $2 billion in 2006, the group says.
Green tea seems to have more health cachet than black tea, perhaps because it has been the focus of more research. Although not as well-studied as green tea, black tea probably is at least as beneficial, says Mukhtar, who drinks two cups of black tea and two of green a day.
Tea polyphenols, compounds with antioxidant activity, may protect against heart disease and a variety of cancers, Mukhtar says. His own research has shown that green and black tea, when substituted for drinking water, inhibits the growth of human prostate cancer cells implanted in mice. In addition, Mukhtar has reported that topical application or ingestion of green tea polyphenols protects against skin cancer in mice.
Human clinical trials by Iman Hakim, a professor at the Arizona Cancer Center at the University of Arizona, suggest that compounds in green tea positively affect genes involved in cancer susceptibility and DNA repair, although not everyone will respond equally well.
Also, Hakim says, an ongoing clinical trial of former and current smokers with chronic obstructive pulmonary disease has found a "significant improvement" in levels of HDL, or good, cholesterol in volunteers given tea as opposed to a placebo drink.
"If tea proves to be good, it might be good to switch the kids to tea," says Hakim, who notes that tea contains less caffeine than coffee does.
John Foxe says he gives his 3-year-old six or seven cups of milky tea a day. Foxe, professor of neuroscience, biology and psychology at City College of the City University of New York, has conducted clinical trials of theanine, an amino acid in tea that, unlike polyphenols, is small enough to cross the blood/brain barrier. It is present in equal amounts in black, green and oolong teas.
"Probably quite a lot of people have heard tea has cardiovascular effects," says Foxe, whose work was financed by Unilever, maker of Lipton tea. "But that's not why people drink tea. They drink it because it makes them feel good."
At an international symposium on tea and human health last month in Washington, Foxe reported that people who drank a solution containing about as much theanine as 10 cups of tea were able to focus better on tasks than those who drank a placebo solution.
More recently, Foxe says, he has found that as little as 100 milligrams of theanine enabled people to focus better on complicated tasks, but only when consumed with 60 milligrams of caffeine — a combination found in roughly four cups of green tea (which contains half as much caffeine as black).
"There was a profound synergistic effect," he says. "My take is, we're all self-medicating with this."
Just because drinking tea might be good for you doesn't mean adding tea extract to cereal and other foods or to dietary supplements is beneficial. "A lot of that is gimmick," Mukhtar says of products that tout tea extract as an ingredient.
A paper published in April suggests one danger of such products with high doses of green tea extracts. "There are quite a few case reports on liver damage due to taking supplements that contain tea extracts," says biochemist Chung Yang, a Rutgers University cancer researcher who co-wrote the paper in the journal Chemical Research Toxicology.
Liver function returned to normal when those affected stopped taking the supplements, Yang and his co-authors write. In addition, they say, studies in rodents and dogs suggest that high doses of tea catechins can damage the kidneys and intestine as well as the liver.
Tea drinkers shouldn't worry, though, Yang emphasizes: "It's very clear, there are no published reports concerning toxicity due to tea consumption."