All tea--black, green or oolong--comes from the same evergreen plant, Camellia sinensis. Green tea leaves are the least fermented.
How it's available: Mostly in the form of loose leaves, but it also comes in tea bags and as an extract in capsules.
The crux: Health interest in green tea focuses on ingredients called antioxidants, which are plentiful in fruits and vegetables and help neutralize harmful molecules called free radicals that are byproducts of metabolism.
Potential for benefit: Tantalizing but unproved. More research is needed to see whether antioxidants in tea help protect against human heart disease and cancer.
Potential for harm: Very slight. More than 16 cups a day could cause caffeine overload and interactions with medical drugs.
Evidence so far: The epidemiology (comparisons of large populations) is mixed, lab studies promising but inconclusive and human tests almost nonexistent.
Unanswered questions: Are tea drinkers healthier than others because of their tea habit, or because they also eat more fruits and vegetables and are less likely to smoke? Will the promising antioxidant results in laboratory animals and test tubes hold true in people?
Convincing claim: "It's a non-caloric beverage. It tastes good. It may even contain things that are beneficial. Two cups of green tea contain about as many flavonoids [a type of antioxidant] as a serving of fruit or vegetables. It's a potentially healthy choice the consumer can make when selecting a beverage." (Jeffrey Blumberg, antioxidants researcher, Tufts University)
Unconvincing claim: "Tea has a miraculous power to prolong life." (East/West Cultural Inc.)
What's next: Researchers at the Department of Agriculture's food laboratories in Beltsville, Md., are conducting chemical tests to determine exactly what is in tea. A few centers are planning clinical trials to see what preventive effect, if any, tea has against disease in people.
Big picture: Even if green tea turns out to have a protective effect against some forms of cancer and heart disease, it is no magic bullet. Nor can tea be expected to offset damage from major health risks such as smoking, poor diet and inactivity.