Drinking the beverage tea has been considered a health-promoting habit since ancient times. The modern medicinal research is providing a scientific basis for this belief. The evidence supporting the health benefits of tea drinking grows stronger with each new study that is published in the scientific literature.
When scientists or research people talk about tea, they mean black, green, white, or oolong teas—all of which are made from the leaves of the Camellia sinensis plant. Herbal brews, like chamomile and peppermint, are not considered tea as they are infusions of other plants with different nutritional characteristics.
What makes the four tea types different from each other is the way the leaves are prepared and how mature they are, which affects both flavor and nutritional content. Black tea is made from leaves that have been withered and then fully oxidized (meaning that chemicals in the leaves are modified through exposure to air). Green tea's leaves are not oxidized. Oolong tea is only partially oxidized, and white tea is not oxidized at all.
Tea, though it has almost no calories, contains a surprising quantity of nutrients and medicinal ingredients. Among the former are vitamins such as thiamin (vitamin B1), riboflavin (vitamin B2), niacin, biotin and inositol. Vitamin E is also present in tea. Tea is also rich in potassium although its content of sodium, a related metal associated with vascular disease when consumed in large quantities, is very low. This makes tea ideal for people suffering from high blood pressure. Tea also contains calcium, zinc and manganese.
All four types of tea are high in polyphenols, a type of antioxidant that seems to protect cells from the DNA damage that can cause cancer and other diseases. It's the polyphenols that have made tea the star of so many studies, as researchers try to figure out whether all that chemical potential translates into real disease-fighting punch. Most research has focused on black tea, which is what about 75% of the world drinks, and green tea, the most commonly consumed variety in China and Japan. Green tea contains an especially high amount of antioxidants—in particular, a type of polyphenol called a catechin, the most active and abundant of which is epigallocatechin-3-gallate (EGCG). That's why there are five times more studies on green than black tea each year.
The most promising claims about tea drinking include the following:
More research needs to be done on other potential benefits. One small study suggested that the catechins and caffeine in green tea may give dieters a small metabolic boost that could amount to burning a few dozen extra calories per day. There's also a slim file on how drinking tea may help ward off osteoporosis and reduce the incidence of cavities, due to the fluoride it contains. And EGCG, that green-tea antioxidant, has been found to increase the number of important immune-boosting cells (called regulatory T-cells)—but only in one animal study.
The vast majority of the research conducted has been observational, meaning scientists can't know if the medical boosts seen in tea drinkers are definitely a result of that habit, or some other factor that makes these people healthier. And many of the studies that have looked at specific compounds in tea have been conducted in labs or on animals, not on people. "These chemicals act as antioxidants in a test tube, but they may not do the same in your body," explains an associate professor in the department of nutrition and exercise science at the School of Biological and Population Health Sciences at Oregon State University.
That said, experts agree that a daily cuppa, or five, won't hurt you, and may well help fight disease. (If you're trying to limit your caffeine intake, go for decaf—it has antioxidants too, though fewer than the caffeinated kind.) "Tea is probably better than a lot of other beverages," says Lona Sandon, RD, assistant professor in the department of clinical nutrition at UT South-western Medical Center and a spokesperson for the American Dietetic Association. "Just make sure you've got other healthy lifestyle habits—you can't count on tea alone to prevent cancer."
Even if we disregard extravagant, scientifically unsupported claims, the established health benefits of tea are numerous. Many of these benefits are preventive, suggesting that a few cups of tea a day can help stave off heart disease, diabetes, hypertension and many forms of cancer.