Okay tea drinkers, your turn. A few months ago we explored the latest research about coffee, almost all of it positive. Scientists have uncovered a similar array of potential health benefits for tea—the world’s most popular beverage, after water. Green tea has gotten the most buzz, but black may be just as healthful. Should you drink a cup—or two or three—a day?
In December a dozen review papers from the Fifth International Scientific Symposium on Tea and Human Health were published in the American Journal of Clinical Nutrition. These drew largely on the more than 2,000 studies done on tea in the past few years. Here’s a wrap-up of this and other tea research.
Tea types and components
Tea comes from Camellia sinensis, an evergreen shrub native to East and South Asia. (“Herbal teas” are made from other plants and are not true tea.) Tea has been consumed for thousands of years in Asia, where it is an important part of many cultures and has long been associated with health benefits. In China and Japan, in particular, its preparation and presentation have been taken to the level of an art form.
Green tea, popular in Asia, is minimally processed—the leaves are steamed, rolled and dried. Black tea is withered, rolled or crushed, and then “fermented” (in this case meaning oxidized, or exposed to oxygen) before being dried, which makes it black and stronger in taste. Oolong tea is partly fermented. White tea is harvested in early spring; the young leaves and silvery white buds are just steamed and dried.
Like coffee, cocoa and many plant foods, tea contains hundreds of biologically active chemicals, notably a wide range of flavonoids and other polyphenols, which can be absorbed and used by the body to varying degrees. Accounting for about one-third of the weight of dried tea leaves, polyphenols have antioxidant and other potentially beneficial properties.
The chemical composition of tea depends on the specific botanical variety, how and where it is grown, and how it is processed. Green tea, for example, is rich in catechins, including the potent antioxidant EGCG. In black tea, the catechins convert into other compounds during fermentation. Oolong tea falls between black and green teas in composition. Various polyphenols have different effects in the body.
Other factors affecting tea’s chemistry include its age and how it’s stored, brewed and served. Longer steeping time results in more polyphenols being released, though steeping for more than three minutes or so usually doesn’t increase these compounds significantly. Decaffeinating tea reduces its catechins. Instant and bottled teas generally have lower levels of polyphenols. Some research suggests that adding milk to tea binds catechins and reduces their effects somewhat. Lemon, in contrast, may enhance the body’s absorption of catechins.
Unless it has been decaffeinated, tea averages about 40 milligrams of caffeine per six-ounce cup (versus 100 milligrams in coffee, on average), depending on the type, brewing time, and other factors. Black tea tends to have more caffeine than green.