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.
Teasing apart tea benefits
So can drinking tea actually improve your health? Studies have yielded inconsistent, though mostly positive, results (be prepared to hear this again). Complicating matters is the fact that the constituents of tea vary, as do the cultural and dietary contexts in which it is consumed, which may also have effects on health. That may help explain, for instance, why Asian studies sometimes find certain benefits while Western ones do not, or vice versa. Your genes and even your colonic microflora (the bacteria living in your large intestine) may also help determine what effects tea has in your body.
Here’s a look at some of the research:
- Cardiovascular health. Of all the potential benefits of tea, those involving cardiovascular disease are “the most promising,” according to Jeffrey Blumberg, Ph.D., professor of nutrition science at Tufts University and chairman of the tea symposium. Many (but not all) observational studies have found that people who consume moderate or high amounts of green or black tea (or flavonoids from all dietary sources) have a reduced risk of cardiovascular disease and, especially, stroke—usually with higher consumption linked to greater benefit. Furthermore, most research has shown that tea can slightly lower LDL (“bad”) cholesterol and blood pressure, as well as improve blood vessel functioning, reduce inflammation, inhibit blood clotting and have other cardiovascular effects. In 2013 a British review of 11 clinical trials concluded that tea seems to improve blood pressure and cholesterol levels, but that longer, better studies are needed to confirm tea’s benefits in terms of cardiovascular disease prevention.
- Cancer. In lab studies, green tea extracts and tea polyphenols have been shown to inhibit a wide variety of cancers; black tea has been less studied. There are many theories as to how tea polyphenols may have anti-cancer effects—for instance, by inducing cell suicide in cancer cells and by inhibiting insulin growth factor (a protein involved with cell proliferation). However, the many human studies looking at the association between tea consumption and various types of cancer—such as breast, colon, prostate, oral, liver, ovarian and lung cancer— have had conflicting results. So farm there is insufficient support for claims that tea can help prevent or treat any cancer.
- Diabetes. Most research has linked green or black tea or compounds in tea (such as catechins) to improved blood sugar control or reduced risk of type 2 diabetes. For instance, a 2012 study in BMJ Open looked at data from 50 countries and found that high consumption of black tea was strongly associated with a reduced diabetes risk. And a 2013 Chinese meta-analysis of clinical trials suggested that green tea helps reduce blood sugar.
- Weight control. There’s some evidence that tea, especially green tea, has a modest weight-loss effect. It’s theorized that tea catechins and caffeine help boost fat burning, at least slightly and temporarily. A 2013 analysis of data from the large National Health and Nutrition Examination Survey linked tea consumption with lower weight and smaller waist size; this was not true of iced tea. But not all studies have found an effect on weight. One reason for the inconsistent findings is that the stimulant effect of caffeine depends largely on whether people are used to it or not; if they consume it regularly, they quickly build up a tolerance. What about concentrated green tea extracts, which are often marketed as diet supplements? A 2012 Cochrane review of 14 clinical trials concluded that they do not cause weight loss in overweight or obese people.
- Brain health and mood. Lab studies support the potential role of tea in improving cognitive function. Studies in people are limited, though. For instance, in a study in Japan in 2006 and one in China in 2008, older people who regularly drank green tea were found to have a reduced risk of cognitive impairment compared to nondrinkers. Tea can increase alertness, thanks largely to its caffeine. It may also help enhance concentration and learning ability because of its theanine (an amino acid), which at the same time promotes relaxation. Of course, the experience of drinking a hot aromatic beverage, along with the associations that come with it, can contribute to the calming effect.
- Parkinson’s disease. A 2008 study in the American Journal of Epidemiology found that people who drank the most black tea—but not green tea—had a much lower risk of Parkinson’s disease than nondrinkers. According to a paper in the Annual Review of Nutrition in 2013, human and lab studies suggest that tea or constituents in it may help protect against Parkinson’s and other neurodegenerative diseases. Coffee seems to be similarly protective.
- Bone health. Lab studies show that tea polyphenols have beneficial effects on factors affecting bone mass and bone strength and thus may help protect against osteoporosis. Bones may also benefit from the fluoride in tea. Most studies of postmenopausal women have linked long-term consumption of tea (usually green and often high intakes) or tea extracts with greater bone density, but there are discrepancies among the results. “More basic research and human studies are warranted,” the researchers at the symposium concluded.
- Dental health. Researchers have found that tea has antibacterial effects and thus may reduce levels of bacteria that cause cavities and contribute to gum disease. Like bones, teeth may benefit from the fluoride in tea. At least two Japanese studies have linked green tea consumption with a reduced risk of tooth loss.
Tea: drinks or supplements?
Tea’s effects in the body are still not fully understood, but don’t expect it to prevent heart disease, cancer or any other conditions on its own. Moreover, it is just one source of flavonoids and other polyphenols—fruits and vegetables, wine, coffee and cocoa are others. Nevertheless, tea can be a healthy addition to your daily diet, especially if it takes the place of high-calorie beverages. All types of tea have something to offer, including small amounts of minerals such as potassium. Without added sugar, tea has negligible calories. Check the labels on bottled and instant teas, however, since some are as sugary as soda.
Skip green tea supplements. It’s not known if they have any benefits or, more importantly, are safe. In any case, they vary widely in composition—ConsumerLab.com found a 15-fold difference in catechin levels in products it tested in 2012. Supplements containing high levels of catechins may interact with medications, and there are continuing concerns about potential liver damage. Some, particularly those marketed for weight loss, may contain very high doses of caffeine.
See also this infographic about the Health Benefits of Tea.