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In the latte-obsessed United States, tea is gaining ground as scientists and the public learn more about its benefits.
A growing body of research suggests that the world's second-most-consumed beverage — only water is more popular — helps prevent cardiovascular disease, burn calories and ward off some types of cancer.
"We don't clearly understand why tea is so beneficial, but we know it is," said Thomas G. Sherman, an associate professor in the Department of Pharmacology and Physiology at Georgetown University Medical Center. "There are lots of epidemiological studies, and so of course people see these studies and want to drink tea and gain these benefits."
Nationally, tea purchases have risen for 20 consecutive years, annual supermarket sales have surpassed $2.2 billion, and away-from-home consumption of tea has grown by at least 10 percent a year for a decade, according to the Tea Association of the USA, a New York-based industry group. On any given day, the association says, 160 million Americans drink tea.
Although coffee is still king in the United States, change is brewing. Department of Agriculture statistics show tea drinking has increased as coffee drinking has declined: Per-person tea consumption was nine gallons in 2009, up from 7.3 gallons in 1980; per-person coffee consumption was 23.3 gallons in 2009, down from 26.7 gallons in 1980, about half what it was in the mid-1940s. And while studies also show that coffee is associated with many health benefits, including helping protect against diabetes and Parkinson's disease, a typical cup has much more jitter-producing caffeine than tea does.
Manelle Martino, co-owner of Capital Teas in Washington, said she has seen the explosion of interest in tea firsthand. Her sales of loose-leaf tea have risen substantially each year since she opened the business in 2007, she said. "We started the tea company with one shop. Now, there are six stores in the D.C. area," she said. "People are becoming more health-conscious. You have baby boomers who are into preserving their youth. You see them wanting to take better care of themselves."
Tea comes from the leaves of the warm-weather evergreen Camellia sinensis, and it is classified into five types: black, white, green, oolong and puerh.
Experts say all are healthful. Many scientists link health benefits to tea's polyphenol antioxidants, which protect against oxidative stress, but others say they don't know exactly which chemicals or combinations of chemicals in tea produce the benefits. Sherman, for example, said there's no evidence connecting tea's antioxidants to beneficial effects, and he pointed to a study showing that black tea reduces LDL, or "bad," cholesterol without affecting antioxidant levels, suggesting something else in tea is causing this.
Numerous epidemiological studies — which establish correlation, not cause and effect — focus on tea's role in reducing cardiovascular disease, the nation's biggest killer.
A 2004 paper in the Archives of Internal Medicine, for instance, looked at hypertension rates among people who drank tea for at least a year. The study, conducted in Taiwan, found that those who drank about four ounces to 20 ounces of tea a day had a 46 percent lower risk of developing high blood pressure than people who didn't drink tea regularly.
Another paper, published in 2002 in the American Journal of Clinical Nutrition, culled through the results of a large study concerning chronic disease and found that people drinking the greatest amount of tea — more than 12 ounces a day — had barely half the risk of heart attack as people who did not drink tea.
More recent cardiovascular research was presented in September at a symposium at the Department of Agriculture in Washington. One study found that black tea reduced blood pressure in all participants and counteracted the detrimental effects of high-fat meals in people with high blood pressure.
"The more tea you drink, the better," Sherman said. "It's astounding, really."
As for cancer prevention, the evidence is less compelling.
A review of studies published in the Journal of Cancer Research and Clinical Oncology last year suggested that consuming 10 Japanese-size cups of green tea a day helps prevent several cancers and protect against recurrence of colorectal cancer. And a 2006 review in the Journal of the American College of Nutrition of the beneficial effects of green tea noted studies linking tea to reduced risk of ovarian, prostate and breast cancers.
But that review also cited some conflicting research. For example, two studies revealed a breast cancer benefit, one showing a decreasing risk among Asian American women with rising tea intake and another showing a lower risk of recurrence among Japanese patients who drank three or more cups a day. But a larger Japanese study of more than 35,000 women concluded tea intake didn't affect the risk of breast cancer.
And scientists at the Johns Hopkins Kimmel Cancer Center announced last week that black and green teas (and coffee) were among the foods and flavorings that affected a gene linked to cancer. But they said this doesn't mean people should stop drinking tea and coffee, only that more research is needed.
Studies have also examined whether tea affects weight loss. One, from 2004, found that caffeine, theanine and perhaps other components in green tea powder suppressed weight gain and fat accumulation in laboratory mice.
There is also some evidence that drinking tea promotes digestive health generally. Gerry Mullin, an associate professor of medicine at Johns Hopkins School of Medicine and author of "The Inside Tract: Your Good Gut Guide to Great Digestive Health," said tea appears to help control glucose and insulin and keep the gastrointestinal system running well.
"At the end of the day, these teas are anti-inflammatory in nature. They have anti-bacterial properties," Mullin said. "They boost the immune system and provide a lot of different benefits."