What’s So Special About Green Tea?
Judging by the headlines on health-news reports, it almost seems that if you drink enough green tea, you could live forever. In scientific journals and the popular press alike, this ancient beverage has been touted for benefits ranging from fighting cancer and heart disease to beautifying your skin and preventing cavities. How much of the recent excitement over green tea is science, not just hype?
Jeffrey B. Blumberg, PhD, director of Tufts’ HNRCA Antioxidants Research Laboratory, has researched the health effects of green tea. “As investigators continue to study the multiple effects that tea has on human health, more research supports tea’s potential in helping reduce the incidence of major diseases,” he says. “In some respects, it is good to think of tea as a plant food, much like fruits and vegetables.”
Despite the headlines, Blumberg adds, the health benefits of tea aren’t limited to the green variety: “Observa - tional studies in Asia show benefits of green tea and observational studies in the US, UK and the Netherlands show benefits of black tea.” Herbal “tea,” however, is not included in such research showing benefits against chronic disease.
Green tea, like other varieties of true tea, comes from the leaves of the Camellia sinensis bush. All tea contains natural antioxidants, especially the class of flavonoids called catechins, thought to benefit the body not only by acting as antioxidants but by modulating biochemical pathways that alter gene expression. Unlike the more familiar (at least until recently) black tea, which is fermented in a process that links many of its catechins into large compounds called oligomers, green tea is minimally processed: The leaves are simply withered and steamed, which preserves a much higher concentration of simple catechins. Green tea also retains more of its principal bioactive catechin, epigallocatechin- 3-gallate (EGCG), popularized as a key ingredient in Coca-Cola's Enviga “weight-loss drink.”
(Oolong tea is partly oxidized, with catechin levels between green and black tea. White tea is made from partly opened buds and young leaves, which are steamed and then dried. So white tea may contain a higher concentration of EGCG than green tea, but it is more expensive and has been less extensively studied.)
Humans have been steeping tea leaves in boiling water for as long as a half-million years, according to archeological evidence. China and India, which were among the first cultures to cultivate tea plants, have traditionally used green tea as a stimulant (one cup contains about 30 milligrams of caffeine), diuretic and astringent. Ancient medical practices credit green tea with protecting the heart, regulating blood sugar, improving digestion, sharpening the mind and even treating flatulence.
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Although the tannins in tea can stain your pearly whites, green tea may actually help prevent cavities. People in Asian countries have traditionally rinsed their mouths with green tea after meals. Studies in both animals and humans suggest there might be something to this practice: In one study, for example, participants replaced brushing and flossing entirely with swishing their mouths with a polyphenol solution similar to green tea. Despite neglecting normal dental health, at the conclusion of the study the participants showed significantly reduced levels of harmful plaque on their teeth. Microbiologists say the polyphenols in tea may inhibit the growth of bacteria that can lead to tooth decay.
Most of the modern evidence for green tea’s health benefits comes from population (epidemiological) studies, rather than from clinical trials. Such observational studies cannot prove cause and effect, however, and it could be that other factors are responsible for the apparent benefits of green-tea consumption. In particular, most of the major studies of green tea have been conducted in Asia, where the beverage is as common as coffee is in the US; Asian people also consume more fish and soy, while eating less red meat than in the typical Western diet. It can be challenging to separate the possible effects of these dietary differences from that of green tea.
On the other hand, Yale University scientists have suggested that green tea might be the explanation for the socalled “Asian paradox”: Why don’t people in Asian countries, where smoking remains more popular than in the US, consistently suffer heart disease and lung cancer at the same rate as Americans? The flavonoids in green tea, researchers suggest, may be partly responsible by maintaining artery function, inhibiting clots and blocking tumor growth.
“Taken altogether, the evidence certainly suggests that incorporating at least a few cups of green tea every day will positively affect your health,” says Diane L. McKay, PhD, another scientist at Tufts’ Antioxidants Research Laboratory who’s studied tea. “It’s not going to cure anything and shouldn’t be consumed as a drug, but it can complement an overall healthy diet.” With that caveat, here’s what’s brewing in research on green tea:
Cardiovascular benefits: In 2006, a large Japanese study found that people who drank five or more cups of green tea daily had a 26% lower risk of death from cardiovascular disease than those averaging less than one cup. The 11-year study of 40,350 adults, ages 40 to 79, showed that the association with green tea was particularly strong for reduced risk of clot-related strokes. Women seemed to benefit the most, possibly because Japanese men still commonly smoke cigarettes.
That same year, however, the US Food and Drug Administration (FDA) rejected a petition to allow green tea makers to promote cardiovascular benefits on their labels. In nixing the request for a “qualified health claim,” the agency said it reviewed more than 100 studies and found “no credible scientific evidence to support qualified health claims about the consumption of green tea or green tea extract and a reduction of a number of risk factors associated with cardiovascular disease.”
| Tea Tips |
To extract the most health-promoting catechins from green tea leaves, steep for a full four minutes. The ideal water temperature for more-delicate green tea is 175 to 185 degrees Fahrenheit—less hot than for oolong (195 degrees) or black tea (212 degrees). Decaffeination reduces the amount of catechins in green tea by 10%-15%, although the amount varies by producer and process.
If you don’t like your tea straight, opt for lemon or other citrus juice instead of milk or cream. It’s not just a question of calories: Purdue University researchers found that adding citrus juice to green tea can boost the level of catechins in the tea that actually make it through your digestive system by as much as 13-fold. The catechins in tea aren’t very stable in non-acidic environments—such as your intestines—so as much as 80% can be lost through digestion. The researchers tested various tea additives using a simulated gastric and small-intestine digestion system, including orange, lemon, grapefruit and lime juice, vitamin C, ordinary milk, soy milk and rice milk.
The citrus juices scored the highest in measurements of four main catechins after a trip through the “digestive system,” followed by vitamin C. Tea with 50% milk also showed higher levels of catechins than plain tea; rice milk improved catechin survival the most of the three milk options.
Subsequent evaluations of the evidence have been somewhat more positive. A 2008 meta-analysis of 133 studies of flavonoids and cardiovascular disease, while mostly negative about other antioxidant-rich foods, concluded that green tea reduced LDL (“bad”) cholesterol levels. To achieve clinically significant reductions, based on that analysis, you’d have to drink two to five cups of green tea daily.
Last fall, a review by the Choleste rol Treatment Center at Concord (NH) Hospital concluded that “preliminary evidence supports the possibility that green tea catechins… may lower LDL.” And a British review found that “a positive effect of green tea catechins on vascular function is becoming apparent.”
Fighting cancer: In 2005, the FDA also rejected a petition to label green tea as protective against breast, prostate and other cancers. Since then, however, evidence has continued to trickle in. McGill University research - ers have shown in the lab that EGCG from green tea inhibits the growth of cancer cells. A Japanese study found that women with breast cancer were less likely to suffer a recurrence if they drank at least two cups of green tea daily. An analysis of 22 studies of green tea and lung cancer concluded that consuming two daily cups was associated with an 18% lower risk. And the Shanghai Women’s Health Study reported that participants who regularly drank green tea at the start of the study were 37% less likely to develop colorectal cancer over six years than those who seldom or never sipped it. Lifelong green-tea drinkers fared even better, with as much as a 57% relative risk reduction.
Most recently, a 2009 review of the effectiveness of green tea in the prevention of cancers found that much of the evidence was inconclusive. “In selected areas,” however, “green tea was effective in slowing the progression of the earlier stages of cancer.”
Winning the battle of the bulge: Helping you lose weight might seem an unlikely benefit of green-tea consumption, but several studies have shown that green tea has slimming power. Most recently, researchers at Provident Clinical Research in Indiana and colleagues, including Tufts’ Blumberg, published results of a study in which 132 participants were split into two groups: Half received a beverage containing 625 milligrams of green tea catechins plus 39 milligrams of caffeine, while the rest drank a control beverage containing only the caffeine. For 12 weeks, participants engaged in at least 180 minutes per week of moderate exercise; calorie intake was kept the same between the two groups. Those in the catechin group tended to lose more weight and showed statistically significantly greater lowering of abdominal fat as well as triglycerides.
A recent review of 15 similar studies concluded that green-tea catechins with caffeine modestly reduced body-mass index (BMI), body weight and waist circumference, but not waist-to-hip ratio.
Protecting against pneumonia: Among the latest proposed green-tea benefits is the possibility that its antioxidants might help combat the growth of viruses and other microorganisms. That’s the suggestion of Japanese researchers who recently reported a link between green tea consumption and reduced risk of dying from pneumonia, at least for women. Scientists at Tohuku University studied 21,493 women and 19,079 men, ages 40 to 79 and free of chronic disease at the start of the research. Over more than 12 years of follow-up, 408 participants died from pneumonia. Women drinking five or more cups of green tea daily were 47% less likely to die of pneumonia, and even those sipping just a cup or so were at 41% lower risk. The same association was not seen in men, however.
Keeping cells young: Finally, the notion that green tea might help you live, if not forever, at least a little longer might not be so far-fetched. In another new study, Hong Kong researchers reported that people who drank an average of three cups of tea daily had longer telomeres—DNA sequences at the end of chromosomes that shorten with aging—than those averaging a quarter-cup daily or less. The study involved 976 men and 1,030 women over the age of 65 and measured both green and black tea consumption; the majority of tea consumed, however, was the green variety popular in China. The cellular difference corresponds to the equivalent of about five years of life, the researchers said, adding: “The antioxidant properties of tea and its constituent nutrients may protect telomeres from oxidative damage in the normal aging process.”
While cautioning against reading too much into these results, Tufts’ Blumberg says the study represents yet another reason to incorporate drinking tea as part of an overall healthy diet.