Jun 11, 2012
More than a decade's worth of research about green tea's health benefits -- particularly its potential to fight cancer and heart disease -- has been more than intriguing, as have limited studies about green tea's role in lowering cholesterol, burning fat, preventing diabetes and stroke, and staving off dementia.
"I believe in green tea based on everything written about it," says Katherine Tallmadge, RD, LD, a nutritionist and spokeswoman for the American Dietetic Association. "Green tea, white tea, black tea -- I like all of them."
Still, real-world evidence is lacking; most of the consistent findings about green tea's health benefits have come out of the lab.
The few large-scale human studies that have focused on green tea's impact on heart disease and cancer are promising, but many of those were conducted in the East, where green tea is a dietary mainstay. The outcomes are likely influenced by other lifestyle factors such as high consumption of fish and soy protein, says cardiologist Nieca Goldberg, MD, a spokeswoman for the American Heart Association and medical director of the New York University Women's Heart Center.
But Goldberg agrees with other health professionals: green tea has important antioxidants and compounds that help in maintaining good health.
Green Tea's Powerful Antioxidants
Green tea's antioxidants, called catechins, scavenge for free radicals that can damage DNA and contribute to cancer, blood clots, and atherosclerosis. Grapes and berries, red wine, and dark chocolate also have potent antioxidants.
Because of green tea's minimal processing -- its leaves are withered and steamed, not fermented like black and oolong teas -- green tea's unique catechins, especially epigallocatechin-3-gallate (EGCG), are more concentrated.
But there's still a question of how much green tea you need to drink to reap its health benefits. EGCG is not readily "available" to the body; in other words, EGCG is not always fully used by the body.
"We must overcome the issue of poor bioavailability [and other issues] in order to get the most out of their benefits," says Tak-Hang Chan, PhD, professor emeritus in the department of chemistry at McGill University in Montreal. Chan has studied the use of a synthetic form of EGCG in shrinking prostate cancer tumors in mice, with success.
Green Tea vs. Cancer
Marji McCullough, ScD, RD, the American Cancer Society's strategic director of nutritional epidemiology, says human studies haven't yet proven what researchers like Chan have discovered in the lab: green tea's EGCG regulates and inhibits cancer growth and kills cells that are growing inappropriately.
"Epidemiologically, one of the challenges is finding populations that drink enough green tea and have for a long time," she says. "With cancer, it's always difficult to find the exposure time," or the point at which cancer cells begin to develop.
Still, it's difficult not to be intrigued by a few human studies that have shown that drinking at least two cups of green tea daily inhibits cancer growth.
One of them, a study conducted in Japan that involved nearly 500 Japanese women with Stage I and Stage II breast cancer, found that increased green tea consumption before and after surgery was associated with lower recurrence of the cancers.
Studies in China have shown that the more green tea that participants drank, the less the risk of developing stomach cancer, esophageal cancer, prostate cancer, pancreatic cancer, and colorectal cancer.
Finally, a recent analysis of 22 studies that probed the correlation between high tea consumption and reduced risk for lung cancer concluded that by increasing your daily intake of green (not black) tea by two cups may reduce the risk of developing lung cancer by 18%.
Is Green Tea Good for Your Heart?
It seems to be, but there are conflicting results of a few epidemiological studies conducted in the East and West.
In a study that involved 500 Japanese men and women, researchers found that drinking at least four cups of green tea every day may be related to the reduced severity of coronary heart disease among the male participants.
A Dutch study of more than 3,000 men and women found that the more tea consumed, the less severe the clogging of the heart's blood vessels, especially in women.
As Goldberg suggests, lifestyle and overall diet are critical to the outcomes of these studies.
But green tea's antioxidants are dilators, she says, because they improve the flexibility of blood vessels and make them less vulnerable to clogging -- and antioxidant-rich blueberries and pomegranates do the same.
"I think people should know these are important studies, that everyday foods that are an option may actually have health benefits," Goldberg says. "I think green tea, because of its antioxidant value, may have heart benefits, but it's not something we regularly prescribe to people, because there isn't as much evidence as there is in exercise's ability to improve heart health."
Green Tea and Weight
Green tea and its extract have been shown to fight obesity and lower LDL "bad" cholesterol -- two risk factors for heart disease and diabetes -- but in very limited studies. One study in the Netherlands and a study in Japan showed that green tea did both.
In the Dutch study, participants who drank caffeinated green tea lost more weight, but even those who typically drank the decaf variety saw a decrease in their waistlines and body weight. Researchers speculated that the caffeine helps with fat oxidation.
In the Japanese study, 240 men and women were given varying amounts of green tea extract for three months. Those who got the highest amount lost fat and weight and had lower blood pressure and lower LDL "bad" cholesterol.
Green Tea Straight Up
Taking weight loss supplements that contain green tea extract probably won't hurt, unless you have liver problems.
But the best way to get the most out of green tea -- even if your main goal is losing weight -- is to drink it.
"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 McKay, PhD, a Tufts University scientist who studies antioxidants. "It's not going to cure anything and it shouldn't be consumed as a drug, but it can complement the rest of the diet."
McCullough bears the same reminder: eat your fruits, vegetables, grains, seeds, and nuts -- and go ahead, drink as much green tea as you want.
"I don't think it can hurt to drink it. I'd focus on dietary sources rather than supplements because there are several compounds in green tea that might need to be consumed together. We just don't know yet," she says.