"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 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.