Today, green tea is getting the majority of the press for its positive effect on health. This is due to the power of its constituents, which include carotenoids, chlorophyll, polysaccharides, fats, vitamins C and E, manganese, potassium and zinc. However, experts agree that it is one type of constituent in particular that provides most of the health benefits. These are polyphenols.
Polyphenols are members of the flavonoid family. They are catechins made of several ringlike structures. Each of these structures has chemicals attached to it called phenol groups, hence the name polyphenols (poly means "many").
Of all three types of tea (green, black, and oolong), green tea contains the most polyphenols: about 15 percent to 30 percent of its weight. The polyphenols in green tea are recognized as anticarcinogenic, and this polyphenol content, along with the naturally occurring vitamin C, helps strengthen blood vessel walls.
Four of these polyphenols are of particular interest: epicatechin (EG), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). In green tea, about half of the polyphenols are EGCG.
EGCG is a powerful antioxidant and has been found to be 20 times stronger than vitamin E in protecting brain lipids, which are very susceptible to oxidative stress. (Chem Pharm Bulletin 38 [1990]: 1049) In animal studies with mice, ECG has been shown to reduce the rate of lung, skin, and stomach cancer. (Preventative Medicine 21)
Green tea is regarded as an antioxidant. The polyphenols, especially EGCG, prevent free radical damage and have even been found to detoxify free radicals produced by the environmental toxin paraquat. (Carcinogenesis 10 [1989]: 1003)
As a whole, human studies indicate that consuming green tea can lower the rate of esophageal cancer, mouth cancers, and gastric cancers. Recent research indicates that green tea may reduce the risk of some forms of stomach cancer. Surveys of Japanese tea drinkers show that those who consume four to six cups of green tea a day have lower levels of breast, esophageal, liver, lung, and skin cancers than those who consume less green tea or none at all.
At a meeting of the American Chemical Society in 1991, researchers reported that even cigarette smokers who consumed green tea had a 45 percent lower risk of cancer than non-tea drinkers. As an anti-tumor agent, green tea has an anti-mutation factor that helps DNA (deoxyribonucleic acid) to reproduce accurately rather than in mutated forms. Green tea's catechin content is believed to be responsible for this effect. Even though Japan has one of the highest rates of smokers in the world, they have one of the lowest rates of lung cancer of any developed nation.
Green tea is a hypotensive, lowering blood pressure and helping to increase blood flow to the heart. Many Asians have long consumed green tea with meals, and this practice is now showing to reduce arterial disease. Many heart attacks are brought on by blood platelet aggregation and green tea prevents the blood from "clumping together" and forming clots that can lead to stroke. One study indicates that 6,000 Japanese women who were nondrinkers and nonsmokers over 40 who drank about five cups of green tea a day had a 50 percent decrease in the risk of stroke. (Natural Health [March/April 1994])
Whereas coffee can elevate cholesterol levels, green tea helps lower them. (HerbalGram 37 [1996]) The catechin content of green tea helps to break down cholesterol and increase its elimination through the bowels. Green tea also helps to keep blood sugar levels moderate.