As medical researchers continue to uncover the health benefits of green tea, one might think that these discoveries are of recent origin. In fact, the Western world’s growing interest in green tea’s disease-preventive effects tends to overshadow what Asian cultures have known for thousands of years: that green tea is one of nature’s most potent agents in protecting the body against a host of illnesses, thus offering real hope to those seeking to live longer, disease-free lives.
Asian populations that regularly consume green tea have lower overall rates of cancer.1,2 In 1994, researchers from the Shanghai Cancer Institute compared green tea drinkers to non-drinkers in a large population study in China. They found that in non-smokers, drinking green tea was associated with fewer cancers of the esophagus.3
Since that time, scientists have been trying to ascertain exactly why green tea drinkers are less likely to develop cancer and how green tea works in the human body. What is known is that research conducted in the last few years suggests that green tea may be effective in helping to prevent a wide variety of cancers in humans, including cancers of the bladder, colon, esophagus, pancreas, rectum, and stomach.4
The active ingredients in green tea thought to be principally responsible for chemoprevention are poly-phenols, the natural antioxidant compounds found in plants. Tea contains four main polyphenols called catechins, which are water-soluble compounds that make up a subgroup of flavonoids, also commonly found in fruits and vegetables, coffee, chocolate, and wine. Catechins are powerful antioxidants that can be easily oxidized in the body; their antioxidant potential has been found to be significantly higher than that of grape juice and red wine.
The catechins present in green tea include epigallocatechin-3 gallate (EGCG), epigallocatechin (EGC), and epicatechin-3 gallate (ECG).5 Of these, EGCG demonstrates the most potent anti-cancer activity. Clinical tests have shown that its antioxidant activity destroys free radicals and reactive oxygen species that can damage DNA, cell membranes, and other cell components, and thus make the body more susceptible to cancer and other degenerative diseases.
Scientists now believe they may have identified the mechanism responsible for EGCG’s anticarcinogenic benefits. Green tea was long believed to target proteins implicated in the formation and proliferation of cancer cells. A 2003 study suggests a strong link between the anti-cancer activities of tea polyphenols and their inhibition of a crucial pathway necessary for the development of many common human malignancies.6
Anti-Cancer Effects Confirmed
Studies released in 2004 support these findings with even greater evidence of green tea’s value in the fight against cancer. One study investigated the effects of treatment with different concentrations of green tea on induced lung tumors in female mice.7 A treatment with 0.6% green tea preparation significantly reduced lung tumor multiplicity and also inhibited angiogenesis, the development of new blood vessels required by tumors in order to grow.
A second study, the Prostate Cancer Prevention Trial, investigated green tea’s positive effects against the most commonly diagnosed visceral cancer in US men, with more than 230,000 newly diagnosed cases in 2004 alone. The trial cited considerable data supporting the use of green tea and other substances as “promising agents” in the prevention of prostate cancer.8 Green tea is among the agents being tested in new, large-scale, phase III chemopreventive clinical trials.
A study published in March 2004 explored the use of dietary components that are capable of inhibiting human cancer cell growth without affecting normal cell growth—specifically, EGCG’s effects on breast cancer cells. EGCG was found to inhibit the actions of telomerase, an enzyme that prolongs the life span of cancer cells by maintaining the end portions of the tumor cell chromosomes. Treatment with EGCG also increased the percentage of apoptotic cells. This prompted the study authors to conclude that EGCG leads to “suppression of [cancer] cell viability and induction of apoptosis, thus providing the molecular basis for the development of EGCG as a novel chemopreventive and pharmacologically safe agent against breast cancer.”12
Further studies have found that drinking green tea suppresses the proliferation of human T-cell lymphotropic virus type 1, which is associated with the causes of adult T-cell leukemia.13 Of greater interest to leukemia sufferers, Mayo Clinic researchers
have discovered that EGCG in green tea helps kill the cells of the most common chronic leukemia in the US. The research, using laboratory cell cultures, showed that EGCG interrupts the communication signals leukemia cells need to survive.14 The leukemia cells studied were taken from patients with B-cell chronic lymphocytic leukemia, most often diagnosed in patients in their sixties. This form of leukemia has no cure, though chemotherapy is administered in the most severe cases. The Mayo Clinic study showed that EGCG from green tea prompted leukemia cells to die in eight of ten patient samples tested in the laboratory.14
Green tea offers further good news for skin cancer sufferers. EGCG has been shown to control the metastasis, or uncontrolled spread, of melanoma (skin cancer) cells to the lung.15 This discovery is especially important, as it is the metastasis of tumors—not the primary tumor itself—that eventually causes death.
The evidence supporting green tea’s role in cancer prevention is so overwhelming that the Chemo-prevention Branch of the National Cancer Institute has initiated a plan for developing tea compounds as chemopreventive agents in further human trials.16
Proven Cardioprotective Benefits
People in China and Japan have always had lower incidences of heart disease than their Western counterparts. Most scientists now accept that green tea is at least in part responsible for this discrepancy. Results from human studies over the past few decades show that green tea consumption is correlated with lower levels of cholesterol, thus modulating one risk factor for cardiovascular disease.17 A study of heart disease risk in men showed that a higher dietary intake of flavonoids, primarily from tea, was associated with decreased mortality from coronary heart disease. Higher flavonoid intake was also correlated with a decreased incidence of myocardial infarction, or heart attack.18
More recent studies have taken these results a step further. An animal study demonstrated that green tea catechins reduce atherosclerosis through antioxidant effects and by lowering blood lipid levels. This study examined how varying doses of green tea affected atherosclerosis. A lower dose decreased atherosclerosis by 26-46%, while a higher dose was even more effective, decreasing it by 48-63%. Green tea also helped improve levels of dangerous low-density lipoprotein (LDL) as well as the ratio of LDL to HDL (high-density lipoprotein). Supplementation with green tea was found to be equally effective at human-equivalent doses.19
A study in Chiba, Japan examined the effects of green tea consumption on coronary artery disease by following 203 patients who had undergone coronary angiography. Green tea consumption was found to be significantly higher in patients without coronary artery disease than in those who had the disease. The researchers concluded that green tea consumption was in fact associated with a lower incidence of heart disease in their study population.20 The more green tea patients consumed, the researchers concluded, the less likely they were to have coronary artery disease.
A team at the Cedars-Sinai Medical Center’s Atherosclerosis Research Center recently examined the effects of green tea on atherosclerosis, using mice with high blood cholesterol levels. The researchers examined the effects of a purified form of EGCG on both new and established plaque in the mice. While new plaque formation was significantly reduced, EGCG had no effect on pre-existing plaques in the aorta.21 According to study author Kuang-Yuh Chyu, MD, the results suggest that antioxidant therapy could have therapeutic benefits if initiated during a critical window early in the formation of plaque.
Oxidative stress has been report-ed to be involved not only in cardiovascular disease, but also in hypertension. Epidemiological studies indicate that consuming green tea can reduce blood pressure. In two studies, Japanese scientists sought to determine whether green tea could lower blood pressure in stroke-prone hypertensive rats.22 The studies found that during the daytime, systolic and diastolic blood pressure were significantly lower in the animals that were fed green tea catechin supplements mixed with water. The data demonstrated that green tea polyphenols moderated blood pressure increases through their antioxidant properties. Moreover, because the amounts used in the experiment correspond to those in approximately one liter of tea, regular consumption of green tea may also provide some protection against hypertension in humans.22