A large body of research support the hypothesis that oxidative damage to DNA, lipids and proteins may contribute to the development of cardiovascular disease, cancer and neurodegenerative diseases. Reactive oxygen and nitrogen species are formed in the body and endogenous antioxidant defenses are not always sufficient to counteract them completely. Diet-derived antioxidants may therefore be particularly important in protecting against chronic diseases. Tea is an important source of antioxidants. Tea provides about 200 mg of flavonoids per cup for a typical brew of black tea.
The flavonoids present in green and black tea are very effective antioxidants in vitro and may therefore be effective antioxidants in the body. The uptake of flavonoids from tea is most efficient for the monomeric catechins found in green tea, namely, epicatechin (EC), epigallocatechin (EGC), epicatechin-3-gallate (ECG) and epigallocatechin gallate (EGCG), the last named being the most important.
Green tea has been found to be beneficial in two of the most life-threatening diseases – heart disease and cancer.
Green Tea and Heart Disease
The vascular endothelium is a critical regulator of vascular homeostasis, and endothelial dysfunction contributes to the pathogenesis and clinical expression of coronary artery disease. Normal endothelium regulates vasomotor tone, platelet activity, leukocyte adhesion and vascular smooth muscle cell proliferation via release of several paracrine factors, including nitric oxide. These endothelial functions are impaired with atherosclerosis and its risk factors.
Endothelial dysfunction in atherosclerosis is associated with increased oxidative stress and may be reversed by the antioxidants present in green and black teas. Both short- and long term tea consumption improved endothelium-dependent flow-mediated dilation of the brachial artery. There is experimental evidence that antioxidant flavonoids improve endothelium-derived nitric oxide bioavailability by enhanced synthesis rather than by decreased superoxide-mediated breakdown. A cross sectional study conducted in Japan, where green tea consumption is prevalent, revealed that increased consumption was associated with decreased serum cholesterol and triglyceride levels and an increased HDL cholesterol.
Studies suggest that tea consumption is associated with lower mortality among individuals with cardiovascular diseases. A prospective cohort study of 1900 patients hospitalized with confirmed acute myocardial infarction was followed for 3.8 years. Of the 1900 patients, 1019 were consumed no tea, 615 were moderate tea drinkers (<14 cups per week) and 266 were heavy tea drinkers (>14 cups per week). During the follow-up period 313 patients died. In age- and sex-adjusted analysis, a graded inverse relationship between tea consumption and total mortality was found. Compared to nondrinkers, the hazard ratios were 0.65 and 0.61 for moderate and heavy tea drinkers, respectively.
One study suggests that daily consumption of tea containing 690 mg catechins for 12 wk reduced body fat.
Green Tea in Cancer
Several animal models by different researchers have demonstrated the cancer preventive properties of green tea. Green tea has shown cancer chemoprevention against UV light, chemically-induced, and genetic models of carcinogenesis. The organ sites include the lung, skin, oral cavity, esophagus, stomach, liver, pancreas, bladder, small intestine colon, mammary gland and prostate. Epidemiological data are, however, mixed .
Most studies have focused on EGCG which has been shown to inhibit growth and induce cell cycle arrest and apoptosis in different cancer cell lines. A number of probable mechanisms have been suggested for the anticancer activity of the tea constituents. Antioxidant properties, or interaction with certain enzymes or proteins implicated in cancer biology such as urokinase, ornithine decarboxylase, NADPH-cytochrome P450 reductase, protein kinase C, steroid 5 alpha reductase, TNF expression and nitric oxide synthase have been proposed as possible mechanisms of action.
Green tea is now acknowledged as a cancer preventive in Japan. For the general population 10 cups of green tea per day is recommended.
