ABSTRACT

Both animal and human studies have suggested a cardioprotective effect for tea.[6] Treatment of nephrectomized rats with 0.25% green tea extract for 4weeks resulted in attenuation of left ventricular hypertrophy and hypertension.[7] Studies with isolated myocytes showed that green tea extract inhibited ouabaininduced reactive oxygen species (ROS) production and cell proliferation. Both green tea and black tea (1-2% in the diet) have been shown to reduce serum cholesterol levels in hypercholesterolemic rats at least in part by inhibiting the absorption of cholesterol from the gut.[8,9] Vinson and colleagues have demonstrated that 1.25% green or black tea (as the drinking fluid) reduced cholesterol (20-29% decrease), triglycerides (20-32% decrease), and lipid peroxides (27-49% decrease) in cholesterol fed hamsters after 15 days of treatment.[6] Treatment of C57bL=6J apolipoprotein (apo) E-deficient mice with 0.08% green tea extract for 14weeks reduced the number of atheromatous areas in the aorta by 23% and aortic cholesterol and triglyceride levels by 27% and 50%, respectively.[10] By

contrast, 8 weeks of treatment with green tea and black tea had no effect on the incidence of atherosclerotic lesions, low-density lipoprotein (LDL) oxidation, and lipid peroxidation in New Zealand white rabbits.[11]

Human Studies

Studies in humans have yielded mixed results. A Phase II randomized controlled study has shown that consumption of 4 cups=day of green tea but not black tea results in a 31% decrease in the urinary concentrations of 8-hydroxydeoxyguanosine, a marker of oxidative stress, in smokers.[12] Hirano et al. have reported that intake of green tea was inversely associated with incidence of myocardial infarction (1-3 cups=day reduced prevalence by 35%), but had no effect on the incidence of coronary artery disease.[13] Another study of 512 subjects in Japan suggested that drinking 2-3 or 4 cups=day was inversely associated with the development of coronary atherosclerosis with odds ratios of 0.5 and 0.4, respectively.[14]

NEURODEGENERATIVE DISORDERS

Green tea and its components, especially EGCG, have been shown in some experiments to inhibit the development of Parkinson’s disease. In a case-control study in China, consumption of 3 or more cups of tea per day was shown to reduce the risk of developing this disease by 28%.[15] Laboratory trials with mice further support a potential protective effect for green tea. Choi et al. reported that oral administration of green tea and EGCG attenuated the development of

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)- induced Parkinson’s disease in mice.[16] EGCG prevented the loss of tyrosine hydroxylase positive cells in the substantia nigrum and decreased the expression of neuronal nitric oxide synthetase (nNOS).[16] Potential mechanisms for the antiparkinsonian effects of EGCG include inhibition of catechol-O-methyltransferase (COMT) and of 6-hydroxydopamine-induced neuronal cell death.[17,18]

DIABETES

Obesity and diabetes have become widespread health problems and contribute to increased risk of other diseases, including heart diseases, and cancer. Consumption of 1.5 L of oolong tea for 30 days has been shown to decrease the plasma glucose levels by 30% in individuals with Type II diabetes.[19] Likewise, green tea, but not an equivalent amount of caffeine, was shown to increase the 24 h energy expenditure and fat oxidation in healthy human volunteers.[20]

Sabu, Smitha, and Kuttan have demonstrated that green tea polyphenols [500mg=kg, intragastric (i.g.)] increased glucose tolerance in normal rats, and daily administration of 50-100mg=kg, i.g. for 15 days reduced the plasma glucose levels by 29-44% in alloxan-treated rats.[21] In vitro, EGCG inhibited interleukin-1b and interferon-g-induced cytotoxicity in insulinoma cells.[22] The author suggested that this effect may be beneficial in preventing islet cell death in Type I diabetes as these mediators are important in the pathology of that disease.