Both animal and human studies have suggested a cardioprotective effect for tea. Treatment of nephrectomized rats with 0.25% green tea extract for 4weeks resulted in attenuation of left ventricular hypertrophy and hypertension. 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. 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. 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. 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. 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. 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. 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%. 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. EGCG prevented the loss of tyrosine hydroxylase positive cells in the substantia nigrum and decreased the expression of neuronal nitric oxide synthetase (nNOS). 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. 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. 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. In vitro, EGCG inhibited interleukin-1b and interferon-g-induced cytotoxicity in insulinoma cells. 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.
All Science Journal Classification (ASJC) codes
- Health Professions(all)
- Pharmacology, Toxicology and Pharmaceutics(all)