Green teaNovember 2005
Antioxidants of the beverage tea in promotion of human health.
Tea that contains many antioxidants is a pleasant and safe drink that is enjoyed by people across the globe. Tea leaves are manufactured as black, green, or oolong. Black tea represents approximately 78% of total consumed tea in the world, whereas green tea accounts for approximately 20% of tea consumed. The concept of "use of tea for promotion of human health and prevention and cure of diseases" has become a subject of intense research in the last decade. Diseases for which tea drinkers appear to have lower risk are simple infections, like bacterial and viral, to chronic debilitating diseases, including cancer, coronary heart disease, stroke, and osteoporosis. Initial work on green tea suggested that it possesses human health-promoting effects. In recent years, the research efforts have been expanded to black tea as well. Research conducted in recent years reveals that both black and green tea have very similar beneficial attributes in lowering the risk of many human diseases, including several types of cancer and heart diseases. For cancer prevention, evidence is so overwhelming that the Chemoprevention Branch of the National Cancer Institute has initiated a plan for developing tea compounds as cancer-chemopreventive agents in human trials. Thus, modern medical research is confirming the ancient wisdom that therapy of many diseases may reside in an inexpensive beverage in a "teapot."
Antioxid Redox Signal . 2004 Jun;6(3):571-82
Update on chemoprevention of prostate cancer.
PURPOSE OF REVIEW: Prostate cancer remains the most commonly diagnosed visceral cancer in men in the United States, with almost 200,000 newly diagnosed cases in 2003. Prevention of this disease would have a major impact on disease-associated cost, morbidity, and mortality for a large segment of the population. A major advance in prevention of prostate cancer came in 2003 with the publication of the Prostate Cancer Prevention Trial. This overview summarizes the results of that trial, the design of other large-scale trials, and advances in understanding of the molecular mechanisms underlying the effect of other promising agents. RECENT FINDINGS: The Prostate Cancer Prevention Trial demonstrated that use of finasteride is associated with a 25% reduction in the 7-year period prevalence of prostate cancer in men over age 55 years with normal digital rectal exam and initial prostate specific antigen <3.0 ng/ml. Use of finasteride was associated with a slightly higher risk of Gleason sum 7-10 tumors, some sexual side effects, and fewer urinary symptoms. A substantial body of new molecular evidence supports the existing body of clinical and epidemiological data leading to testing of vitamin E and selenium as preventative agents in men at risk for prostate cancer. Epidemiologic and molecular evidence also makes cyclooxygenase-2 inhibitors, lycopene, soy, and green tea promising agents. SUMMARY: Results of a population-based, randomized phase III trial demonstrates that finasteride can prevent prostate cancer. A large amount of data supports the use of other agents as potential preventatives, including selenium, vitamin E, vitamin D, other 5-alpha-reductase inhibitors, cyclooxygenase-2 inhibitors, lycopene, and green tea. Some of these agents are being tested in new large-scale phase III clinical trials.
Curr Opin Urol . 2004 May;14(3):143-9
Medicinal benefits of green tea: part I. Review of noncancer health benefits.
Tea, in the form of green or black tea, is one of the most widely consumed beverages in the world. Extracts of tea leaves also are sold as dietary supplements. However, with the increasing interest in the health properties of tea and a significant rise in scientific investigation, this review covers recent findings on the medicinal properties and noncancer health benefits of both green and black tea. In Part II, a review of anticancer properties of green tea extracts is presented. Green tea contains a unique set of catechins that possess biological activity in antioxidant, anti-angiogenesis, and antiproliferative assays potentially relevant to the prevention and treatment of various forms of cancer. Although there has been much focus on the biological properties of the major tea catechin epigallocatechin gallate (EGCg) and its antitumor properties, tea offers other health benefits; some due to the presence of other important constituents. Characteristics unrelated to the antioxidant properties of green and black teas may be responsible for tea's anticancer activity and improvement in cardiac health and atherosclerosis. Theanine in green tea may play a role in reducing stress. Oxidized catechins (theaflavins in black tea) may reduce cholesterol levels in blood. Synergistic properties of green tea extracts with other sources of polyphenolic constituents are increasingly recognized as being potentially important to the medicinal benefits of black and green teas. Furthermore, due to presumed antioxidant and antiaging properties, tea is now finding its way into topical preparations. Each of these aspects is surveyed.
J Altern Complement Med. 2005 Jun;11(3):521-8
The antifolate activity of tea catechins.
A naturally occurring gallated polyphenol isolated from green tea leaves, (-)-epigallocatechin gallate (EGCG), has been shown to be an inhibitor of dihydrofolate reductase (DHFR) activity in vitro at concentrations found in the serum and tissues of green tea drinkers (0.1-1.0 micromol/L). These data provide the first evidence that the prophylactic effect of green tea drinking on certain forms of cancer, suggested by epidemiologic studies, is due to the inhibition of DHFR by EGCG and could also explain why tea extracts have been traditionally used in "alternative medicine" as anticarcinogenic/antibiotic agents or in the treatment of conditions such as psoriasis. EGCG exhibited kinetics characteristic of a slow, tight-binding inhibitor of 7,8-dihydrofolate reduction with bovine liver DHFR (K(I) = 0.109 micromol/L), but of a classic, reversible, competitive inhibitor with chicken liver DHFR (K(I) = 10.3 micromol/L). Structural modeling showed that EGCG can bind to human DHFR at the same site and in a similar orientation to that observed for some structurally characterized DHFR inhibitor complexes. The responses of lymphoma cells to EGCG and known antifolates were similar, that is, a dose-dependent inhibition of cell growth (IC50 = 20 micromol/L for EGCG), G0-G1 phase arrest of the cell cycle, and induction of apoptosis. Folate depletion increased the sensitivity of these cell lines to antifolates and EGCG. These effects were attenuated by growing the cells in a medium containing hypoxanthine-thymidine, consistent with DHFR being the site of action for EGCG.
Cancer Res. 2005 Mar 15;65(6):2059-64
Green tea and tea polyphenols in cancer prevention.
The cancer-preventive effects of green tea and its main constituent (-)-epigallocatechin gallate [(-)-EGCG] are widely supported by results from epidemiological, cell culture, animal and clinical studies in the recent decade. In vitro cell culture studies show that tea polyphenols potently induce apoptotic cell death and cell cycle arrest in tumor cells but not in their normal cell counterparts. Green tea polyphenols affect several signal transduction pathways, including growth factor-mediated, the mitogen-activated protein kinase (MAPK)-dependent, and ubiquitin/proteasome degradation pathways. Epidemiological studies have suggested that the consumption of green tea lowers the risk of cancer. Various animal studies have revealed that treatment by green tea inhibits tumor incidence and multiplicity in different organ sites such as skin, lung, liver, stomach, mammary gland and colon. Phase I and II clinical trials were carried out recently to explore the anticancer effects of green tea in patients with cancer. At this time, more mechanistic research, animal studies, and clinical trials are necessary to further evaluate the role of green tea in cancer prevention.
Front Biosci . 2004 Sep 1;9:2618-31
Body weight loss and weight maintenance in relation to habitual caffeine intake and green tea supplementation.
OBJECTIVE: Investigation of the effect of a green tea-caffeine mixture on weight maintenance after body weight loss in moderately obese subjects in relation to habitual caffeine intake. RESEARCH METHODS AND PROCEDURES: A randomized placebo-controlled double blind parallel trial in 76 overweight and moderately obese subjects, (BMI, 27.5 +/- 2.7 kg/m(2)) matched for sex, age, BMI, height, body mass, and habitual caffeine intake was conducted. A very low energy diet intervention during 4 weeks was followed by 3 months of weight maintenance (WM); during the WM period, the subjects received a green tea-caffeine mixture (270 mg epigallocatechin gallate + 150 mg caffeine per day) or placebo. RESULTS: Subjects lost 5.9 +/-1.8 (SD) kg (7.0 +/- 2.1%) of body weight (p < 0.001). At baseline, satiety was positively, and in women, leptin was inversely, related to subjects' habitual caffeine consumption (p < 0.01). High caffeine consumers reduced weight, fat mass, and waist circumference more than low caffeine consumers; resting energy expenditure was reduced less and respiratory quotient was reduced more during weight loss (p < 0.01). In the low caffeine consumers, during WM, green tea still reduced body weight, waist, respiratory quotient and body fat, whereas resting energy expenditure was increased compared with a restoration of these variables with placebo (p < 0.01). In the high caffeine consumers, no effects of the green tea-caffeine mixture were observed during WM. DISCUSSION: High caffeine intake was associated with weight loss through thermogenesis and fat oxidation and with suppressed leptin in women. In habitual low caffeine consumers, the green tea-caffeine mixture improved WM, partly through thermogenesis and fat oxidation.
Obes Res . 2005 Jul;13(7):1195-204
Effects of purified green and black tea polyphenols on cyclooxygenase- and lipoxygenase-dependent metabolism of arachidonic acid in human colon mucosa and colon tumor tissues.
The effects of green and black tea polyphenols on cyclooxygenase (COX)- and lipoxygenase (LOX)-dependent arachidonic acid metabolism in normal human colon mucosa and colon cancers were investigated. At a concentration of 30 microg/mL, (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), and (-)-epicatechin-3-gallate (ECG) from green tea and theaflavins from black tea inhibited LOX-dependent activity by 30-75%. The formation of 5-, 12-, and 15-LOX metabolites was inhibited to a similar extent. Tea polyphenols also inhibited COX-dependent arachidonic acid metabolism in microsomes from normal colon mucosa, with ECG showing the strongest inhibition. The formation of thromboxane (TBX) and 12-hydroxyheptadecatrienoic acid (HHT) was decreased to a greater extent than other metabolites. The inhibitory effects of tea polyphenols on COX activity, however, were less pronounced in tumor microsomes than in normal colon mucosal microsomes. Theaflavins strongly inhibited the formation of TBX and HHT, but increased the production of prostaglandin E(2) (PGE(2)) in tumor microsomes. The enhancing effect of theaflavins on PGE(2) production was related to the COX-2 level in the microsomes. Although theaflavin inhibited ovine COX-2, its activity in the formation of PGE(2) was stimulated by theaflavin when ovine COX-2 was mixed with microsomes, suggesting that theaflavin affects the interaction of COX-2 with other microsomal factors (e.g. PGE synthase). The present results indicate that tea polyphenols can affect arachidonic acid metabolism in human colon mucosa and colon tumors, and this action may alter the risk for colon cancer in humans.
Biochem Pharmacol . 2001 Nov 1;62(9):1175-83
Inhibition of collagenases from mouse lung carcinoma cells by green tea catechins and black tea theaflavins.
Theaflavin and theaflavin digallate, which are components of black tea were examined by in vitro invasion assay with mouse Lewis lung carcinoma LL2-Lu3 cells, which are highly metastatic. The compounds inhibited invasion by the tumor cells. Gelatin zymography showed that the cells secreted matrix metalloproteinases (MMPs), probably including MMP-2 and MMP-9, which may be involved in tumor cell invasion and metastasis. Theaflavin and theaflavin digallate also inhibited MMPs from the culture medium of these tumor cells, as did (-)-epigallocatechin gallate. These results suggest that theaflavin, theaflavin digallate, and (-)-epigallocatechin gallate inhibit tumor cell invasion by inhibiting type IV collagenases of the LL2-Lu3 cells.
Biosci Biotechnol Biochem . 1997 Sep;61(9):1504-6
Scavenging effect of extracts of green tea and natural antioxidants on active oxygen radicals.
With the use of the spin trapping methods, the scavenging effects of the extracts of green tea and other natural foods are studied. In stimulated polymorphonuclear leukocytes (PMN) system, water extract fraction 6 (F6) from green tea and green tea polyphenols (GTP) have the strongest scavenging effect on the active oxygen radicals, much stronger than vitamin C (Vc) and vitamin E (VE). Rosemary antioxidants (RA) and Curcumin (Cur) have weaker scavenging effects than Vc, but stronger than VE. In Fenton Reaction, Cur has the strongest scavenging effect (69%) on hydroxyl radicals. In irradiation, riboflavin system F6(74%) and GTP(72%) have very strong scavenging effects that are weaker than Vc, but much stronger than VE (23%). With the use of spin probe oxymetry, the oxygen consumption in respiratory burst of stimulated PMN were measured when the antioxidants existed in these systems. The results demonstrated that these antioxidants did not affect the respiratory burst of human polymorphonuclear leukocytes stimulated with PMA.
Cell Biophys . 1989 Apr;14(2):175-85
Effects of tea polyphenols and flavonoids on liver microsomal glucuronidation of estradiol and estrone.
Administration of 0.5 or 1% lyophilized green tea (5 or 10 mg tea solids per ml, respectively) as the sole source of drinking fluid to female Long-Evans rats for 18 days stimulated liver microsomal glucuronidation of estrone, estradiol and 4-nitrophenol by 30-37%, 15-27% and 26-60%, respectively. Oral administration of 0.5% lyophilized green tea to female CD-1 mice for 18 days stimulated liver microsomal glucuronidation of estrone, estradiol and 4-nitrophenol by 33-37%, 12-22% and 172-191%, respectively. The in vitro addition of a green tea polyphenol mixture, a black tea polyphenol mixture or (-)-epigallocatechin gallate inhibited rat liver microsomal glucuronidation of estrone and estradiol in a concentration-dependent manner and their IC50 values for inhibition of estrogen metabolism were approximately 12.5, 50 and 10 microg/ml, respectively. Enzyme kinetic analysis indicates that the inhibition of estrone glucuronidation by 10 microM (-)-epigallocatechin gallate was competitive while inhibition by 50 microM (-)-epigallocatechin gallate was noncompetitive. Similarly, several flavonoids (naringenin, hesperetin, kaempferol, quercetin, rutin, flavone, alpha-naphthoflavone and beta-naphthoflavone) also inhibited rat liver microsomal glucuronidation of estrone and estradiol to varying degrees. Naringenin and hesperetin displayed the strongest inhibitory effects (IC50 value of approximately 25 microM). These two hydroxylated flavonoids had a competitive mechanism of enzyme inhibition for estrone glucuronidation at a 10 microM inhibitor concentration and a predominantly noncompetitive mechanism of inhibition at a 50 microM inhibitor concentration.
J Steroid Biochem Mol Biol . 1998 Feb;64(3-4):207-15
Inhibitory effects of antioxidants on formation of heterocyclic amines.
It is important to search for effective antioxidants to suppress formation of mutagenic and carcinogenic heterocyclic amines (HCAs), like 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), because these HCAs are considered to be probable human carcinogens. The effects of various food-derived antioxidants on MeIQx formation were examined by their addition (0.2 mmol each) to mixtures of creatine (0.4 mmol), glycine (0.4 mmol) and glucose (0.2 mmol), and heating at 128 degreesC for 2 h. Glycine was replaced by l-phenylalanine in the case of PhIP formation. Among the 14 kinds of antioxidants tested, green tea catechins and the major component [(-)-epigallocatechin gallate], two flavonoids (luteolin and quercetin) and caffeic acid were found to clearly suppress the formation of both MeIQx and PhIP, being 3.2-75% of the level of the controls. These phenolic antioxidants also reduced the total mutagenicity of the heated mixtures. The results suggest that foodstuffs containing catechins, flavonoids and caffeic acid may suppress the formation of HCAs in cooked foods.
Mutat Res . 1f998 Jun 18;402(1-2):237-45
Selective inhibition of steroid 5 alpha-reductase isozymes by tea epicatechin-3-gallate and epigallocatechin-3-gallate.
Inhibitors of 5 alpha-reductase may be effective in the treatment of 5 alpha-dihydrotestosterone-dependent abnormalities, such as benign prostate hyperplasia, prostate cancer and certain skin diseases. The green tea catechins, (-)epigallocatechin-3-gallate and (-)epicatechin-3-gallate, but not (-)epicatechin and (-)epigallocatechin, are potent inhibitors of type 1 but not type 2 5 alpha-reductase. (-)Epigallocatechin-3-gallate also inhibits accessory sex gland growth in the rat. These results suggest that certain tea gallates can regulate androgen action in target organs.
Biochem Biophys Res Commun. 1995 Sep 25;214(3):833-8