Tea & Health

Tea Expo 2000, 23-26 November 2000

Paris-France

 

Tea polyphenols: prevention of cancer and optimizing health

Am J Clin Nutr 2000 Jun;71 Suppl(6):1698S-1702S  

By Mukhtar et al. Department of Dermatology, Case Western Reserve University, Cleveland.

The tea plant Camellia sinesis is cultivated in >30 countries. Epidemiologic observations and laboratory studies have indicated that polyphenolic compounds present in tea may reduce the risk of a variety of illnesses, including cancer and coronary heart disease. Most studies involved green tea, however; only a few evaluated black tea. Results from studies in rats, mice, and hamsters showed that tea consumption protects against lung, forestomach, esophagus, duodenum, pancreas, liver, breast, colon, and skin cancers induced by chemical carcinogens. Other studies showed the preventive effect of green tea consumption against atherosclerosis and coronary heart disease, high blood cholesterol concentrations, and high blood pressure. Because the epidemiologic studies and research findings in laboratory animals have shown the chemopreventive potential of tea polyphenols in cancer, the usefulness of tea polyphenols for humans should be evaluated in clinical trials. One such phase 1 clinical trial is currently under way at the MD Anderson Cancer Center in collaboration with Memorial Sloan-Kettering Cancer Center. This study will examine the safety and possible efficacy of consuming the equivalent of >/=10 cups (>/=2.4 L) of green tea per day. The usefulness of tea polyphenols may be extended by combining them with other consumer products such as food items and vitamin supplements. This "designer-item" approach may be useful for human populations, but it requires further study.

IS WHITE TEA BETTER THAN OTHER TEAS AS A POTENTIAL ANTICARCINOGEN?

"White tea" does not refer to black tea with milk, but rather to a specific form of tea in which the leaves and buds are simply steamed and dried. In this sense, white tea represents the least processed form of tea, since green, oolong and black teas undergo withering before various degrees of oxidation. White tea also contains a higher proportion of buds, which are covered with fine 'silvery' hairs that impart a light white/grey color to the tea. White tea brews to a pale yellow/light red color, and has a slightly sweet flavor with no 'grassy' undertones sometimes associated with green tea.

Researchers at the Linus Pauling Institute tested four types of white tea for their ability to inhibit mutations in bacteria, and subsequently examined the protective properties in a rat colon cancer model. In the former studies using bacteria, white teas were generally more effective than green tea in inhibiting mutagenicity (mutagenicity is a result of unrepaired/misrepaired DNA damage and an early step in the process leading to cancer).

White teas contained many of the expected polyphenols, some of which were present at higher concentrations than in green tea brewed under the same conditions. Other constituents, such as caffeine, also were present at higher levels in white tea.

Rats were given white tea (tea was brewed for 5 min, using 2g/100ml hot water) in the drinking water for up to 8 weeks. A second group was given the equivalent amount of caffeine alone. In weeks 3 and 4, animals were given a carcinogen from cooked meat ("PhIP"). After 2 weeks of treatment, and prior to PhIP dosing, enzyme changes were detected in the liver, white tea being slightly more effective in this regard than caffeine alone. Overall, the altered enzyme profiles, and profiles of metabolites excreted in the urine, suggested that the carcinogen was more rapidly metabolized and detoxified. At the end of the study, rats given white tea had significantly fewer PhIP-induced pre-cancerous lesions in the colon (called aberrant crypt foci, or ACF). However, rats given caffeine alone also had fewer ACF.

These data are highly preliminary, and cannot be extrapolated to human cancer prevention or treatment. They indicate that white tea, like other forms of tea, can block the DNA damage caused by some compounds using a test tube assay with bacteria as indicator organisms. The animal studies scored pre-cancerous changes in the colon, not actual tumors, and raised the possibility that any potential extra 'benefit' from white tea (versus other teas) might simply be related to higher caffeine levels. Finally, animal studies in which inhibition of colon tumor formation has been demonstrated cannot be simply extrapolated to protection in people.

Green tea polyphenol epigallocatechin-3-gallate differentially modulates nuclear factor kappaB in cancer cells versus normal cells

Arch Biochem Biophys, April 2000

By A.Gupta et al.

Green tea has shown remarkable anti-inflammatory and cancer chemopreventive effects in many animal tumor bioassays, cell culture systems, and epidemiological studies. Many of these biological effects of green tea are mediated by epigallocatechin 3-gallate (EGCG), the major polyphenol present therein. We have earlier shown that EGCG treatment results in apoptosis of several cancer cells, but not of normal cells (J. Natl. Cancer Inst. 89, 1881-1886 (1997)). The mechanism of this differential response of EGCG is not known. In this study, we investigated the involvement of NF-kappaB during these differential responses of EGCG. EGCG treatment resulted in a dose-dependent (i) inhibition of cell growth, (ii) G0/G1-phase arrest of the cell cycle, and (iii) induction of apoptosis in human epidermoid carcinoma (A431) cells, but not in normal human epidermal keratinocytes (NHEK). Electromobility shift assay revealed that EGCG (10-80 microM) treatment results in lowering of NF-kappaB levels in both the cytoplasm and nucleus in a dose-dependent manner in both A431 cells and NHEK, albeit at different concentrations. EGCG treatment was found to result in a dose-based differential inhibition of TNF-alpha- and LPS-mediated activation of NF-kappaB in these cells. The inhibition of NF-kappaB constitutive expression and activation in NHEK was observed only at high concentrations. The immunoblot analysis also demonstrated a similar pattern of inhibition of the constitutive expression as well as activation of NF-kappaB/p65 nuclear protein. This inhibition of TNF-alpha-caused NF-kappaB activation was mediated via the phosphorylative degradation of its inhibitory protein IkappaBalpha. Taken together, EGCG was found to impart differential dose-based NF-kappaB inhibitory response in cancer cells vs normal cells; i.e., EGCG-mediated inhibition of NF-kappaB constitutive expression and activation was found to occur at much higher dose of EGCG in NHEK as compared to A431 cells. This study suggests that EGCG-caused cell cycle deregulation and apoptosis of cancer cells may be mediated through NF-kappaB inhibition.

Substance Found In Green Tea Prevents Blood Vessel Growth

Nature (1999;398:381-382)

Green tea's cancer-fighting benefits may be due to a component of the drink that prevents angiogenesis, a Swedish study has found.

The substance epigallocatechin-3-gallate -- or EGCG -- prevented the growth of new blood vessels, researchers reported in Nature (1999;398:381-382). Yihai Cao, M.D., Ph.D., and Renhai Cao, Ph.D., of the Karolinska Institute in Stockholm, Sweden, examined a group of four mice that consumed green tea as their sole drinking fluid and a second group of four mice that drank water. The concentration of EGCG in the plasma of the green-tea-drinking group was similar to the level found in humans who have consumed two or three cups of the tea, according to the report.

The researchers then used vascular endothelial growth factor to stimulate corneal neovascularization in the mice in order to compare growth of new blood vessels. They found that the mice given the green tea had between 35 percent and 70 percent less blood-vessel growth than the water-drinking-mice. These results suggest that EGCG's role in preventing the growth of new blood vessels could be key to green tea's association with lower cancer incidence, according to the researchers.

Green tea also contains polyphenols, chemicals that are believed to act as powerful antioxidants. Several recent studies have shown an association between a lower incidence in cancer and consumption of green tea, a beverage consumed principally in countries like Japan, China and India. In one study, researchers from the Shanghai Cancer Institute in China and the National Cancer Institute in Bethesda, MD, found that green-tea drinking was linked to a 60 percent reduction in esophageal cancer risk among nonsmoking men and women.