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Green Tea Shows More Anti-Cancer Promise
8/20/2007
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Green tea boosts anti-cancer antioxidant enzyme in clinical trial; mouse study shows that green tea kills colon cancer in the cradle

by Craig Weatherby


Green tea enjoys a considerable reputation as a healthful drink. And growing evidence links tea and its catechin-class polyphenol antioxidants to a wide range of health benefitslower risk of diabetes and certain cancers, better weight control and heart health, and protection against senility.


Its reputation as an anti-cancer food is based on a large body of evidence, mostly of two kinds:

  • Experimental (cell and animal) research, which provide indications of possible benefit and details how whole foods or isolated food factors affect the body;
  • Epidemiological (population) studies, in which researchers look for statistical associations between foods and health in large groups of people.

The situation hasn’t changed substantially since researchers at The State University of New Jersey described it five years ago (Yang CS et al 2002):

  • “The inhibition of tumorigenesis (early tumor growth) by green or black tea preparations has been demonstrated in… [animals] on different organ sites such as skin, lung, oral cavity, esophagus, fore stomach, stomach, small intestine, colon, pancreas, and mammary gland.
  • “Epidemiological studies, however, have not yielded clear conclusions concerning the protective effects of tea consumption against cancer formation in humans.
  • “The discrepancy between the results from humans and animal models could be due to 1) the much higher doses of tea used in animals in comparison to human consumption, 2) the differences in causative factors between the cancers in humans and animals, and 3) confounding factors limiting the power of epidemiological studies to detect an effect. It is possible that tea may be only effective against specific types of cancer caused by certain… factors.”

Most importantly, we lack clinical studies that would pinpoint precisely which kinds of cancer are most susceptible to tea’s preventive potential, and which at-risk populations would derive the most benefit.


A few such clinical studies are underway or planned, and in the meantime, two new studies shed more light on the matter.


The intriguing results of a clinical study illuminate yet another potentially anti-cancer effect of green tea, while an animal experiment indicates that green tea helps nip colon tumors in the bud.


EGCG: Marketers' efforts
seem to exceed the evidence

The four primary polyphenols found in green and white tea leaves are epigallocatechin, epicatechin gallate, epicatechin, and epigallocatechin gallate or EGCG: some of which also occur in abundance in raw, non-Dutched cocoa and dark chocolate.


Without making health claims, major marketers of bottled beverages trumpet that they fortify theirs with added EGCG, because they know that media stores have made people aware of its anti-aging, heart-health, anti-diabetic, and anti-cancer potential.


But it appears that the body absorbs very little EGCG. As a recent study found, “…EGCG is unstable under physiologic conditions and has poor bioavailability” (Landis-Piwowar KR et al 2007).


(Roche Vitamins, which owns the patent on purified EGCG and markets it under the trade name Teavigomakes no claim that any EGCG is absorbed at all.)


In fact, the same scientists produced a modified version of EGCG that is much better absorbed and more durable in the body. Of course, the resulting compound would be a costly, patented drug.


In the meantime, based on the results of many studies of tea polyphenols and the behavior of other plant polyphenols ingested in isolation from their food context, it seems likely that the impact of multiple different catechinsas found in tea and dark chocolatewould be greater that than of EGCG alone in a sweet bottled drink.

Clinical trial: Green tea normalizes key antioxidant enzyme

Researchers from the University of Arizona decided to test the effects of green tea catechins on people’s body levels of a key antioxidant enzyme called glutathione S-transferase (GST).


GST can neutralize many of the free radicals generated internally as part of inflammation or acquired from air water, and food. Specifically, GST protects cellular DNA from cancer-causing oxidative damage induced by free radicals.


However, people’s levels of GST vary widely in accordance with genetic variations and environmental stresses.


The researchers recruited 42 healthy volunteers, took blood samples, and asked them to avoid tea for a full month.


Following the tea-free period, the participants all consumed 800mg of a key catechin-class polyphenolepigallocatechin gallate or EGCGper day for four weeks.


The Arizona team drew a second round of blood samples, and found that GST activity increased by an average of 17 percent.


But the increase was much higher in people who had low GST levels at the outset of the study, in whom GST activity rose by up to 80 percent.


This study indicates that green tea catechins can increase levels of GST in people who have low levels, and that green teaor a green tea extractcould help them strengthen their internal anti-cancer, anti-aging defenses.


Green tea curbs budding colon cancer in mice

According to a study in mice, green tea stops nascent colon tumors from growing.


It had no effect on more advanced tumors, but this was expected, since the anti-cancer potential of foods rich in anti-cancer compounds lies in prevention, not cure.


Scientists at University of South Carolina School of Medicine treated mice with a chemical (azoxymethane or AOM) that causes colon tumors.


At the age of eight weeks, the mice were divided into four groups, given either water or green tea solution as their only source of water:

  • Water only for 4 weeks
  • Water only for 8 weeks
  • Green tea solution only for 4 weeks
  • Green tea solution only for 8 weeks

They found that formation of colon tumors was inhibited significantly in the green tea groups, with no effect against larger tumors.


Test results showed that green tea decreased levels of body chemicals that signal the onset of colon cancer.


The tea had no effect on the pro-inflammatory COX-2 enzyme linked to development of colon cancer (Aspirin’s damping effect on COX-2 enzymes may explain why it seems to help prevent colon tumors).


While green tea polyphenols inhibit the COX-2 enzyme, they do so only when the doses far exceed those in normal green tea and in the green tea solution used in this mouse study.


We know that EGCG and other polyphenols in green tea work to prevent cancer growth by influencing multiple genetic switches and signaling pathways in cells, in ways known to discourage cancer.


In terms of observable physical effects, green tea polyphenols inhibit growth of blood supply to tumors, the spread of tumors, and cancer cell growth while inducing apoptosis (suicide) in cancer cells, thereby providing “…strong cancer chemopreventive effects” (Shankar S et al 2007).



Sources

  • Chow HH, Hakim IA, Vining DR, Crowell JA, Tome ME, Ranger-Moore J, Cordova CA, Mikhael DM, Briehl MM, Alberts DS. Modulation of human glutathione s-transferases by polyphenon e intervention. Cancer Epidemiol Biomarkers Prev. 2007 Aug;16(8):1662-6.
  • Issa AY, Volate SR, Muga SJ, Nitcheva D, Smith T, Wargovich MJ. Green tea selectively targets initial stages of intestinal carcinogenesis in the AOM-ApcMin mouse model. Carcinogenesis. 2007 Jul 17; [Epub ahead of print]
  • Shukla Y. Tea and cancer chemoprevention: a comprehensive review. Asian Pac J Cancer Prev. 2007 Apr-Jun;8(2):155-66.
  • Shankar S, Ganapathy S, Srivastava RK. Green tea polyphenols: biology and therapeutic implications in cancer. Front Biosci. 2007 Sep 1;12:4881-99. Review.
  • Carlson JR, Bauer BA, Vincent A, Limburg PJ, Wilson T. Reading the tea leaves: anticarcinogenic properties of (-)-epigallocatechin-3-gallate. Mayo Clin Proc. 2007 Jun;82(6):725-32. Review.
  • Yang CS, Maliakal P, Meng X. Inhibition of carcinogenesis by tea. Annu Rev Pharmacol Toxicol. 2002;42:25-54. Review.
  • Shankar S, Ganapathy S, Srivastava RK. Green tea polyphenols: biology and therapeutic implications in cancer. Front Biosci. 2007 Sep 1;12:4881-99. Review.
  • Landis-Piwowar KR, Huo C, Chen D, Milacic V, Shi G, Chan TH, Dou QP. A novel prodrug of the green tea polyphenol (-)-epigallocatechin-3-gallate as a potential anticancer agent. Cancer Res. 2007 May 1;67(9):4303-10.



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