Green tea stops the growth of colon tumors in their infancy, says a new study using mice, but merely drinking green tea offers no benefits against larger tumors.If the study, published in the journal Carcinogenesis, can be translated to humans, then this could have implications for the beverage and its extracts as a preventative against colon cancer.
"Our results suggest that green tea specifically targets initial stages of colon carcinogenesis; the time of administration of green tea is pivotal for effective chemoprevention," wrote the researchers, led by Ala Issa from University of South Carolina. "Beverage levels of green tea are not likely to inhibit the progress of any large adenomas or adenocarcinomas existing prior to the tea administration," they added.
The results add to an ever-growing body of science linking consumption to a wide range of health benefits, including lower risk of certain cancers, increased weight loss, improved heart health, and protection against Alzheimer's.
Green tea contains between 30 and 40 percent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 percent. The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
Issa and collaborators from the Medical University of South Carolina and the University of Columbia used ApcMin mice as a model for colon cancer and treated the animals with azoxymethane (AOM), a chemical that selectively induces cancer in the colon. At the age of eight weeks, the mice were divided into two groups - one to receive water or a green tea solution (0.6 percent) as the only beverage source for a further four to eight weeks.
The researchers report that consumption of the green tea solution significantly inhibited the formation of new tumors in the colon, but had no effect against larger tumors. A mechanistic study indicated that green tea decreased the total levels of the early carcinogenesis biomarker beta-catenin and its downstream target cyclin D1. No effect on the cyclooxygenase-2 (COX-2) enzyme that plays a role in inflammatory responses and has been linked to cancer development.
"Despite its appeal as a target for chemoprevention by green tea, inhibition of COX-2 expression seems to be achieved at concentrations far exceeding the physiological concentrations of catechins present in the green tea beverage," wrote the researchers. "Therefore, inhibition of COX-2 cannot explain the vast number of epidemiological data that shows negative association between drinking green tea and the risk for cancers such as colorectal cancer," they stated.
A large body of epidemiological data already indicates that the beverage and its extracts may offer significant benefits for lowering the risk of colon cancer.
A.Y. Issa, S.R. Volate, S.J. Muga, D.Nitcheva, T. Smith and M.J. Wargovich. Green tea selectively targets initial stages of intestinal carcinogenesis in the AOM-ApcMin mouse model; Carcinogenesis. 2007 Jul 17; Published online ahead of print, doi:10.1093/carcin/bgm161.