Green Tea to Prevent Cancer?
Green Tea for Cancer Prevention: A Mixed Bag
Can something as simple as drinking green tea prevent cancer?
A large and growing body of evidence — both preclinical and human trials — suggests that there is a protective effect across different types of cancer. However, the data are not definitive.
The preventive effects of tea for a number of cancer types have been demonstrated in laboratory models, including cancers of the gastrointestinal tract, lung, prostate, breast, and skin. The proposed mechanisms of action include antioxidant effects, inhibition of growth-factor signaling, and enhancement of chemotherapy agents.
Much of the research to date, both experimental and clinical, and suggestions for future studies are outlined in 2 reviews published online October 30 in the American Journal of Clinical Nutrition.
Tea contains a large number of bioactive compounds, including catechins, flavonols, lignans, and phenolic acids, notes Jian-Min Yuan, MD, PhD, from the division of epidemiology and community health at the University of Minnesota in Minneapolis. In his review, he asserts that recent epidemiologic studies "neither confirm nor refute a definitive cancer-preventive role of green tea intake."
The large number of experimental studies conducted have consistently shown the inhibitory activities of green tea extract and/or green tea polyphenols against tumorigenesis at various organ sites, Dr. Yuan notes. However, results from epidemiologic studies have been inconsistent.
For example, Dr. Yuan recently reviewed epidemiologic studies looking at the association between esophageal cancer and green tea consumption in Asian populations, which have a high incidence of esophageal cancer and a high consumption of green tea (Mol Nutr Food Res. 2011;55:886-904). Of the 15 such studies, 6 reported a significantly reduced risk for esophageal cancer associated with high amounts of tea consumption, 4 reported a lower but nonsignificant risk, 3 reported a significantly positive association between tea consumption and esophageal cancer risk, and 2 reported no association.
Other reviews have reported similar inconsistencies for different cancer types, including colon, liver, breast, and prostate.
In the future, "we should conduct phase 2 intervention studies to understand the biologic mechanism of green tea polyphenols or other constituents in humans, given that current knowledge on tea and cancer protection is primarily derived from in vitro and in vivo animal experiments," Dr. Yuan told Medscape Medical News.
It would not be difficult to design and conduct phase 2 studies to learn the biologic mechanisms related to some of the known cancer-risk pathways in humans, Dr. Yuan contends. "The limiting factor is funding," he noted. "Of course, the final word on the effect of tea polyphenols on cancer prevention should come from phase 3 randomized large intervention trials that evaluate cancer incidence and mortality as study outcomes. Such trials would require a large number of subjects (more than 50,000), a long study period (at least 5 years), and a large number of institutions and investigators."
Dr. Yuan points out that a number of uncontrolled variables could account for inconsistent results from epidemiologic studies. Human exposure to tea polyphenols is relatively low — in the range of 1 to 2 orders lower than those generally used in in vitro and in vivo experimental studies. There is also the "residual confounding effect of cigarette smoking and alcohol consumption," along with the "adverse effect of the high temperature of tea beverages, which could mask or complicate the tea–cancer risk association."
The heterogeneity of the amount of tea consumed and the eating habits of different populations can also contribute to the inconsistencies seen in the results. These confounders help emphasize the need for randomized intervention studies that can ultimately provide "definitive data to determine the beneficial or deleterious effects of green tea consumption on cancer development in humans," Dr. Yuan writes.