Tea may protect against endometrial cancer, but more research is needed before it’s clear if the antioxidant-rich beverage offers a real benefit, a recent analysis found.

Tea is the second most-consumed beverage in the world, after water, and multiple studies have looked into whether or not the drink brewed from the plant Camellia sinesis protects against various types of cancer. Animal studies have shown that the polyphenols found in tea may have a tumor-shrinking effect, but results focusing on endometrial cancer haven’t shown a clear benefit.

Endometrial cancer - which forms in the lining of the uterus - is the fourth most common cancer in American women. The National Cancer Institute says there are 42,000 new cases in the United States each year, and nearly 7,800 deaths.

Researchers from the National Shanghai Center for New Drug Safety Evaluation and Research in China analyzed several published studies looking at the role of green and black tea in the prevention of endometrial cancer.

While the researchers found that existing research indicates that drinking tea - particularly green tea - may offer some protection against endometrial cancer, they cautioned that the limited number of overall studies means that more investigation is needed.

The analysis of existing research, published in the American Journal of Obstetrics and Gynecology, included seven studies. The researchers first compared people who consumed tea regularly with those with the lowest or no tea consumption, and then compared low-consumption, moderate-consumption and high-consumption tea drinkers.

After accounting for the different ways the studies measured tea drinking, the researchers found that an increase in tea consumption of two cups daily was associated with a 25-percent reduced risk of developing endometrial cancer. The association was significant for green tea but not for black tea. There was also a protective effect shown in the Chinese and Japanese studies but not the American studies.

The researchers cautioned that the risk reduction seen in Asian studies but not the American studies may be the result of some other unexamined factor, such as diet, lifestyle or genetic differences. For example, American tea drinkers tend to drink black tea while most of tea drinkers in China and Japan drink green tea.

Finally, simply measuring tea exposure is difficult. Tea consumption in the different studies was measured by cups consumed daily, but cup size could vary among participants and across different countries.

If tea does offer a protective effect against endometrial cancer, it’s likely due to a number of factors, the researchers suggested. Endometrial cancer is associated with late menopause or infertility, and the caffeine in tea can affect hormone levels.

Also, earlier research indicates that tea contains antioxidants that may affect cancer development. Tea also contains phytoestrogens, compounds that might protect against endometrial cancer because they could interfere with estrogen receptors.

It’s difficult to apply the results of this analysis globally because the studies involved only looked at three countries, and because the researchers only found seven studies to review. However, the researchers said that because tea drinkers showed some evidence of a reduced risk of endometrial cancer, further investigation of the possible connection is worthwhile.

SOURCE: American Journal of Obstetrics and Gynecology, December 2009.