Research continues to link tea with lowered risk of several types of cancer, yet many questions remain unanswered. Studies have identified several pathways through which substances in tea can fight cancer development. And several population studies link tea consumption with lower cancer risk. But many specific details remain unknown.
A new report in the Journal of Nutrition identifies many different types of studies that show how tea could help in the prevention of cancer.
Laboratory research with human cancer cells show that a natural phytochemical in tea, called EGCG, can block several enzymes necessary for these cells to reproduce and flourish. But many of these experiments have involved concentrations of EGCG higher than those found in human tea drinkers, so it’s difficult to know how much of this benefit we can actually experience from tea.
Further laboratory studies show that EGCG and other polyphenol substances in tea are strong antioxidants. Antioxidants prevent and repair DNA damage that could otherwise lead to cancer, and also help to protect our heart and blood vessels.
Animal studies have consistently shown that tea and polyphenols in tea can inhibit cell damage linked with cancers of the skin, lung, colon, liver and pancreas. Scientists say that we now need research that includes good markers of oxidation damage in animals and humans, to further identify the effectiveness of antioxidants from green and black tea.
Many (but not all) human studies show important increases in antioxidant capacity with the consumption of one to six cups of tea per day. This research suggests that greater antioxidant potential in the blood after drinking green or black tea better protects our cells’ DNA, but again, scientists say we need better studies that can clearly measure such damage.
Tea’s potential protection against colon cancer may not be surprising, since its antioxidants would be closely available to protect digestive tract cells. Yet another report in the Journal of Nutrition examines 30 different studies of tea consumption in 12 countries. It concludes that support for tea as a strong anti-cancer influence is not consistent.
One of the problems encountered in looking at large populations is that it’s not always easy to tell whether the effects we see are really due to tea. For example, in some studies, people who drink tea may also eat more vegetables and fruits, or exercise more regularly.
A new study of the potential link of green tea consumption with lower risk of breast cancer does suggest a protective benefit. Asian-American women who averaged at least three six-ounce cups of green tea per week were about half as likely to develop breast cancer as those who drank none.
One of the strengths of this study is that researchers statistically adjusted for the effects of many other influences on breast cancer risk, such as family history, smoking, exercise, body size, menstrual and childbearing history, and even consumption of soy and dark green vegetables. Adjusting for these influences reduces the chance that the protection seen was really due to some other factor. But it is possible, for example, that these Asian-American women who drank green tea more often also followed traditional Asian diets with less meat and more fish, or made other lifestyle choices that offered protection.
Many consumers also wonder whether they need to drink green tea to get health benefits. Green tea is higher in antioxidants than black tea. Some population studies have shown green tea consumption most strongly linked with lower cancer or DNA damage, but others have shown no significant difference in impact.