‣ Tea contains polyphenol compounds, particularly catechins, which are antioxidants and whose biological activities may be relevant to cancer prevention.
‣ Few clinical trials of tea consumption and cancer prevention have been conducted and their results have been inconclusive.
‣ Results of epidemiologic studies examining the association between tea consumption and cancer risk have been inconclusive.
‣ Inconsistencies in study findings regarding tea and cancer risk may be due to variability in tea preparation, tea consumption, the bioavailability of tea compounds (the amounts that can be absorbed by the body), lifestyle differences, and individual genetic differences.
‣ The National Cancer Institute does not recommend for or against the use of tea to reduce the risk of any type of cancer.
What is tea?
Tea is one of the most ancient and popular beverages consumed around the world. Black tea accounts for about 75 percent of the world’s tea consumption (1). In the United States, United Kingdom (UK), and Europe, black tea is the most common tea beverage consumed; green tea is the most popular tea in Japan and China (2). Oolong and white tea are consumed in much lesser amounts around the world (2).
Tea is made from the leaf of the plant Camellia sinensis. Shortly after harvesting, tea leaves begin to wilt and oxidize. During oxidation, chemicals in the leaves are broken down by enzymes, resulting in darkening of the leaves and the well-recognized aroma of tea. This oxidation process can be stopped by heating, which inactivates the enzymes. The amount of oxidation and other aspects of processing determine a tea’s type. Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized. In contrast, green tea is made from unwilted leaves that are not oxidized. Oolong tea is made from wilted, bruised, and partially oxidized leaves, creating an intermediate kind of tea. White tea is made from young leaves or growth buds that have undergone minimal oxidation. Dry heat or steam can be used to stop the oxidation process, and then the leaves are dried to prepare them for sale.
Tea is brewed from dried leaves and buds (either in tea bags or loose), prepared from dry instant tea mixes, or sold as ready-to-drink iced teas. So-called herbal teas are not really teas but infusions of boiled water with dried fruits, herbs, and/or flowers.
What are the ingredients of tea?
Tea is composed of polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds (chemicals that readily produce vapors and contribute to the odor of tea), fluoride, aluminum, minerals, and trace elements (3). The polyphenols, a large group of plant chemicals that includes the catechins (4), are thought to be responsible for the health benefits that have traditionally been attributed to tea, especially green tea. The most active and abundant catechin in green tea is epigallocatechin-3-gallate (EGCG). The active catechins and their respective concentrations in green tea infusions are listed in the table below.
TABLE 1. Catechin Concentrations of Green Tea Infusions.
|Catechin in Green Tea Infusion||Catechin Concentration
(mg/8 fl oz)
|Note: mg = milligram; L = liter; fl oz = fluid ounce. See reference 5.|
Black tea contains much lower concentrations of these catechins than green tea (6). The extended oxidation of black tea increases the concentrations of thearubigins and theaflavins, two types of complex polyphenols (2). Oolong tea contains a mixture of simple polyphenols, such as catechins, and complex polyphenols (2). White and green tea contain similar amounts of EGCG but different amounts of other polyphenols (7).