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Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, MA 02111
3To whom correspondence should be addressed. E-mail: jeffrey.blumberg{at}tufts.edu.
The Third International Scientific Symposium on Tea and Human Health: Role of Flavonoids in the Diet was held September 23, 2002 at the U.S. Department of Agriculture in Washington, D.C. The symposium was organized by the Tea Council of the USA and cosponsored by the American Cancer Society, American College of Nutrition, American Health Foundation, American Society for Nutritional Sciences, Food and Agriculture Organization and the Linus Pauling Institute at Oregon State University. The symposium was cochaired by Martijn Katan (Wageningen University) and Jeffrey Blumberg (Tufts University). The first meeting in this series, Physiological and Pharmacological Effects of Camellia sinesis (Tea): Implications for Cardiovascular Disease, Cancer, and Public Health, was chaired by John Weisburger (American Health Foundation) and held at the Ford Foundation, New York, in 1991 (1). Dr. Weisburger also chaired the Second International Scientific Symposium on Tea and Human Health at the USDA in 1998 (2). The scientific publications indicating a role for tea in the promotion of health and prevention of disease have grown substantially during the last decade (Fig. 1) as have critical reviews of this topic (3–14). Indeed, the number of studies completed since 1998 suggested that it would be useful to again review the state-of-the-science regarding tea and health. Because most of this research has been directed toward the potential preventive and therapeutic roles of tea in cancer and heart disease, the leading causes of morbidity and mortality in most industrialized countries, this symposium focused principally on these two conditions while trying also to place tea as a whole food contributing substantially to dietary flavonoid intakes.
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B and AP-1 and reduction of protein tyrosine kinase activity and c-jun mRNA expression, have also been suggested as relevant chemopreventive pathways for tea (19). Although it is reasonable to focus on the bioactivity of catechins from tea, as is the case in this symposium, it is important to appreciate the complex phytochemical composition of the beverage that includes other polyphenols, amino acids, caffeine, carbohydrates, chlorophyll, fluoride, minerals, protein and still undefined compounds (20). Indeed, the recent report by Kamath et al. (21) suggests that ethylamine and its precursor L-theanine, not catechins, are the constituents in tea responsible for priming human 
T cells to promote natural resistance to microbial infections and perhaps tumors. The implication that tea may reduce the risk for cancer and cardiovascular disease is significant with their estimated impact directly on medical costs and indirectly through productivity losses estimated at $157 and $352 billion per annum, respectively, in the United States alone. However, despite the focus on these diseases here, intriguing recent evidence suggests that tea may also play a beneficial role in other conditions, including dental caries (22,23), osteoporosis (24,25), cognitive function (26) and weight maintenance (27).
One challenge in interpreting the observational evidence concerning tea and health is appreciating the diversity of this beverage. For example, the relative catechin content of tea is dependent upon how the leaves are processed prior to drying as well as geographic location and growing conditions. The flavonoid concentration of any particular tea beverage depends upon the type of tea (e.g., blended, decaffeinated or instant) and preparation (e.g., amount used, brew time and temperature). The highest concentration of flavonoids are found in brewed hot tea (541–692 µg/mL), less in instant preparations (90–100 µg/mL) and lower amounts in iced and ready-to-drink tea (28,29). Decaffeinating reduces the catechin content of black tea by 10–20%. Despite some early controversy, research results are largely consistent in demonstrating that the addition of milk to tea does not interfere with catechin absorption (30).
The conduct of clinical studies with tea can also present challenges to investigators. For example, not only are the kind of tea (i.e., green, oolong and black) and preparation important, but the frequency and timing of intake are critical factors because they affect the pharmacokinetics and ultimate disposition of the polyphenols within tissues. Furthermore, interindividual variations in the bioavailability of tea polyphenols can be substantial and may be due in part to differences in colonic microflora and genetic polymorphisms among the enzymes involved in polyphenol metabolism (31). The effect of tea drinking may also differ by genotype, e.g., individuals with the E2 allele of ApoE possess a reduced plasminogen activator inhibitor (PAI-1) activity following consumption of black tea (32).
Other than water, tea is the most popularly consumed beverage worldwide with a per capita consumption estimated at about 120 mL/d. Black tea is consumed principally in Europe, North America and North Africa (except Morocco) whereas green tea is drunk throughout Asia; oolong tea is popular in China and Taiwan. All tea is produced from the leaves of the tropical evergreen C. sinensis. Black tea is made via a postharvest "fermentation," an auto-oxidation catalyzed by polyphenol oxidase. After picking, leaves for green tea are steamed to inactivate polyphenol oxidase prior to drying. Oolong tea is produced by a partial oxidation of the leaf, intermediate between the process for green and black tea. Approximately 77% of the tea produced and consumed worldwide is black, 21% is green and 2% is oolong.
The Dietary Guidelines for Americans provide detailed information about healthful food patterns but offer little advice concerning beverage consumption beyond including milk within the dairy group and suggesting alcohol intake be moderate if and when it is consumed. While the totality of the evidence from research on tea is promising, more research is necessary to fully understand its contributions to human health. While no single food item can be expected to provide a significant effect on public health, it is important to note that a modest effect between a dietary component and a disease having a major impact on the most prevalent causes of morbidity and mortality, i.e., cancer and heart disease, should merit substantial attention. While nutritional guidelines for public health should always be conservative, with the potential benefits and efficacy of changes defined in the near absence of risk, there is no evidence to suggest any adverse consequence from tea consumption in an otherwise healthful diet.
Dietary recommendations must be developed such that people will accept the changes proffered and try, if only with partial success, to incorporate them into their lives. Human studies now suggest tea may contribute to a reduction in the risk of cardiovascular disease and some forms of cancer as well as to the promotion of bone and oral health. Because tea is already one of the most popular beverages worldwide, future studies, designed to accurately assess tea consumption and tea polyphenol status, should be directed to quantifying its role in the primary and secondary prevention of chronic diseases.
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FOOTNOTES |
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2 The contents of this publication do not necessarily reflect the views or policies of the USDA nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. government.
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LITERATURE CITED |
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