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D J Baer and S C Chen, US Department of Agriculture, Beltsville, MD, USA

Published by Elsevier Ltd.

Throughout the world, tea is one of the most consumed beverages, second only to water. Based on data from the Tea Association of the United States, the estimated wholesale value of the tea industry increased 273% between 1990 and 2002, and it is now valued in excess of $5 billion. The greatest increase has been in the ready-to-drink market (925% increase). One reason for the popularity in tea consumption is the interest of consumers to improve their health. Although tea is a poor source of most classically defined essential nutrients, it is a good and important source of many phytochem-icals that have been presumptively linked to reduction of risk of cancer, cardiovascular disease, and diabetes. Many of these phytochemicals have strong antioxidant properties, and presumably these antioxidant properties are associated with improved health.

Epidemiological studies have provided the evidence suggesting an association between the consumption of plant-based foods and diets with a reduced risk of diseases, especially cardiovascular disease and cancer. These findings, together with experimentally data, have spurred further research activities and encouraged the consumption of fruits, vegetables, and other foods and beverages with high concentrations of nutrient (e.g., vitamin C, vitamin E, and carotenoids) and nonnutrient antioxidants (e.g., polyphenols). Epi-demiologic studies that focus on food consumption indicate great potential benefit from consumption of foods high in antioxidant phytonutrients. Experimental studies include in vitro and cell culture studies; ex vivo studies using biological material from experimental animals and humans (e.g., determination of susceptibility of low-density lipoprotein to oxidation); and investigations using animal models of cancer, cardiovascular disease, diabetes, arthritis, macular degeneration, and the diseases of most other organ systems. Many experimental studies focus on the antioxidant compounds that are found in foods and not on the foods per se as they are typically consumed. Although disease prevention based on inference from these studies seems promising, results from dietary intervention studies are inconclusive or contradictory. Moreover, the number of well-designed, carefully controlled dietary intervention studies, which show cause and effect, is limited, and this represents a critical gap in our understanding of these compounds as they relate to health promotion and disease prevention.

More than 8000 phenolic compounds have been identified in plants. The structure of these compounds ranges from simple monomers to highly complex, heterogeneous polymers with molecular sizes in excess of 30 kDa. Of these phenolic compounds, more than 5000 are structurally defined as polyphenolic 'flavonoids.' Although there is an abundance of these compounds in our food supply, many studies have focused on the activity of flavo-noids from tea, wine, and cocoa because these are the most common sources of flavonoids in typical American diets. Compared to most foods, tea contains a much higher concentration of flavonoids (in the form of flavan-3-ols), which are provided with a minimal amount of additional energy. The high concentration of water-soluble polyphenols, limited contribution of other nutrients (especially other anti-oxidants), ease of product preparation and administration, and subject acceptability make tea an ideal source for these potentially important compounds for human intervention studies.

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