A high proportion of the human population has consumed caffeine-containing beverages for many centuries. Authenticated usage of tea in China dates from a.d. 350. The beverage reached Europe in 1600. Coffee came into use as a beverage in Arabia around a.d. 1000 and was brought to Europe in the seventeenth century. Cocoa was consumed by the Aztecs in Mexico. In many parts of South America maté has been a source of substantial caffeine intake. Caffeine content of several contemporary food products is shown in Table 1.
Variations in caffeine content in specific plant species result from varietal diversity, climatic changes in the growing areas, and horticultural techniques. In tea, youngest leaf has the highest concentration (17). Processing conditions also affect caffeine content. High coffee roasting temperatures result in caffeine loss by sublimation (18). There is a higher level of caffeine in tea than in coffee beans, but 200 cups of tea beverage are obtained per pound of tea leaves, whereas only about 40 to 60 cups of coffee are usually prepared per pound of coffee beans.
By far, the greatest sources of dietary caffeine are coffee and tea, but cola beverages provide an increasing portion of the intake. The total per capita intake of caffeine by adults in the United States is about 3 mg/kg of body weight with about 2 mg/kg coming from coffee and most of the rest from tea. For caffeine users, average intake is approximately 4 mg/kg (19). The consumption by heavy users is of interest because of concern with possible health effects. Mean daily intake (mg/kg body weight) in the United States by consumers in the 90th to 100th percentiles of consumption varies with age level as follows:
Tea and soft drinks are the major caffeine sources for users under the age of 18. Coffee becomes the major source for those over that age (20).
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