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Vitamin K is an essential cofactor for the posttranslational synthesis of gamma-carboxyglutamic acid (Gla) in several proteins involved in the regulation of blood clotting. It is also a cofactor of a number of other vitamin K-dependent proteins found in tissues that are not involved in blood coagulation (eg, osteocalcin, rephrocalcin, matrix Gla protein (1).
In a literature review on the dietary sources of vitamin K (2), we discovered large discrepancies in the reported vitamin K-1 (phylloquinone) content of tea and coffee. though tea and coffee have been listed as important dieta contributors of vitamin K-1 (3-5), some investigators (6,7) have reported negligible amounts in the brews. It is important to resolve this difference because some studies have reported that foods rich in vitamin K-1 interfere with anticoagulant therapy (8,9). Dietary guidelines have been written that restrict these foods for persons taking warfarin for the management, of thromboembolic disease(10). Brewed tea has been used therapeutically in children the low prothrombin levels, and its effectiveness has been tributed to the vitamin K-1 extracted during the brewing process (3). Other reports (11-13, however, identify tea catechines and other secondary chemicals as the active components that affect blood coagulation.
To resolve the discrepancies reported for the vitamin K-1 content of tea and coffee, we conducted a detailed study of the content of this vitamin in the brews and of tea leaves and ground coffee beans before and after the brewing process.
METHODS
Single samples of 14 types of tea (including 2 samples each of green and black tea; 2 types of ground, generic-brand coffee beans (1 with caffeine and 1 commercially decaffeinated); and 2 types of instant coffee (1 with caffeine and 1 commercially decaffeinated) were randomly purchased from commercial stores within the Boston, Mass, area. Because of the growing popularity of herbal and flavored teas, samples representing these different types of teas were included in the analysis. All samples were stored at room temperature and protected from light. Each leaf or ground bean sample was analyzed in triplicate, whereas brewed tea and coffee samples were analyzed in duplicate. All samples were analyzed within 6 months of purchase.
SAMPLE PREPARATION
The vitamin K-1 content of all dry tea leaf and coffee...