Original Contribution BLACK TEA INCREASES THE RESISTANCE OF HUMAN PLASMA TO LIPID PEROXIDATION IN VITRO, BUT NOT EX VIVO ANTONIO CHERUBINI,* † M. FLINT BEAL,* and BALZ FREI † *Neurochemistry Laboratory, Massachusetts General Hospital, Harvard Medical School, Boston, MA, and † Linus Pauling Institute, Oregon State University, Corvallis, OR, USA (Received 2 December 1998; Revised 9 March 1999; Accepted 10 March 1999) Abstract—A number of in vitro studies have shown that polyphenols and flavonoids in tea exert significant antioxidant activity. However, epidemiologic and experimental studies have produced conflicting results. The purpose of the present study was to compare the antioxidant activity of black tea in vitro with that ex vivo. Black tea polyphenols (BTP), black tea extract (BTE), or their major polyphenolic antioxidant constituent, epigallocatechin gallate (EGCG), were added to human plasma and lipid peroxidation was induced by the water-soluble radical generator 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Following a lag phase, lipid peroxidation was initiated and occurred at a rate that was lowered in a dose-dependent manner by BTP. Similarly, EGCG and BTE added to plasma in vitro strongly inhibited AAPH-induced lipid peroxidation. The lag phase preceding detectable lipid peroxidation was due to the antioxidant activity of endogenous ascorbate, which was more effective at inhibiting lipid peroxidation than the tea polyphenols and was not spared by these compounds. In contrast, when eight healthy volunteers consumed the equivalent of six cups of black tea, the resistance of their plasma to lipid peroxidation ex vivo did not increase over the next 3 h. These data suggest that, despite antioxidant efficacy in vitro, black tea does not protect plasma from lipid peroxidation in vivo. The striking discrepancy between the in vitro and ex vivo data is most likely explained by the insufficient bioavailability of tea polyphenols in humans. © 1999 Elsevier Science Inc. Keywords—Tea, Flavonoids, Antioxidants, Lipid peroxidation, Human study, Free radicals INTRODUCTION Oxidative stress appears to play an important role in several human chronic diseases, including atherosclero- sis and cancer [1–3]. Therefore, antioxidants may be useful in the prevention and treatment of these condi- tions. Flavonoids are a group of polyphenolic substances ubiquitous in foods of plant origin, and are present in some beverages, such as wine and tea. Tea, the second most common beverage in the world after water, is produced from the leaves of the plant Camellia sinensis. There are two major types of tea: green tea, which is consumed almost exclusively in Asian and North African countries; and black tea, which represents approximately 80% of the world consumption of tea [4]. Green tea has a high content of flavanols, i.e., catechins, which can exist in two different configurations, with the “epi” con- figuration being the more common. The gallocatechins are catechins containing three hydroxy groups on the B ring, whereas the catechin gallates are esterified on the pyran ring with gallic acid. Black tea undergoes exten- sive fermentation during which the majority of the cat- echins are enzymatically oxidized, with formation of theaflavins, bisflavanols and thearubigens. In vitro studies have consistently demonstrated that tea flavonoids have significant antioxidant activity due to their ability to scavenge reactive oxygen species and chelate metal ions [5– 8]. Therefore, the consumption of tea, as well as other flavonoid-containing foods and beverages [8], has been proposed as a useful practice to limit oxidative damage in the body. However, studies of tea consumption in animals and humans have produced conflicting results [9 –17], suggesting that the evidence for antioxidant activity of tea flavonoids in vitro cannot be extrapolated to the in vivo situation, as the bioavail- ability of these compounds may be limited. In order to Address correspondence to: Balz Frei, Ph.D., Director, Linus Pauling Institute, Oregon State University, 571 Weniger Hall, Corvallis OR 97331-6512, USA; Tel: (541) 737-5078; Fax: (541) 737-5077; E-Mail: balz.frei@orst.edu. Free Radical Biology & Medicine, Vol. 27, Nos. 3/4, pp. 381–387, 1999 Copyright © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0891-5849/99/$–see front matter PII S0891-5849(99)00064-7 381