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Supplement: Proceedings of the Third International Scientific Symposium on Tea and Human Health

Antioxidant Activity of Tea Polyphenols In Vivo: Evidence from Animal Studies¹

Linus Pauling Institute, Oregon State University, Corvallis, OR 97331

²To whom correspondence should be addressed. E-mail: Balz.Frei{at}oregonstate.edu.

	ABSTRACT

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
Tea is particularly rich in polyphenols, including catechins, theaflavins and thearubigins, which are thought to contribute to the health benefits of tea. Tea polyphenols act as antioxidants in vitro by scavenging reactive oxygen and nitrogen species and chelating redox-active transition metal ions. They may also function indirectly as antioxidants through 1) inhibition of the redox-sensitive transcription factors, nuclear factor-B and activator protein-1; 2) inhibition of "pro-oxidant" enzymes, such as inducible nitric oxide synthase, lipoxygenases, cyclooxygenases and xanthine oxidase; and 3) induction of phase II and antioxidant enzymes, such as glutathione S-transferases and superoxide dismutases. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo. Animal studies offer a unique opportunity to assess the contribution of the antioxidant properties of tea and tea polyphenols to the physiological effects of tea administration in different models of oxidative stress. Most promising are the consistent findings in animal models of skin, lung, colon, liver and pancreatic cancer that tea and tea polyphenol administration inhibit carcinogen-induced increases in the oxidized DNA base, 8-hydroxy-2'-deoxyguanosine. In animal models of atherosclerosis, green and black tea administration has resulted in modest improvements in the resistance of lipoproteins to ex vivo oxidation, although limited data suggest that green tea or green tea catechins inhibit atherogenesis. To determine whether tea polyphenols act as effective antioxidants in vivo, future studies in animals and humans should employ sensitive and specific biomarkers of oxidative damage to lipids, proteins and DNA.

KEY WORDS: • tea • polyphenol • antioxidant • biomarker • oxidative damage

The potential for the consumption of tea or tea polyphenols to prevent or ameliorate chronic disease is currently the subject of considerable scientific investigation (1). Although a number of mechanisms have been proposed for the beneficial effects of tea in different models of chronic disease, the radical scavenging and antioxidant properties of tea polyphenols are frequently cited as important contributors (2). Much of the evidence supporting an antioxidant function for tea polyphenols is derived from assays of their antioxidant activity in vitro. However, evidence that tea polyphenols are acting directly or indirectly as antioxidants in vivo is more limited. Animal studies offer a unique opportunity to assess the contribution of the antioxidant properties of tea polyphenols to the physiological effects of tea administration in different models of oxidative stress. The purpose of this article is to review the experimental evidence from animal studies thus far that tea polyphenols function as effective antioxidants in vivo.

	Tea polyphenol content

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
Fresh tea leaves are rich in flavanol monomers known as catechins. The principal catechins found in tea are (-)-epicatechin (EC)² (3), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG). EGCG is the most abundant catechin in tea (3). Tea leaves also contain polyphenol oxidase enzymes in separate layers of the leaf. When tea leaves are rolled or broken during industry manufacture, catechins come in contact with polyphenol oxidase, resulting in their oxidation and the formation of flavanol dimers and polymers known as theaflavins and thearubigins, respectively (4). Tea leaves destined to become black tea are rolled and allowed to ferment (oxidize), resulting in relatively high concentrations of theaflavins and thearubigins and relatively low concentrations of catechins. Green tea is withered and then steamed to inactivate polyphenol oxidase. Consequently, green tea contains relatively high concentrations of catechins and low concentrations of theaflavins and thearubigins. Tea also contains small amounts of flavonols (kaempferol, quercetin and myricitin) in the form of glycosides. The flavonol content is less affected by processing, and flavonols are present in comparable amounts in green and black teas (5).

	Potential mechanisms for the antioxidant effects of tea

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
Radical and oxidant scavenging.

Numerous studies have demonstrated that tea catechins and polyphenols are effective scavengers of physiologically relevant reactive oxygen and nitrogen species in vitro, including superoxide [O·₂^- (6,7)], peroxyl radicals, singlet oxygen (8), peroxynitrite [ONOO^- (9,10)], and hypochlorous acid (11). Several structures appear to be important for these antioxidant activities of tea polyphenols (Fig. 1), including an ortho-3'4'-dihydroxyl (catechol) group or 3'4'5'-trihydroxyl (gallate) group in the B ring, a gallate group esterified at the 3 position of the C ring, and hydroxyl groups at the 5 and 7 positions of the A ring (12).

View larger version (9K): [in this window] [in a new window]   FIGURE 1 Functional groups important to the antioxidant activity of catechin monomers, dimers (theaflavins) and polymers (thearubigins): example, epicatechin gallate.

Even with very high intakes of tea or tea extracts, plasma and intracellular concentrations of tea catechins and polyphenols in humans are likely to be 100 to 1000 times lower than those of other physiological antioxidants, such as ascorbate, urate and glutathione (Table 2). Thus, the relative importance of tea catechins and polyphenols as radical and oxidant scavengers in vivo may be minor, based on their reduction potentials and concentrations achieved in plasma and tissues.

The ability of tea polyphenols to chelate metal ions, such as iron and copper, may contribute to their antioxidant activity by preventing redox-active transition metals from catalyzing free radical formation (16). These metal-chelating properties likely explain the ability of tea polyphenols to inhibit copper-mediated LDL oxidation and other transition metal-catalyzed oxidations in vitro (17). However, it is not clear whether metal chelation is a physiologically relevant antioxidant activity, because most transition metal ions are bound to proteins in vivo where they cannot participate in metal-catalyzed free radical formation.

Inhibition of redox-sensitive transcription factors.

Green and black tea, as well as individual catechins and tea polyphenols, can inhibit the activation of the redox-sensitive transcription factors, nuclear factor-B (NF-B) and activator protein-1 (AP-1), in cultured cells. Although other antioxidants also can inhibit these redox-sensitive transcription factors, recent research indicates that tea catechins and polyphenols are acting as kinase inhibitors in complex signaling pathways. Interestingly, the kinase inhibiting activities of tea polyphenols may not be directly related to their ability to function as hydrogen donators or antioxidants (18).

Inhibition of "pro-oxidant" enzymes.

Stimulation of inflammatory cells such as macrophages by bacterial endotoxins or inflammatory cytokines results in increased expression of inducible nitric oxide synthase (iNOS) and subsequent production of large amounts of nitric oxide (NO^·). Nitric oxide reacts very rapidly with O·₂^- to form ONOO^- and other NO-derived oxidants capable of damaging DNA and proteins (19). Green tea and black tea (10,20), as well as individual catechins (21,22) and theaflavins (23), can inhibit lipopolysaccharide-induced iNOS gene expression and iNOS activity in cultured macrophages. Green tea catechins and black tea theaflavins appear to downregulate iNOS by inhibiting NF-B activation (22,23).

Through their peroxidase activity, lipoxygenases and cyclooxygenases are capable of co-oxidizing molecules other than their regular substrates, with the potential for increasing oxidative damage in some tissues (24). Green and black tea polyphenols were found to inhibit cyclooxygenase (COX)-2 and 5-, 12-, and 15-lipoxygenase activities in human colon mucosa cells and human colon cancer cells (25). Feeding green tea polyphenols to mice inhibited ultraviolet light-induced increases in epidermal COX activity (26), whereas topical application of green tea (27) and black tea polyphenols (28) inhibited phorbol ester-induced increases in epidermal COX and lipoxygenase activities. Precancerous lesions of colon mucosa (aberrant crypts) and COX-2 activity were lower in azoxymethane-treated rats given 2% green tea extract in their drinking water compared with controls (29).

Tea polyphenols may also inhibit the formation of reactive oxygen species by inhibiting the enzyme, xanthine oxidase. Xanthine oxidase catalyzes the oxidation of both hypoxanthine and xanthine to uric acid, while reducing O₂ to O·₂^- and H₂O₂. Green tea catechins can inhibit the activity of xanthine oxidase in vitro, with EGCG exerting the most potent effect (30). In cultured human leukemia cells, EGCG from green tea and theaflavin gallates from black tea also inhibited xanthine oxidase activity (31).

Induction of phase II enzymes.

Phase II detoxification enzymes promote the excretion of potentially toxic or carcinogenic chemicals. Most phase II enzymes contain cis-acting regulatory elements called antioxidant response elements (ARE). Glutathione S-transferases (GST) are a family of phase II enzymes that catalyze the conjugation of glutathione to electrophiles, thereby reducing their ability to react with and damage nucleic acids and proteins (24). Green tea polyphenol extract (32) as well as individual green tea catechins (33) have been found to increase ARE-mediated reporter gene activity in transfected HepG2 cells. Feeding rats green tea leaves significantly increased liver GST activity (34), and providing mice with green tea polyphenols in their drinking water also significantly increased GST activity in the liver and small intestine (35).

Limitations of in vitro research on antioxidant activity of tea polyphenols.

The bioavailability of tea catechins appears to be relatively low. When healthy volunteers were given a single serving of 4.5 g of green tea solids dissolved in 500 mL of water, peak plasma concentrations of individual catechins (conjugated and unconjugated) were <2 µmol/L (36). Average peak plasma catechin concentrations (conjugated and unconjugated) in healthy volunteers given a single dose of 1.5 mmol of pure EGC, ECG or EGCG were 5.0, 3.1 and 1.3 µmol/L, respectively (37). These values represent peak plasma levels after high doses of green tea or pure catechins. Average plasma catechin concentrations are likely to be considerably lower. Because theaflavins and thearubigins are difficult to detect in blood or urine, there is little information regarding the biotransformation or bioavailability of black tea polyphenols in humans or animals.

Upon ingestion, tea catechins are rapidly and extensively metabolized in the intestines, liver and kidneys. The major biotransformation reactions of tea catechins are glucuronidation, sulfation and methylation (18). Following tea ingestion 4'-O-methyl-EGC and its glucuronidated and sulfated metabolites were found in human plasma at concentrations 4–6 times higher than unconjugated EGC (38). Tea catechins are also metabolized by intestinal microflora. Bacterial ring fusion metabolites of EGC and EC have been detected in human urine and plasma in amounts several times higher than their precursors (39). Studies in cultured cells indicate that catechin metabolites have different antioxidant and biological activities than their precursors (40).

A great deal of research has evaluated the antioxidant and biological activities of green and black tea as well as their individual catechins and polyphenols in vitro. Until recently, relatively little of the in vitro research published employed physiologically relevant concentrations of catechins. Evidence that catechins are extensively metabolized in vivo and that the antioxidant and biological activities of catechin metabolites may differ from those of their parent compounds emphasizes the importance of demonstrating the antioxidant effects of tea and tea polyphenols in vivo.

	Antioxidant activity of tea and tea polyphenols in animal models of oxidative stress

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
Endogenous antioxidants and antioxidant enzymes.

The addition of green tea catechins to plasma (41) or LDL (42) resulted in sparing of endogenous -tocopherol during in vitro oxidation. In hypercholesterolemic rabbits, green and black tea administration increased plasma -tocopherol concentrations after 8 and 17 wk of tea administration but not after 21 wk (43). The total plasma antioxidant capacity was not affected by green or black tea administration over the 21-wk study period. In rats, administration of green tea catechins prevented decreases in plasma and erythrocyte -tocopherol concentrations resulting from a diet high in PUFA (6), but green tea flavonoid administration to marginally vitamin C-deficient Osteogenic Disorder Shionogi (ODS) rats did not increase plasma -tocopherol concentrations (44).

Tea administration prevented decreases in tissue glutathione (GSH) concentrations in three out of four animal studies. Consumption of black tea leaves prevented carbon tetrachloride-induced liver depletion of GSH in male rats (45) but not in female rats (46). Similarly, providing green tea extract in the drinking water of male rats prevented decreases in liver GSH concentrations induced by ethanol administration (47). In mice infected with Mycobacterium tuberculosis, oral administration of green tea extract attenuated decreases in erythrocyte GSH concentrations caused by the infection (48).

Administration of tea and tea polyphenols has been reported to prevent or attenuate decreases in antioxidant enzyme activities in a number of animal models of oxidative stress. Providing hairless mice with green tea polyphenols in their drinking water significantly inhibited UVB-induced decreases in epidermal catalase and glutathione reductase activities (26). Oral administration of green tea extract to mice infected with M. tuberculosis attenuated infection-associated decreases in erythrocyte superoxide dismutase (SOD) activity (48), while oral administration of either black or green tea extract resulted in increased serum SOD activity in mice exposed to the carcinogen, 3-methylcolanthrene (49). Providing rats with green tea extract in their drinking water attenuated ethanol-associated decreases in serum and liver SOD as well as liver glutathione peroxidase (GPX) and catalase activities (50). An electrical muscle stimulation protocol that elicited oxidative damage to muscle proteins in rats did not result in significant changes in muscle SOD and GPX activities, hence it is not surprising that the activity of these enzymes did not differ between rats given a diet containing 0.1% EGCG and those given a control diet (51).

In contrast to studies in animal models of oxidative stress, studies in healthy humans have not found tea or tea polyphenol consumption to result in significant changes in plasma antioxidant concentrations or antioxidant enzyme activities (2). Although consumption of tea or tea polyphenols by humans frequently results in modest transient increases in the total antioxidant capacity of plasma, as measured by the ferric-reducing antioxidant potential (FRAP), oxygen radical absorbance capacity (ORAC), or Trolox-equivalent antioxidant capacity (TEAC) assays, recent research suggests that concomitant increases in plasma urate accounts for much, if not all, of the increased plasma antioxidant capacity (37,52).

Ex vivo lipoprotein oxidation.

In animal models of atherosclerosis, the majority of studies suggests that tea administration increases the resistance of lipoproteins to ex vivo oxidation, usually by prolonging the lag phase of copper-mediated lipid peroxidation (Table 3). In hamsters fed normal or high cholesterol diets, the lag phase of copper-mediated LDL + VLDL oxidation was significantly increased in those animals given green or black tea in their drinking water (53). Feeding green tea flavonoids (8 g/kg of diet) to marginally vitamin C-deficient ODS rats resulted in a significantly increased lag phase of copper-mediated LDL oxidation (44). Feeding green tea polyphenols dose-dependently decreased the concentration of thiobarbituric acid-reacting substances (TBARS) in LDL of cholesterol-fed rats after 4 h of copper-mediated oxidation, with the highest dose (2.5%) conferring similar resistance to that of the antioxidant, probucol (54).

Atherosclerosis.

Although LDL oxidation is thought to represent an early event in the development of atherosclerosis, the physiological relevance of assays of ex vivo lipoprotein oxidation has been questioned. Indeed, in animal models of atherosclerosis, the effect of tea or tea polyphenol administration on atherosclerotic lesion formation does not always reflect the results of ex vivo lipoprotein oxidation (table 3). Providing green tea extract in the drinking water of mice fed an atherogenic diet dose-dependently decreased the accumulation of aortic cholesterol compared with control mice (59). In apolipoprotein E (ApoE)-deficient mice, aortic atherosclerotic lesion area and aortic cholesterol accumulation were also lower in those animals given green tea catechins in their drinking water (60). Administration of red wine, quercetin or catechin resulted in significantly smaller atherosclerotic lesion areas in the aortas of ApoE-deficient mice when compared with mice administered an ethanol-containing placebo, despite the fact that only red wine and quercetin administration significantly increased the resistance of LDL to ex vivo oxidation (61). In hypercholesterolemic rabbits given green tea, black tea, vitamin E or ß-carotene for 21 wk, aortic atherosclerotic lesion areas were 31% smaller in animals given green tea than in control animals, although this difference was not statistically significant (P = 0.11) (43). Black tea administration to LDL-receptor deficient mice did not affect aortic fatty streak lesion area, although fatty streak lesion areas in animals supplemented with antioxidants (vitamin C, vitamin E and ß-carotene) were 60% smaller than those of control animals (56). Thus, limited data suggest that green tea or catechin administration inhibits atherogenesis in some animal models of atherosclerosis.

Biomarkers of lipid peroxidation.

Assessment of TBARS is often used to measure plasma and tissue concentrations of malondialdehyde (MDA), a decomposition product of oxidized lipids, and as an index of plasma and tissue lipid peroxidation. Most of the numerous animal studies that have measured plasma or tissue TBARS have reported significant decreases with tea or tea polyphenol administration. However, the utility of the TBARS assay as a measure of lipid peroxidation in vivo is questionable due to its lack of specificity for MDA in biological samples and its susceptibility to artifactual ex vivo oxidation (62). Consequently, studies using only TBARS to assess lipid peroxidation are not further considered in this review of biomarkers of in vivo lipid peroxidation.

Animal studies employing relevant measures of lipid peroxidation are limited (Table 4). Basal levels of lipid hydroperoxides (LOOH) measured iodometrically in LDL were decreased in ApoE-deficient mice fed red wine, quercetin or catechin in their drinking water compared with an ethanol-containing placebo (61). In contrast, there were no differences in the basal levels of LOOH in LDL from hypercholesterolemic rabbits given green or black tea in their drinking water compared with controls (43). In rats injected with the colon carcinogen 1,2-dimethylhydrazine, phosphatidylcholine hydroperoxides were significantly lower in the colonic mucosa of those rats receiving green tea extract in their drinking water (Fig. 2) (63). Lipid hydroperoxides may decompose to form aldehydes such as MDA and 4-hydroxynonenal (4-HNE). In rats on a high fat diet, green tea administration prevented ethanol-induced increases in 4-HNE adducts to liver proteins and significantly decreased ethanol-induced liver necrosis (64).

View larger version (23K): [in this window] [in a new window]   FIGURE 2 Providing green tea extract (GTE) in the drinking water of rats for 10 d prior to subcutaneous injection with the colon carcinogen, 1,2-dimethylhydrazine (DMH) significantly inhibited increases in 8-hydroxy 2'-deoxyguanosine [8-OHdG (72)] and phosphatidylcholine hydroperoxides [PCOOH (63)] in colon mucosa. Black bars represent mean values in animals given water and gray bars represent mean values in animals given GTE. Mean values represent eight independent cases for 8-OHdG and five independent cases for PCOOH. Bars marked with an asterisk (*) represent values that are significantly different from those for animals given water (P < 0.05). Reproduced with permission from Elsevier Science and Masao Inagake.

Biomarkers of oxidative damage to proteins.

Oxidative damage to proteins may result in chemical modification of amino acids, aggregation or crosslinking of proteins or protein fragmentation. Of three different animal studies that assessed the effects of oral tea administration on oxidative damage to proteins in vivo, each used a different model of oxidative stress and a different measure of oxidative protein damage (Table 5). Supplementing the diets of rats with 1% EGCG significantly inhibited increases in muscle protein carbonyl content induced by electrical muscle stimulation (51). Protein glycation results from the reactions between primary amino groups of proteins and reducing sugars, such as glucose. Oxidation and structural rearrangement of glycated proteins results in the formation of advanced glycation end products, such as N-(carboxymethyl)lysine and pentosidine. Old rats (up to 22 mo-of-age) given green tea extract in their drinking water starting at 6 mo-of-age were found to have decreased aortic collagen-linked Maillard-type fluorescence, a marker for advanced glycation endproducts (68). As mentioned above, oral administration of green tea prevented ethanol-induced increases in 4-HNE adducts to liver proteins (64). The only controlled study to examine the effect of tea polyphenol consumption on oxidative damage to proteins in humans compared a low flavonoid diet with the same diet fortified with green tea extract over a 3-wk period (69). Levels of oxidatively modified plasma and hemoglobin proteins were not significantly different between the two diets.

The anticarcinogenic effects of tea and tea polyphenols have been amply demonstrated in a number of animal models involving tumors of the lung, digestive tract, prostate, bladder, mammary glands and skin (18). Data from animal studies also support a role for tea in the prevention of oxidative damage to DNA bases induced by chemical carcinogens (Table 6). Although enzymes present in mammalian cells can recognize and excise oxidatively damaged DNA bases, mutations may occur if excision and repair processes cannot keep pace with the rate of oxidative damage. Topical EGCG inhibited the epidermal formation of the oxidized DNA bases, thymidine glycol, 5-hydroxymethyl-2'-deoxyuridine and 8-hydroxy 2'-deoxyguanosine (8-OHdG) in mice treated with phorbol ester-type tumor promoters (70).

Reactive oxygen and nitrogen species generated upon reperfusion of ischemic tissue appear to play a critical role in ischemia-reperfusion injury. By trapping free radicals with the spin-trapping reagent, -(-4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN), and measuring the resulting adduct with electron spin resonance spectroscopy, Zhong and colleagues demonstrated that ischemia-reperfusion of the liver in rats resulted in a twofold increase of 4-POBN/radical adducts detected in bile (79). Feeding rats 0.1% green tea extract or 0.085% EC significantly decreased bile 4-POBN/ radical adducts to levels comparable to those of sham-operated rats. Brain injury due to ischemia-reperfusion is also thought to result, at least in part, from oxidative damage. Providing gerbils with 0.5% or 2% solutions of green tea extract in their drinking water for 3 wk dose-dependently inhibited increases in brain 8-OHdG levels following ischemia-reperfusion (80). Although a similar trend in brain 8-OHdG levels was observed when rats were provided with a 0.5% solution of green tea extract 3 wk prior to the induction of ischemia-reperfusion, the inhibition was not statistically significant (81).

Single cell alkaline gel electrophoresis, also known as the comet assay, is a sensitive assay of oxidative and nonoxidative DNA damage. The comet assay may be adapted to measure oxidative damage to DNA bases by measuring DNA strand breaks induced by treatment with relevant repair enzymes, e.g., FapyGua glycosylase for oxidized purine lesions and endonuclease III for oxidized pyrimidine lesions (82). Only one published animal study has used the comet assay to assess the effect of tea polyphenols on oxidative DNA damage. Rats consuming a high fat diet were given a red wine polyphenol preparation, thearubigins extracted from black tea or water by gavage for 10 d prior to killing (83). Single-strand breaks and oxidized pyrimidine bases in the DNA of colonic mucosal cells did not differ between thearubigin-treated animals and those treated with water. Oxidized purine and oxidized pyrimidine bases were significantly lower in the colonic mucosa cells of animals treated with red wine polyphenols than those treated with water, although single-strand breaks did not differ between the two groups. Thus, research on the role of tea or tea polyphenol administration in the prevention of oxidative damage to DNA in animal models of oxidative stress other than that induced by chemical carcinogens is limited and the protective effects of tea are less consistent. Evidence from controlled human trials that tea or tea polyphenol consumption inhibits oxidative DNA damage is lacking (69).

	The addition of milk to tea

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
In a number of countries, tea is commonly consumed with milk. Interactions between tea polyphenols and proteins found in milk have been found to diminish total antioxidant capacity in vitro (84), but it is presently unclear whether consuming tea with milk substantially alters the biological activities of tea flavonoids in vivo. The addition of milk to black tea did not significantly alter areas under the curve for plasma catechins (85) or flavonols (86) in human volunteers, suggesting that adding milk to tea does not substantially affect the bioavailability of tea catechins or flavonols. Two studies in humans found that the addition of milk decreased (87) or eliminated (88) increases in plasma antioxidant capacity induced by tea consumption, whereas another found no effect (89). Few studies have examined the effects of adding milk to tea in animal models. Although at least two studies in animal models found that adding milk to black tea did not diminish its inhibitory effect on tumorigenesis induced by chemical carcinogens (90,91), it is not known whether these effects were related to the antioxidant activity of tea polyphenols.

	SUMMARY

TOP ABSTRACT Tea polyphenol content Potential mechanisms for the... Antioxidant activity of tea... The addition of milk... SUMMARY LITERATURE CITED

 
Data from animal studies provide some support for the notion that tea polyphenols act as antioxidants in vivo. Administration of green tea extract and, in one case, black tea extract attenuated decreases in SOD activity caused by infection, ethanol or the carcinogen, 3-methylcolanthrene. Although green and black tea administration improved the resistance of lipoproteins to ex vivo oxidation in several animal models, the improvement was generally much less than that conferred by supplementation with other antioxidants. Animal studies examining the effect of tea administration on biomarkers of in vivo lipid peroxidation other than TBARS are limited. However, there is some evidence from mouse models of atherosclerosis that green tea catechin consumption is antiatherogenic. Very limited data suggest that green tea or EGCG administration may protect proteins from oxidative damage. Most promising are the consistent findings that tea or tea polyphenol administration prevented carcinogen-induced increases in the oxidized DNA base, 8-OHdG, in animal models of skin, lung, colon, liver and pancreatic cancer.

Why are the results of animal and human studies different?

Although the consumption of tea or tea polyphenols results in modest transient increases in plasma antioxidant capacity in humans, limited research has not generally revealed significant decreases in biomarkers of in vivo oxidative damage (2). Tea concentrations used in animal and human studies are often similar, but animals generally receive much higher doses relative to body weight than humans. Findings that tea or tea polyphenol administration inhibits increases in 8-OHdG induced by chemical carcinogens provide the most consistent evidence that tea and tea polyphenols have antioxidant effects in vivo. In contrast, observational studies in humans do not generally support a significant cancer chemoprotective effect of black or green tea consumption (2). Although observational studies in humans have a number of limitations, it is also possible that animal models employing chemical carcinogens may not be entirely relevant to the causes of oxidative stress and cancer in humans. While genetic variability is limited in animal models, wide genetic variations in the response of humans to oxidative stress may obscure small changes in biomarkers induced by tea polyphenols. To determine whether increased consumption of tea or tea polyphenols prevents oxidative damage to biomolecules and associated pathology in humans, research in humans and animals should employ sensitive and specific markers of oxidative damage to lipids, proteins and DNA, such as F₂-isoprostanes, protein carbonyls and the comet assay. To increase the applicability of data from animal studies, animal models selected to assess the antioxidant activity of tea polyphenols should be relevant to likely sources of oxidative stress and associated diseases in humans.

	FOOTNOTES

 
¹ Presented as part of "The Third International Scientific Symposium on Tea and Human Health: Role of Flavonoids in the Diet," given at the United States Department of Agriculture, September 23, 2002. This conference was sponsored 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 and was supported by a grant from the Tea Council of the U.S.A. Guest editor for this symposium was Jeffrey Blumberg, PhD, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, Boston, MA 02111.

³ Abbreviations used: 4-HNE, 4-hydroxynonenal; 4-POBN, -(-4-pyridyl-1-oxide)-N-tert-butylnitrone; 8-OHdG, 8-hydroxy-2'-deoxyguanosine; AP-1, activator protein-1; ApoE, apolipoprotein E; ARE, antioxidant response element; COX, cyclooxygenase; E°’, standard one electron reduction potential; EC, (-)-epicatechin; ECG, (-)-epicatechin gallate; EGC, (-)-epigallocatechin; EGCG, (-)-epigallocatechin gallate; FRAP, ferric reducing antioxidant potential; GSH, glutathione; GST, glutathione-S-transferase; GPX, glutathione peroxidase; iNOS, inducible nitric oxide synthase; LOOH, lipid hydroperoxide; MDA, malondialdehyde; NF-B, nuclear factor-B; NO, nitric oxide; O·₂^-, superoxide; ODS, Osteogenic Disorder Shinogi; ONOO^-, peroxynitrite; ORAC, oxygen radical absorbance capacity; SOD, superoxide dismutase; TBARS, thiobarbituric acid reacting substances; TEAC, trolox-equivalent antioxidant capacity.

	LITERATURE CITED

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