Increasing evidence is highlighting the role antioxidants may have in protecting against certain conditions such as heart disease, stroke, and cancers. It has been proposed that the mechanisms leading to the diseases may be promoted by free radicals and that antioxidants may oppose the action of these molecules. In addition to the well known antioxidants such as vitamins C and E, there is growing research demonstrating the potentially beneficial effects of plant-derived antioxidants polyphenols, found in fruits, vegetables, nuts, cereal and drinks such as tea and red wine.
Free radicals explained
Free radicals are unstable molecules that include the hydrogen atom, nitric oxide (NO) and molecular oxygen (O2). These naturally occur in the body as a result of chemical reactions during normal cellular processes. They can also be formed in response to excessive pollution, too much UV sunlight and exposure to cigarette smoke. In an attempt to stabilize, they attack other molecules in the body potentially leading to cell damage and triggering the formation of another free radical resulting in a chain reaction. Some scientists believe that this type of free radical action has been implicated in certain chronic and ageing diseases such as cancer, heart disease, stroke, rheumatoid arthritis, cataracts and Alzheimer's disease.
Protective mechanisms of antioxidants
Antioxidants are compounds that help to inhibit the many oxidation reactions caused by free radicals thereby preventing or delaying damage to the cells and tissues. Their mechanisms of action include-
- Scavenging reactive oxygen and nitrogen free radical species
- Decreasing the localized oxygen concentration thereby reducing molecular oxygen's oxidation potential
- Metabolizing lipid peroxides to non-radical products
- Chelating metal ions to prevent the generation of free radicals
In this way antioxidants limit the free radical damage from-
- Oxidizing Low Density Lipoprotein (LDL) cholesterol, which may increase the risk of ahtersclerosis
- Promoting platelet adhesion, which can lead to thrombosis thereby increasing the risk of heart disease or stroke
- Damaging the cell's DNA which may lead to cancer
- Blocking the normal endothelial cell function and vasodilatatioin in response to nitric oxide, a potential mechanism for heart disease and cancer - Triggering inflammation
- Impairing immune function
Some antioxidants are synthesized within the cells themselves (endogenous) and others need to be provided in the diet.
Sources of dietary antioxidants
Traditionally dietary antioxidants were thought of as Vitamin E and C and the carotenoid - carotene. In recent years there has been particular interest in the antioxidant activity and health benefits of other phytochemicals.
Tea has one of the Highest total flavonoid contents of all plants at 15% of the leaf by dry weight and is also the major source of flavonoids in the UK diet, providing approximately 80% of dietary flavonoids for the population as a whole.
The types and amounts of flavonoids present in tea will differ dependent on the variety of leaf, growing environment , processing, manufacturing, particular size of ground tea leaves and infusion preparation typically 93% of the total tea phenolic compounds are flavonoids. Green teas contain more of the simple flavonoids called catechins, while the oxidization that the leaves undergo to make black tea converts these simple flavonoids to the more complex varieties called theaflavins and thearubigins.
Tea flavonoids are water-soluble and one study has shown that a cup of UK tea that has been allowed to brew for 40-60 seconds will typically deliver approximately 140mg of flavonoids whilst a second carried out by the UK Tea Trade Technical Committee using typical UK consumer brewing conditions and encompassing the range of blends and bag weights commonly on sale in the UK gives a figure of 125mg 235ml serving. The longer the tea is left to brew, the higher the concentration of flavonoids.
Tea flavonoids demonstrate antioxidant activity and while not a replacement for fruit and vegetables, the antioxidant activity of tea has been compared to that of fruit and vegetables in a number of studies. One study concluded that at the typical UK daily consumption of 3 cups a day, tea has approximately the same antioxidant power as eating six apples.
Another study found that one or two cups of tea has the same "radical scavenging capacity as five portions of fruit and vegetables or 400mg vitamin C equivalents.
Health benefits of tea flavonoids
For many years it has been known that the plant polyphenols are antioxidant in vitro, in fact many common flavonoids are several times more potent than Vitamin C or E. this growing interest in the antioxidant activity of phenolic compounds has led to increased research into their potential health benefits e. g.
-Heart Disease and Stroke
-Several reports indicate that tea flavonoids inhibit the oxidation of LDL cholesterol in vitro
-A reduction in blood lipids has been demonstrated in animal studies
-Certain tea flavonoids exhibit anti-inflammatory actions in animals
-Athersclerosis is a disease with a strong inflammatory component
-Improvements in blood vessel function, specifically the vascular endothelium, has been seen in patients with established CHD
-Several in vitro studies and one human trial have found that platelet aggregation can be inhibited by various flavonoids.
The antioxidant activity of tea flavonoids may account for the results of a number of epidemiologocal studies suggesting that they may have a protective role in conditions such as cardiovascular disease.
-In vitro studies have demonstrated that the initiation stage of cancer can be prevented by the action of tea flavonoids
-Tea polyphenols have been shown to inhibit DNA synthesis of leukemia cells and lung carcinoma cells
-animal studies have shown that tea and its flavonoids protect against may types of cancer e. g. skin tumors in mice, lung cancer in mice and digestive cancer in mice and rats
-Tea extracts exhibit inhibitory effects against Salmonella typhi Campylobacter jejuni, Campylobacter coli, Helicobacter pylori, Shigella, Clostridium. Pseudomonas, Candida and others
-Green tea and various catechins have exhibited inhibitory effects on the growth of cryogenic bacteria by preventing the adherence and growth of bacteria at the tooth surface.
Absorption of tea flavonoids
Until recently the majority of the research demonstrating the antioxidant activity of tea flavonoids was either using animal models or laboratory cellular studies. Emerging evidence is concluding that the body does in fact absorb some of these antioxidants, e. g. when green tea extract is consumed by healthy human volunteers; various catechins are found in the plasma in a dose-dependent concentration varying between 0.2-2.0% of the ingested amount with a maximum concentration being achieved 1.4 to 2.4 hours after consumption. Some studies have shown that plasma antioxidant activity peaks 30-60 minutes after tea consumption and returns close to baseline by 90 minutes. Further research is currently being undertaken on the metabolism, distribution and excretion of tea flavonoids and its metabolites.
The addition of milk to tea, as enjoyed by the majority of the UK population, does not appear to affect the bioavailability of the tea flavonoids.
It is well known that fruit and vegetables are good sources of antioxidants; however, what is less well known is the amount of antioxidants present in tea. The major group of antioxidants in tea are flavonoids that appear to be digested, absorbed and metabolized by the body. There is a wealth of evidence demonstrating that tea and flavonoids exhibit beneficial effects in animal and in vitro studies and provide a promising area of research for the future human studies.
So as well as eating more fruit and vegetables, antioxidant intake can be topped up by drinking more tea, helping to promote overall health and well-being.
1. Lakenbrink C at el. (2000) Flavonoids and other polyphenols in consumer brews of tea and other caffeinated beverages. J Agric Food Chem, 49, 2848-2852
2. Personal communication from UK Tea Trade Technical Committee
3. 3. Astill C, et al (2001) Factors affecting the caffeine and poyphenol contents of black and green tea infusions. J Agric Food Chem; 49(11): 5340-7
4. Englehardt, U et al (1999) Caffeinated Beverages Symposium, 219 the American Chemical Society Meeting. Anaheim, USA
5. Sarkar A, et al (2001) Black tea is a powerful chemopreventor of reactive oxygen and nitrogen species comparison with its individual catechin constituents in green tea. Biochem Biophysss. Res. Commun; 284(1): 173-178
6. Karakaya S, et al (2001) Antioxidant activity of some foods containing phenic compounds. Int J Food Science; 52(6): 501-8
7. Paquay JB, et al (2001) Protection against nitric oxide toxicity by tea. J Agric Food Chem 48(11): 5768-5772
8. Laung LK, et al (2001) Theaflavins in black tea and catechins in green tea equally affective antioxidants. J Nutr 131(9); 2248-51
9. National Drinks Survey (2001)
10.Papanga G, et al (1999) The polyphenolic content of fruit and vegetables and their antioxidant activities. What does a serving constitute? Free Rad Res; 30(2): 153-162
11.Du Toit R, et al (2001) Comparison of the antioxidant content of fruits, vegetables, and teas measured as Vitamin C equivalents. Toxicology; 186(1-2): 83-9
12.Vinson JA, et al (1995) Plant flavonoids, especially tea flavonols, are powerful antioxidants using an In vitro oxidation model for heart disease. J Agric Food Chem.; 43(11): 2800-2802
13.Rice- Evans CA, et al (1995) The relavent antioxidant activities of plant derived polyphenolic flavonoids. Free Rad Res.; 2214(4): 375-383
14.De Whalley, et al (1990) Flavonoid inhibit the oxidative modification of Low Density Lipoproteins by macrophages. Biochem Pharmacol 39; 1743-1750
15.Yoshida H, et al (1999) Inhibitory effect of tea falvonoids on the ability of cells to oxidize low density lipoprotein. Biochem Pharmacol 58; 1985-703 Biochem Pharmacol 39; 1742-1750
16.Pearson DA, eat al (1998) Inhibition of endothelia cell mediated low density lipoprotein oxidation by green tea extracts. J Agric Food Chem 46; 1445-9
17.Zhu QY, et al (2000) Interaction between flavonoids and alpha-tocopherol in human low density lipoprotein. J Nutr Biochem 11; 14-21
18.Yang TTC, et al (1997) Hypochelesterolemic effect of Chinese tea. Pharmacol res 35; 505-12
19.Lin YL, et al (1998) Hypolipidemic effect of green tea leaves through induction of antioxidant and Phase II enzymes including superoxidase dismutase, catalase, and glutathione S-transferase in rats. J Agric food Chem 46; 1893-9
20.Vinson JA, et al (1998) Effect of green and black tea supplementation on lipids, lipid oxidation and fibrinogen in hamster, mechanisms for the epidermiological benefits of tea drinking. FEBS Lett 433; 44-6
21.Tijburg LBM (1997) Tea falvonoids and cardiovascular diseases a review. Crit Rev Food Sci. Nutr 37; 771-85
22.Hofbauer R, et al (1999) The green tea extract epigallocatechin is able to reduce nautrophil transmigration through monolayers of endothelia cells. Wiener Klinischa Wochenschrift 111; 278-82
23.Duffy SJ, et al (2001) Short and low-term black tea consumption reverses endothelial dysfunction in patients with coronary artery disease. Circulation, 10, 104,2, 151
24.Corvazier E, et al (1985) Interference of some falvonoids and non-steroidal anti-inflammatory drugs with oxidative metabolism of arachidonic acid by human platelets and neutraphils. Biochemica at Biophysica Acta 835; 315-321
25.Polatte A, et al (1995) N-3 fatty acid-induced lipid paroxidation in human platelets is prevented by catechins. Thrombosis and Haamotasis 95; 945-9
26.Kelly C, et al (1998) Modulation of human platelet function by food falvonoids. Biochemical Soc transaction 24; 197
27.Tzang SH, et al (1991) Inhibition of platelet aggregation by some flavonoids. Thromb Res. 64: 91-100
28.Hodgson JM, et al (2001) Affects of regular ingestion of black tea on haemostasis and cell adhesion molecules in humans. Eur J Clin Nutr; 55 (10): 88 1-8
29.Sesso HD, et al (1999) Coffee and tea intake and the risk of myocardial infraction. Am J Epidemiol, Jan 15, 149(2), 162-167
30.Keli SO, et al (1996) Dietary flavonoids, antioxidant vitamins, and incidence of stroke: the Zulphen study. Arch Intern Med. 156(6) 637-642
31.Knekt P, at al. Flavonoid intake and coronary mortality in Finland: a cohort study. BMJ, 312, 478-81 1996
32.Stensvold I, et al Tea consumption, Relationship to cholesterol, blood pressure and coronary and total mortality. Preventive Med. 21,546,1992
33.Yochum L, et al Dietary flavonoid intake and risk of cardiovascular disease in postmenopausal women. Am J Epidermiol, 149(10), 943-9, 1999
34.Hertog MGL, et al (1993) Flavonoid intake and long-term risk of coronary heart disease and cancer in the seven countries study. The Zutphen Elderly study, Lancet, 155.381
35.Hertog MGL, et al (1995) Flavonoid intake and long-term risk of coronary heart disease and cancer in the seven countries study, Hertog, et al, Arch Int Med 155-382
36.Geleijnse JM, et al (1999) Tea falvonoids may protect against artherosderosie: the Rotterdam study. Arch Intern Med 159; 2170-4
37.Gordon MH (1998) Dietary antioxidants in disease prevention. Nat Prod Rep 13, 265-73
38.Katiyar s, et al (1996) Tea in chemoprevention of cancer; epidemiological and experimental studies (review), Int J Oncol 8; 221-38
39.Yamada J, et al (1994) Antimutagenic activity of water extracts of black tea and oolong tea. Bioeci. Biotech. biochem 12; 2197-200
40.Yan GC, et al (1995) Antioxidant activity of various tea extracts in relation to their antimutagenicity. J Agric Food Chem 43; 27032
41.Kuroda Y, et al (1999) Antimutagenic and ant carcinogenic activity of tea polypohenols. Mut Res 438; 69-97
42.Surono IS, et al (1996) Bacterial mutagenicity of terasi and anti-mutagenicity of Indonesian jasmine tea against terasi. Int J food Microbiol 32; 49-58
43.Bu-abbas, A, et al (1997) Fractionation of green tea extracts, correlation Antimutagenic affect with flavored content. J Sci Agric 75; 453-62
44.Hour TC, et al (1999) Inhibition of eleven mutagens by various tea extracts, (-) epigallocatechin-3-gaflate, gallic acid and caffeine. Food Chem Toxicol 37;559-79
45.Steele VE, et al (2000) Comparative chemo preventive mechanisms of green tea and black tea and selected polyphenols extracts measured by in vitro bioassays. Carcinogenesis 21; 63-7
46.Yang GY, et al (1998) Inhibition of growth and induction of apoptosis in human cancer cell lines by tea polyphenols, Carcinogenesis 19; 611-6
47.Smith DM, et al (2001) Green tea induces polyphenols epigallocatechin inhibits DNA replication and consequently induces leukemia cell apoptosis. Int J Mol Med; 7(6): 645-62
48.Wang ZY, et al (1992) inhibitory effect of green tea in the drinking water on tumorignenisis by ultraviolet light and 12-O-tetradecanoylphorbol-13-acetate in the skin of SKH-1 mice. Cancer Res 52; 1182-70
49.Wang ZY, et al (1992) Inhibitory effect of green tea on the growth of established skin papillomas in mice. Cancer Res 52; 6657-665
50.Conney H, et al (!999) Inhibitory effect of green and black tea on tumor growth. Proc Soc Exper Biol Med 220; 229-33
51.Cao J, et al (1998) Chemo preventive effects of green and black tea on pullonary and hepstic carcinogenesis. Fund Appl Toxicol 29; 244-50
52.Xu Y, et al (1992) Inhibition of tobacco-specific nitrosamine-induced lung tumorignenisis in A/J mice by green tea and its major polyphenols as antioxidants. Cancer Res52; 3875-9
53.Landau JM, et al (1998) Inhibition of spontaneous formation of lung tumors and rhabdomycoaarcomas in A/J mice by black and green tea. Carcinogenesis 19; 501-7
54.Steale VE, et al (1999) Preclinical efficacy studies of green and black tea extracts. Proc Soc Exper Biol Med 220; 210-2
55.Diker KS, et al (1994) The bactericidal activity of tea against Helicobacter pylori. Lett Apl Microbiol 19; 299-300
56.Maity S, et al (1998) Role of glutathione in the nitiucer effect of hot water extract of black tea (Camellia sinensis). Jap J Pharmacol 78; 285-292
57.Toda M, et al (1991) The protective activity of tea against infection by Vibrio cholerae 01. J Appl Microbiol 70; 109-12
58.Diker KS, et al The bactericidal activity of tea against Campylobacter jajuni and Campylobacter coli. Lett Appl Microbiol 12, 34-4
59.Otake S, et al (1991) Anticaries effects of polyphenolic compounds from Japanese green tea. Caries Res, 25(6): 438-43
60.Sakanaka S, et al (1990) Inhibitory effects of green tea polyphenols on glucan synthesis and cellular adherence of cariogenic Streptococci. Agri Biol Chem 54; 2925-9
61.Van Het Hof K, et al (1998) Bioavailability of catechins from tea, the effect of milk. Eur J Clin Nut, 52, 358
62.Leanan R, et al (2000) A single dose of tea with or without milk increases plasma antioxidant activity in humans. Eur J Clin Nutr, 54,87
63.Warden BA, et al (2001) Catechins are bioavailable in men and women drinking black tea throughout the day. J Nutr, 131(6): 1731-7
64.Sharzad S, et al (2001) Pharmokinetics of gallic acid and its relative bioavailability from tea I healthy humans. J Nutr, 131(4): 1207-10
65.Nakagawa K, et al (1997) Dose-dependent incorporation of tea catechins, (-) epigallocatechin-3-galate and (-) spigallocatechin, into human plasma. Biosc Biotech Biochem 61; 1981-5
66.Pietta PG, et al (1998) Catechin metabolitea after intake of green tea infusions. Biofacto 8; 111-8
67.Yang C, et al (1999) Inhibition of carcinogeneisis by tea; bioavailability of tea polyphenols and mechanisms of action. Proc Soc Exper Biol Med 220; 213-7
68.Sarafini M, et al (1996) In vivo antioxidant effect of green and black tea in man. Eur J Clin Nutr, 50: 29-32
69.Benzie FF, et al (1999) Consumption of green tea causes rapid increase in plasma antioxidant power in humans. Nutr and Cancer, 34(1): 83-87
70.Hollman PC, et al (2001) addition of milk does not effect the absorption of flavonoids from tea in man. Free Radical Res 34,3,297