Abstract

Flavonoids and other polyphenolic compounds have powerful antioxidant effects in vitro in many test systems, but can act as pro-oxidants in some others. Whether pro-oxidant, antioxidant, or any of the many other biological effects potentially exerted by flavonoids account for or contribute to the health benefits of diets rich in plant-derived foods and beverages is uncertain. Phenolic compounds may help to protect the gastrointestinal tract against damage by reactive species present in foods or generated within the stomach and intestines. The overall health benefit of flavonoids is uncertain, and consumption of large quantities of them in fortified foods or supplements should not yet be encouraged.

1. Introduction

The traditional “trio” of antioxidants (ascorbate, β-carotene, α-tocopherol) has had a bad press recently, with human intervention trials giving mostly-negative results, and some meta-analyses and other studies suggesting that these agents not only fail to protect against disease, but also that some of them may accelerate development of cancers or cardiovascular disease in certain subjects [1–6]. Does this mean that the concept that free radicals and other reactive species contribute to the development of age-related diseases such as cancer is incorrect? Possibly, but I do not believe so (reviewed in [7]). Instead, I think it to be more likely that the antioxidant administrations simply failed to protect against oxidative damage (discussed in detail in [2,7–9]). For example, high doses of α-tocopherol are poorly-effective at decreasing levels of lipid peroxidation in humans, as measured by reliable biomarkers such as F2-isoprostanes [7,10,11]. Whether other tocopherols, or tocotrienols, work better remains to be determined. α-Tocopherol acts much more effectively as an antioxidant in mice than it does in humans, and its ability to protect against atherosclerosis and neurodegeneration in mice is correspondingly greater [12–16].

2. Fruits and vegetables

Among the few things uncontested by nutritionists are that increased consumption of grains, fruits, and vegetables, decreased saturated fat intake, a moderate degree of exercise and a judicious consumption of red wine (or other alcoholic beverages) seem associated with a lower risk of developing cardiovascular disease, some forms of cancer, and perhaps Alzheimer's disease [15,17–23]. However, foods and beverages derived from plants are chemically complex, and protective effects could arise from many components or mixtures of components present, including fibre, immunostimulatory agents, inducers of antioxidant or xenobiotic-metabolizing enzymes, monounsaturated fatty acids, agents that modulate cholesterol synthesis, B-vitamins, folic acid (which may minimize homocysteine levels), agents modulating nitric oxide production, cyclooxygenase inhibitors and even the humble ethanol molecule itself [2,17–28]. Are fruits and vegetables beneficial because of these other components? Or is it that fruits and vegetables act (in whole or in part) by antioxidant actions, but that the active antioxidants are not vitamin C, α-tocopherol or β-carotene? Some studies suggested yes to the latter question [29–31]. For example, Verhagen et al. [29] found that urinary excretion of 8-hydroxy-2′-deoxyguanosine (8OHdG), a putative biomarker of oxidative damage to DNA and DNA precursors [32,33], was decreased by feeding human volunteers Brussels sprouts, but not by giving them α-tocopherol, ascorbate, or β-carotene [34].

Several other authors have shown that consumption of antioxidant-rich foods decreases levels of oxidative damage in vivo in humans (reviewed in [35]). Others have found little effect (e.g. [36]), and some registered increases in biomarkers of oxidative protein damage, such as 2-aminoadipic and γ-glutamyl semialdehydes [37]. One must be cautious in all such studies to rule out confounding effects of refeeding fasted individuals, as opposed to the effects of antioxidants in the food, on biomarkers of oxidative damage. Thus Vissers et al. [38] showed that olive oil administration to human volunteers decreased the propensity of low-density lipoproteins (LDL) subsequently isolated from their blood to undergo oxidation in vitro, but feeding oil without antioxidants had the same effect. In 2000, we reported [39] that dark soy sauce has powerful antioxidant abilities in vitro. Recently, we attempted to see if dark soy sauce decreased oxidative damage in vivo in human volunteers, and indeed it was able to decrease levels of F2-isoprostanes [40]. We administered the soy sauce with rice, using a placebo colouring on the same amount of rice as a control. The rice meal (devoid of antioxidants) also had effects on F2-isoprostanes and urinary 8OHdG excretion [40], although the soy sauce meal did better than the placebo in lowering F2-isoprostane levels. Similarly, Richelle et al. [41] and Lee et al. [42] suggested that fasting may raise plasma F2-isoprostane levels. At the moment, the balance of evidence does suggest that antioxidant effects contribute to the benefits of a high intake of fruits and vegetables (reviewed in [35,43]) but the extent of their contribution is uncertain. More work needs to be done on the effect of diet on oxidative damage, using suitable controls.

3. Pro-oxidant effects

Some authors have claimed that “antioxidants” can stimulate oxidative damage in vivo, especially ascorbate, alleged in several studies to increase oxidative DNA damage (reviewed in [44]). Indeed, it was suggested that mega-doses of ascorbate might kill cancer cells in vivo by oxidizing to produce H2O2[45]. We found small and transient increases in oxidative DNA damage in human volunteers fed mixtures of ascorbate, β-carotene and α-tocopherol, but there was wide variation between experiments [8]. Overall, the available data that ascorbate, β-carotene or α-tocopherol are pro-oxidant in vivo are (in my view) equivocal and inconclusive. Nor is the evidence that they are antioxidants a great deal better.

4. Enter the flavonoids

Flavonoids and other polyphenols have powerful antioxidant activities in vitro, being able to scavenge a wide range of reactive species, including hydroxyl radicals, peroxyl radicals hypochlorous acid and (sometimes) superoxide radical, O2 (reviewed in [46]). Flavonoids can also inhibit biomolecular damage by peroxynitrite in vitro[47–49], although they are less good at doing this in the presence of physiological levels of HCO3/CO2[49,50]. Peroxynitrite reacts fast with CO2/HCO3 to form reactive products that flavonoids appear to scavenge less well. Many flavonoids chelate transition metal ions such as iron and copper, decreasing their ability to promote reactive species formation [46,51,52].

Two observations drew attention to the potential biological importance of flavonoids. First, phenolics in red wine were shown to be able to inhibit the oxidation of LDL in vitro and this was suggested as an explanation of the “French paradox” [19,53]. Second, the Zutphen study, an epidemiological study in the Netherlands, suggested an inverse correlation between the incidence of coronary heart disease and stroke and the dietary intake of flavonoids, especially quercetin [54]. Since then, several other epidemiological studies have confirmed similar associations, although a few have not, and there is little evidence that flavonoids protect against cancer [55]. Some suggestions of protection against neurodegenerative disease have been made [22,23,43,56–58], although it is unclear to what extent flavonoids can enter the human brain [59].

Thus could flavonoids be major contributors to the disease-protective effects of fruits and vegetables? If so, is this due to antioxidant effects? Many polyphenols are absorbed, although rarely completely, and most of the remainder are broken down in the colon to generate high levels of monophenols [60,61]. Are the amounts of polyphenols absorbed sufficient to exert significant antioxidant effects? Several studies administering flavonoid-rich foods and beverages and measuring biomarkers of oxidative damage suggest yes, but others no (discussed in [35,62]). “Feeding effects” alluded to earlier could account for some of the apparent positive effects. Are significant antioxidant effects likely in vivo? Plasma levels of unconjugated flavonoids rarely exceed 1 μM and the metabolites tend to have lower antioxidant activity because radical-scavenging –OH groups are blocked by methylation, sulphation, or glucuronidation [60,61]. Since plasma total antioxidant capacities (TAC) are often in the range of 1 mM or more (reviewed in [7]), it seems difficult to imagine how an additional 1 μM polyphenol could exert a powerful antioxidant effect in vivo. Some studies have shown effects of flavonoid-rich foods in raising plasma TAC in humans. But one must be cautious here; many such foods can increase plasma uric acid levels, and urate is detected by several TAC assays [62–64]. Since elevated urate may be a risk factor for some diseases, the alleged “antioxidant benefit” may not be what it seems [62]. Finally, flavonoids and other phenols are complex molecules and have multiple potential actions other than antioxidant ones, including inhibiting telomerase, glutamate dehydrogenase, cyclooxygenase, lipoxygenase, xanthine oxidase, matrix metalloproteinases, angiotensin-converting enzyme, proteasome, cytochrome P450 and sulphotransferase enzyme activities, affecting signal transduction pathways and interacting with sirtuins [25,35,43,56,57,65–73]. Flavonoids may also interact with cellular drug transport systems, compete with glucose for transmembrane transport, interfere with regulation of the cell cycle, inhibit protein glycation, modulate paraoxonase, myeloperoxidase and thyroid peroxidase activities, increase endothelial nitric oxide production and affect platelet function [74–82]. Again, it is uncertain whether some of these effects occur in vivo, given the low concentration of bioavailable polyphenols.

5. Do polyphenols work pre-absorption?

It has been proposed [83] that antioxidant and other protective effects of flavonoids and other phenolic compounds could occur before absorption, i.e. within the stomach, intestines and colon (Fig. 1). This could account for the suggested ability of flavonoid-rich foods to protect against gastric, and possibly colonic, cancer, although again it must not be assumed that any protective effect of flavonoid-rich foods is attributable to antioxidant actions of the flavonoids, or to flavonoids at all, rather than to other components in the foods. However, ingestion of green tea was reported to rapidly decrease prostaglandin E2 concentrations in human rectal mucosa, consistent with inhibition of cyclooxygenase activity, a potential anti-cancer mechanism [25]. The levels of individual flavonoids in faecal water are fairly low (μM or less), but monophenols (many derived from polyphenol breakdown) are present at much higher concentrations [84]. By contrast to the colon, polyphenols are likely to be present in the stomach and intestines at high (≥mM) concentrations after consumption of polyphenol-rich foods and beverages.

Fig. 1

Dietary antioxidants and the gastrointestinal (GI) tract. *Except when supplements are taken. This figure refers to normal dietary intake. +, There is considerable intersubject variability in the efficiency of GI uptake of vitamin E. Much H2O2 may be removed in the oral cavity by catalase and peroxidases in saliva, and by H2O2 diffusion into the oral and oesophageal epithelium followed by its rapid catabolism. Adapted from [7] with permission from Oxford University Press. RNS Reactive nitrogen species, OH hydroxyl radical, RO alkoxyl radical, RO2 peroxyl radical, RS reactive species, LOX lipoxygenase, COX-2 cyclooxygenase-2.

Fig. 1

Dietary antioxidants and the gastrointestinal (GI) tract. *Except when supplements are taken. This figure refers to normal dietary intake. +, There is considerable intersubject variability in the efficiency of GI uptake of vitamin E. Much H2O2 may be removed in the oral cavity by catalase and peroxidases in saliva, and by H2O2 diffusion into the oral and oesophageal epithelium followed by its rapid catabolism. Adapted from [7] with permission from Oxford University Press. RNS Reactive nitrogen species, OH hydroxyl radical, RO alkoxyl radical, RO2 peroxyl radical, RS reactive species, LOX lipoxygenase, COX-2 cyclooxygenase-2.

Why should antioxidant protective effects of polyphenols be important to the stomach and intestines? The gastrointestinal (GI) tract is constantly exposed to reactive species. Some are released by the GI tract itself, eg. superoxide and H2O2 production by NADPH oxidases and “dual oxidases” in epithelial cells [85–87]. Some reactive species are present in food and beverages, and yet others are generated by chemical reactions of dietary components within the stomach [83,88]. Sources of reactive species include H2O2 in beverages [89], the mixtures of ascorbate and Fe2+ in the stomach (dietary iron, dietary ascorbate, and ascorbate normally present in gastric juice [90]), and ingested haem proteins, which can promote oxidation of dietary lipids [88]. Other reactive species that can be present in foods include lipid peroxides, cytotoxic aldehydes, and isoprostanes [91–95]. Nitrite is present at high levels in saliva and in foods [96]. It is converted to HNO2 by gastric acid, and HNO2 can then form nitrosating and DNA-deaminating species [97]. Activation of immune cells naturally present in the GI tract by diet-derived bacteria and toxins can also increase ROS production [98].

Flavonoids and other phenolic compounds might exert direct protective effects in the gastrointestinal tract, by scavenging reactive species and/or preventing their formation. For example, polyphenols can inhibit haem protein-induced peroxidation in the stomach [88,99] and decrease DNA base deamination or nitrosamine formation by HNO2-derived reactive nitrogen species [97,100]. Phenols might also increase levels of toxin-metabolizing or antioxidant defence enzymes in the GI tract, and chelate transition metal ions [83]. Dietary iron is usually not completely absorbed, especially among subjects on Western diets. Unabsorbed dietary iron enters the faeces, where it could represent a pro-oxidant challenge to the colon and rectum [101]. Indeed, diets rich in fat and low in fibre may aggravate this pro-oxidant effect [102]. Phenolic compounds, by chelating iron, may help to alleviate pro-oxidant actions of colonic iron.

6. Flavonoids as pro-oxidant xenobiotics

Why do hot beverages often contain high levels of H2O2? Simply because the polyphenols within them oxidize readily at high temperatures [89,103–105]. Polyphenols can also oxidize readily in cell culture media, and several claims of the cytotoxic effects of flavonoids on malignant, and other, cells in culture may have been led astray by this artefact. Flavonoids oxidize especially readily in Dulbecco's Modified Eagle's medium (DMEM), but also do so in most other cell culture media, at a slower rate [106,107]. Oxidation generates H2O2, quinones and semiquinones that can contribute to (and sometimes entirely account for) cytotoxicity [106–110]. For example, the apparent toxicity of green tea to PCI2 cells appeared entirely due to oxidation products generated in the culture medium [109]. This is not to say that all the observed cellular effects of flavonoids are artefacts; indeed, they may exert different effects on different cell types, those on vascular endothelial cells and other cells of the vascular system perhaps being especially important physiologically [43,82]. However, most work with cultured cells has failed to separate real effects from artefacts and may need to be repeated under conditions that slow or prevent phenol oxidation.

The mechanism of polyphenol oxidation in cell culture media is unclear. Metal ions may be involved (since DMEM is rich in iron ions, added to it as ferric nitrate), but it is not simply an iron-catalysed oxidation of polyphenols [110]. Ascorbate also oxidizes in DMEM to make H2O2, but mixtures of ascorbate and flavonoids generate less H2O2 than would be expected from the rate of its generation by either compound alone [110].

Since there are transition metal ions in the GI tract, it is possible that polyphenols could oxidize there as well. This might even be good for you, generating a pro-oxidant challenge that raises levels of xenobiotic-metabolizing and antioxidant defence enzymes in the GI tract.

7. A caution about supplements

Flavonoid-rich foods appear good for us, although to what extent (if any) the flavonoids contribute to this benefit is uncertain. Other possible protective components in foods were listed in Section 1. So should we consume flavonoid supplements or the flavonoid-enriched foods (e.g. cocoa, chocolate) now coming onto the market in some countries? I would be cautious until we know more [2,77,111]. To me, dietary polyphenols are typical xenobiotics, metabolized as such and rapidly removed from the circulation. They may be beneficial in the gut in the correct amounts. But too much may not be good and thus, I suggest that one should be content with eating a good diet for now.

References

[1]
Bjelakovic
G.
Nikolova
D.
Simonetti
R.G.
Gluud
C.
Antioxidant supplements for prevention of gastrointestinal cancers: a systematic review and meta-analysis
Lancet
 
2004
364
1219
1228
[2]
Halliwell
B.
Polyphenols; antioxidant treats for healthy living or covert toxins
J Sci Food Agric
 
2006
86
1992
1995
[3]
Lawlor
D.A.
Davey Smith
G.
Kundu
D.
Bruckdorfer
K.R.
Ebrahim
S.
Those confounded vitamins: what can we learn from the differences between observational versus randomised trial evidence?
Lancet
 
2004
363
1724
1727
[4]
Lee
D.H.
Folsom
A.R.
Harnack
L.
Halliwell
B.
Jacobs
D.R.
Jr.
Does supplemental vitamin C increase cardiovascular disease risk in women with diabetes?
Am J Clin Nutr
 
2004
80
1194
1200
[5]
Miller
E.R.
III
Pastor-Barriuso
R.
Dalal
D.
Riemersma
R.A.
Appel
L.J.
Guallar
E.
Meta-analysis: high-dosage vitamin E supplementation may increase all-cause mortality
Ann Intern Med
 
2005
142
37
46
[6]
Neuhouser
M.L.
Patterson
R.E.
Thornquist
M.D.
Omenn
G.S.
King
I.B.
Goodman
G.E.
Fruits and vegetables are associated with lower lung cancer risk only in the placebo arm of the beta-carotene and retinol efficacy trial (CARET)
Cancer Epidemiol Biomarkers Prev
 
2003
12
350
358
[7]
Halliwell
B.
Gutteridge
J.M.C.
Free radicals in biology and medicine, 4th ed.
2006
Clarendon Press
[8]
Halliwell
B.
Establishing the significance and optimal intake of dietary antioxidants: the biomarker concept
Nutr Rev
 
1999
57
104
113
[9]
Halliwell
B.
The antioxidant paradox
Lancet
 
2000
355
1179
1180
[10]
Meagher
E.A.
Barry
O.P.
Lawson
J.A.
Rokach
J.
FitzGerald
G.A.
Effects of vitamin E on lipid peroxidation in healthy persons
JAMA
 
2001
285
1178
1182
[11]
Morrow
J.D.
Quantification of isoprostanes as indices of oxidant stress and the risk of atherosclerosis in humans
Arterioscler Thromb Vasc Biol
 
2005
25
279
286
[12]
Pratico
D.
Tangirala
R.K.
Rader
D.J.
Rokach
J.
FitzGerald
G.A.
Vitamin E suppresses isoprostane generation in vivo and reduces atherosclerosis in ApoE-deficient mice
Nat Med
 
1998
4
1189
1192
[13]
Cyrus
T.
Yao
Y.
Rokach
J.
Tang
L.X.
Pratico
D.
Vitamin E reduces progression of atherosclerosis in low-density lipoprotein receptor-deficient mice with established vascular lesions
Circulation
 
2003
107
521
523
[14]
Conte
V.
Uryu
K.
Fujimoto
S.
Yao
Y.
Rokach
J.
Longhi
L.
et al
Vitamin E reduces amyloidosis and improves cognitive function in Tg2576 mice following repetitive concussive brain injury
J Neurochem
 
2004
90
758
764
[15]
Martin
I.
Grotewiel
M.S.
Oxidative damage and age-related functional declines
Mech Ageing Dev
 
2006
127
411
423
[16]
Zhao
L.
Pratico
D.
Rader
D.J.
Funk
C.D.
12/15-Lipoxygenase gene disruption and vitamin E administration diminish atherosclerosis and oxidative stress in apolipoprotein E deficient mice through a final common pathway
Prostaglandins Other Lipid Mediat
 
2005
78
185
193
[17]
Rimm
E.B.
Ascherio
A.
Giovannucci
E.
Spiegelman
D.
Stampfer
M.J.
Willett
W.C.
Vegetable, fruit, and cereal fiber intake and risk of coronary heart disease among men
JAMA
 
1996
275
447
451
[18]
Lloyd
T.
Chinchilli
V.M.
Rollings
N.
Kieselhorst
K.
Tregea
D.F.
Henderson
N.A.
et al
Fruit consumption, fitness, and cardiovascular health in female adolescents: the Penn State Young Women's Health Study
Am J Clin Nutr
 
1998
67
624
630
[19]
Szmitko
P.E.
Verma
S.
Antiatherogenic potential of red wine: clinician update
Am J Physiol Heart Circ Physiol
 
2005
288
H2023
H2030
[20]
de Lorgeril
M.
Salen
P.
Martin
J.L.
Monjaud
I.
Delaye
J.
Mamelle
N.
Mediterranean diet, traditional risk factors, and the rate of cardiovascular complications after myocardial infarction: final report of the Lyon Diet Heart Study
Circulation
 
1999
99
779
785
[21]
Pietinen
P.
Rimm
E.B.
Korhonen
P.
Hartman
A.M.
Willett
W.C.
Albanes
D.
et al
Intake of dietary fiber and risk of coronary heart disease in a cohort of Finnish men. The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study
Circulation
 
1996
94
2720
2727
[22]
Lau
F.C.
Shukitt-Hale
B.
Joseph
J.A.
The beneficial effects of fruit polyphenols on brain aging
Neurobiol Aging
 
2005
26
Suppl 1
128
132
[23]
Staehelin
H.B.
Micronutrients and Alzheimer's disease
Proc Nutr Soc
 
2005
64
565
570
[24]
Brouwer
I.A.
van Dusseldorp
M.
West
C.E.
Meyboom
S.
Thomas
C.M.
Duran
M.
et al
Dietary folate from vegetables and citrus fruit decreases plasma homocysteine concentrations in humans in a dietary controlled trial
J Nutr
 
1999
129
1135
1139
[25]
August
D.A.
Landau
J.
Caputo
D.
Hong
J.
Lee
M.J.
Yang
C.S.
Ingestion of green tea rapidly decreases prostaglandin E2 levels in rectal mucosa in humans
Cancer Epidemiol Biomarkers Prev
 
1999
8
709
713
[26]
Al-Abed
Y.
Mitsuhashi
T.
Li
H.
Lawson
J.A.
FitzGerald
G.A.
Founds
H.
et al
Inhibition of advanced glycation endproduct formation by acetaldehyde: role in the cardioprotective effect of ethanol
Proc Natl Acad Sci U S A
 
1999
96
2385
2390
[27]
Kris-Etherton
P.M.
Yu-Poth
S.
Sabate
J.
Ratcliffe
H.E.
Zhao
G.
Etherton
T.D.
Nuts and their bioactive constituents: effects on serum lipids and other factors that affect disease risk
Am J Clin Nutr
 
1999
70
504S
511S
[28]
Myzak
M.C.
Dashwood
R.H.
Chemoprotection by sulforaphane: keep one eye beyond Keap1
Cancer Lett
 
2006
233
208
218
[29]
Verhagen
H.
de Vries
A.
Nijhoff
W.A.
Schouten
A.
van Poppel
G.
Peters
W.H.
et al
Effect of Brussels sprouts on oxidative DNA-damage in man
Cancer Lett
 
1997
114
127
130
[30]
Rehman
A.
Bourne
L.C.
Halliwell
B.
Rice-Evans
C.A.
Tomato consumption modulates oxidative DNA damage in humans
Biochem Biophys Res Commun
 
1999
262
828
831
[31]
Sanchez-Moreno
C.
Cano
M.P.
de Ancos
B.
Plaza
L.
Olmedilla
B.
Granado
F.
et al
Mediterranean vegetable soup consumption increases plasma vitamin C and decreases F2-isoprostanes, prostaglandin E2 and monocyte chemotactic protein-1 in healthy humans
J Nutr Biochem
 
2006
17
183
189
[32]
Halliwell
B.
Whiteman
M.
Measuring reactive species and oxidative damage in vivo and in cell culture: how should you do it and what do the results mean?
Br J Pharmacol
 
2004
142
231
255
[33]
Cooke
M.S.
Evans
M.D.
Lunec
J.
DNA repair: insights from urinary lesion analysis
Free Radic Res
 
2002
36
929
932
[34]
Prieme
H.
Loft
S.
Nyyssonen
K.
Salonen
J.T.
Poulsen
H.E.
No effect of supplementation with vitamin E, ascorbic acid, or coenzyme Q10 on oxidative DNA damage estimated by 8-oxo-7,8-dihydro-2′-deoxyguanosine excretion in smokers
Am J Clin Nutr
 
1997
65
503
507
[35]
Halliwell
B.
Rafter
J.
Jenner
A.
Health promotion by flavonoids, tocopherols, tocotrienols, and other phenols: direct or indirect effects? Antioxidant or not?
Am J Clin Nutr
 
2005
81
268S
276S
[36]
McAnulty
S.R.
McAnulty
L.S.
Morrow
J.D.
Khardouni
D.
Shooter
L.
Monk
J.
et al
Effect of daily fruit ingestion on angiotensin converting enzyme activity, blood pressure, and oxidative stress in chronic smokers
Free Radic Res
 
2005
39
1241
1248
[37]
Dragsted
L.O.
Pedersen
A.
Hermetter
A.
Basu
S.
Hansen
M.
Haren
G.R.
et al
The 6-a-day study: effects of fruit and vegetables on markers of oxidative stress and antioxidative defense in healthy nonsmokers
Am J Clin Nutr
 
2004
79
1060
1072
[38]
Vissers
M.N.
Zock
P.L.
Leenen
R.
Roodenburg
A.J.
van Putte
K.P.
Katan
M.B.
Effect of consumption of phenols from olives and extra virgin olive oil on LDL oxidizability in healthy humans
Free Radic Res
 
2001
35
619
629
[39]
Long
L.H.
Kwee
D.C.
Halliwell
B.
The antioxidant activities of seasonings used in Asian cooking. Powerful antioxidant activity of dark soy sauce revealed using the ABTS assay
Free Radic Res
 
2000
32
181
186
[40]
Lee
C.Y.
Isaac
H.B.
Wang
H.
Huang
S.H.
Long
L.H.
Jenner
A.M.
et al
Cautions in the use of biomarkers of oxidative damage; the vascular and antioxidant effects of dark soy sauce in humans
Biochem Biophys Res Commun
 
2006
344
906
911
[41]
Richelle
M.
Turini
M.E.
Guidoux
R.
Tavazzi
I.
Metairon
S.
Fay
L.B.
Urinary isoprostane excretion is not confounded by the lipid content of the diet
FEBS Lett
 
1999
459
259
262
[42]
Lee
C.Y.
Jenner
A.M.
Halliwell
B.
Rapid preparation of human urine and plasma samples for analysis of F2-isoprostanes by gas chromatography-mass spectrometry
Biochem Biophys Res Commun
 
2004
320
696
702
[43]
Issa
A.Y.
Volate
S.R.
Wargovich
M.J.
The role of phytochemicals in inhibition of cancer and inflammation: new directions and perspectives
J Food Compos Anal
 
2006
405
419
[44]
Duarte
T.L.
Lunec
J.
Review: when is an antioxidant not an antioxidant? A review of novel actions and reactions of vitamin C
Free Radic Res
 
2005
39
671
686
[45]
Chen
Q.
Espey
M.G.
Krishna
M.C.
Mitchell
J.B.
Corpe
C.P.
Buettner
G.R.
et al
Pharmacologic ascorbic acid concentrations selectively kill cancer cells: action as a pro-drug to deliver hydrogen peroxide to tissues
Proc Natl Acad Sci U S A
 
2005
102
13604
13609
[46]
Rice-Evans
C.
Wake up to flavonoids
2000
London
Royal Society of Medicine Press
[47]
Pannala
A.S.
Rice-Evans
C.A.
Halliwell
B.
Singh
S.
Inhibition of peroxynitrite-mediated tyrosine nitration by catechin polyphenols
Biochem Biophys Res Commun
 
1997
232
164
168
[48]
Heijnen
C.G.
Haenen
G.R.
van Acker
F.A.
van der Vijgh
W.J.
Bast
A.
Flavonoids as peroxynitrite scavengers: the role of the hydroxyl groups
Toxicol In Vitro
 
2001
15
3
6
[49]
Santos
M.R.
Mira
L.
Protection by flavonoids against the peroxynitrite-mediated oxidation of dihydrorhodamine
Free Radic Res
 
2004
38
1011
1018
[50]
Ketsawatsakul
U.
Whiteman
M.
Halliwell
B.
A reevaluation of the peroxynitrite scavenging activity of some dietary phenolics
Biochem Biophys Res Commun
 
2000
279
692
699
[51]
Hajji
H.E.
Nkhili
E.
Tomao
V.
Dangles
O.
Interactions of quercetin with iron and copper ions: complexation and autoxidation
Free Radic Res
 
2006
40
303
320
[52]
Mira
L.
Fernandez
M.T.
Santos
M.
Rocha
R.
Florencio
M.H.
Jennings
K.R.
Interactions of flavonoids with iron and copper ions: a mechanism for their antioxidant activity
Free Radic Res
 
2002
36
1199
1208
[53]
Frankel
E.N.
Kanner
J.
German
J.B.
Parks
E.
Kinsella
J.E.
Inhibition of oxidation of human low-density lipoprotein by phenolic substances in red wine
Lancet
 
1993
341
454
457
[54]
Hertog
M.G.
Feskens
E.J.
Hollman
P.C.
Katan
M.B.
Kromhout
D.
Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study
Lancet
 
1993
342
1007
1011
[55]
Neuhouser
M.L.
Dietary flavonoids and cancer risk: evidence from human population studies
Nutr Cancer
 
2004
50
1
7
[56]
Mandel
S.
Amit
T.
Reznichenko
L.
Weinreb
O.
Youdim
M.B.
Green tea catechins as brain-permeable, natural iron chelators–antioxidants for the treatment of neurodegenerative disorders
Mol Nutr Food Res
 
2006
50
229
234
[57]
Schroeter
H.
Boyd
C.
Spencer
J.P.
Williams
R.J.
Cadenas
E.
Rice-Evans
C.
MAPK signaling in neurodegeneration: influences of flavonoids and of nitric oxide
Neurobiol Aging
 
2002
23
861
880
[58]
Zbarsky
V.
Datla
K.P.
Parkar
S.
Rai
D.K.
Aruoma
O.I.
Dexter
D.T.
Neuroprotective properties of the natural phenolic antioxidants curcumin and naringenin but not quercetin and fisetin in a 6-OHDA model of Parkinson's disease
Free Radic Res
 
2005
39
1119
1125
[59]
Zini
A.
Del Rio
D.
Stewart
A.J.
Mandrioli
J.
Merelli
E.
Sola
P.
et al
Do flavan-3-ols from green tea reach the human brain?
Nutr Neurosci
 
2006
9
57
61
[60]
Manach
C.
Donovan
J.L.
Pharmacokinetics and metabolism of dietary flavonoids in humans
Free Radic Res
 
2004
38
771
785
[61]
Williamson
G.
Barron
D.
Shimoi
K.
Terao
J.
In vitro biological properties of flavonoid conjugates found in vivo
Free Radic Res
 
2005
39
457
469
[62]
Halliwell
B.
Plasma antioxidants: health benefits of eating chocolate?
Nature
 
2003
426
787
[63]
Lotito
S.B.
Frei
B.
The increase in human plasma antioxidant capacity after apple consumption is due to the metabolic effect of fructose on urate, not apple-derived antioxidant flavonoids
Free Radic Biol Med
 
2004
37
251
258
[64]
Henning
S.M.
Niu
Y.
Liu
Y.
Lee
N.H.
Hara
Y.
Thames
G.D.
et al
Bioavailability and antioxidant effect of epigallocatechin gallate administered in purified form versus as green tea extract in healthy individuals
J Nutr Biochem
 
2005
16
610
616
[65]
Howitz
K.T.
Bitterman
K.J.
Cohen
H.Y.
Lamming
D.W.
Lavu
S.
Wood
J.G.
et al
Small molecule activators of sirtuins extend Saccharomyces cerevisiae lifespan
Nature
 
2003
425
191
196
[66]
Van Hoorn
D.E.
Nijveldt
R.J.
Van Leeuwen
P.A.
Hofman
Z.
M'Rabet
L.
De Bont
D.B.
et al
Accurate prediction of xanthine oxidase inhibition based on the structure of flavonoids
Eur J Pharmacol
 
2002
451
111
118
[67]
Laughton
M.J.
Evans
P.J.
Moroney
M.A.
Hoult
J.R.
Halliwell
B.
Inhibition of mammalian 5-lipoxygenase and cyclo-oxygenase by flavonoids and phenolic dietary additives. Relationship to antioxidant activity and to iron ion-reducing ability
Biochem Pharmacol
 
1991
42
1673
1681
[68]
Naasani
I.
Oh-Hashi
F.
Oh-Hara
T.
Feng
W.Y.
Johnston
J.
Chan
K.
et al
Blocking telomerase by dietary polyphenols is a major mechanism for limiting the growth of human cancer cells in vitro and in vivo
Cancer Res
 
2003
63
824
830
[69]
Katiyar
S.K.
Matrix metalloproteinases in cancer metastasis: molecular targets for prostate cancer prevention by green tea polyphenols and grape seed proanthocyanidins
Endocr Metab Immune Disord Drug Targets
 
2006
6
17
24
[70]
Chen
D.
Daniel
K.G.
Chen
M.S.
Kuhn
D.J.
Landis-Piwowar
K.R.
Dou
Q.P.
Dietary flavonoids as proteasome inhibitors and apoptosis inducers in human leukemia cells
Biochem Pharmacol
 
2005
69
1421
1432
[71]
Actis-Goretta
L.
Ottaviani
J.I.
Fraga
C.G.
Inhibition of angiotensin converting enzyme activity by flavanol-rich foods
J Agric Food Chem
 
2006
54
229
234
[72]
Moon
Y.J.
Wang
X.
Morris
M.E.
Dietary flavonoids: effects on xenobiotic and carcinogen metabolism
Toxicol In Vitro
 
2006
20
187
210
[73]
Li
C.
Allen
A.
Kwagh
J.
Doliba
N.M.
Qin
W.
Najafi
H.
et al
Green tea polyphenols modulate insulin secretion by inhibiting glutamate dehydrogenase
J Biol Chem
 
2006
281
10214
10221
[74]
Morris
M.E.
Zhang
S.
Flavonoid–drug interactions: effects of flavonoids on ABC transporters
Life Sci
 
2006
78
2116
2130
[75]
Singh
R.P.
Agarwal
R.
Natural flavonoids targeting deregulated cell cycle progression in cancer cells
Curr Drug Targets
 
2006
7
345
354
[76]
Shimizu
M.
Kobayashi
Y.
Suzuki
M.
Satsu
H.
Miyamoto
Y.
Regulation of intestinal glucose transport by tea catechins
Biofactors
 
2000
13
61
65
[77]
Ferreira
A.C.
Lisboa
P.C.
Oliveira
K.J.
Lima
L.P.
Barros
I.A.
Carvalho
D.P.
Inhibition of thyroid type 1 deiodinase activity by flavonoids
Food Chem Toxicol
 
2002
40
913
917
[78]
Aviram
M.
Dornfeld
L.
Kaplan
M.
Coleman
R.
Gaitini
D.
Nitecki
S.
et al
Pomegranate juice flavonoids inhibit low-density lipoprotein oxidation and cardiovascular diseases: studies in atherosclerotic mice and in humans
Drugs Exp Clin Res
 
2002
28
49
62
[79]
Steffen
Y.
Schewe
T.
Sies
H.
Myeloperoxidase-mediated LDL oxidation and endothelial cell toxicity of oxidized LDL; attenuation by (−)-epicatechin
Free Radic Res
 
2006
40
1076
1085
[80]
Rechner
A.R.
Kroner
C.
Anthocyanins and colonic metabolites of dietary polyphenols inhibit platelet function
Thromb Res
 
2005
116
327
334
[81]
Wu
C.H.
Yen
G.C.
Inhibitory effect of naturally occurring flavonoids on the formation of advanced glycation endproducts
J Agric Food Chem
 
2005
53
3167
3173
[82]
Schroeter
H.
Heiss
C.
Balzer
J.
Kleinbongard
P.
Keen
C.L.
Hollenberg
N.K.
et al
(−)-Epicatechin mediates beneficial effects of flavanol-rich cocoa on vascular function in humans
Proc Natl Acad Sci U S A
 
2006
103
1024
1029
[83]
Halliwell
B.
Zhao
K.
Whiteman
M.
The gastrointestinal tract: a major site of antioxidant action?
Free Radic Res
 
2000
33
819
830
[84]
Jenner
A.M.
Rafter
J.
Halliwell
B.
Human fecal water content of phenolics: the extent of colonic exposure to aromatic compounds
Free Radic Biol Med
 
2005
38
763
772
[85]
Kuwano
Y.
Kawahara
T.
Yamamoto
H.
Teshima-Kondo
S.
Tominaga
K.
Masuda
K.
et al
Interferon-gamma activates transcription of NADPH oxidase 1 gene and upregulates production of superoxide anion by human large intestinal epithelial cells
Am J Physiol Cell Physiol
 
2006
290
C433
C443
[86]
Ha
E.M.
Oh
C.T.
Bae
Y.S.
Lee
W.J.
A direct role for dual oxidase in Drosophila gut immunity
Science
 
2005
310
847
850
[87]
El Hassani
R.A.
Benfares
N.
Caillou
B.
Talbot
M.
Sabourin
J.C.
Belotte
V.
et al
Dual oxidase2 is expressed all along the digestive tract
Am J Physiol Gastrointest Liver Physiol
 
2005
288
G933
G942
[88]
Kanner
J.
Lapidot
T.
The stomach as a bioreactor: dietary lipid peroxidation in the gastric fluid and the effects of plant-derived antioxidants
Free Radic Biol Med
 
2001
31
1388
1395
[89]
Halliwell
B.
Long
L.H.
Yee
T.P.
Lim
S.
Kelly
R.
Establishing biomarkers of oxidative stress: the measurement of hydrogen peroxide in human urine
Curr Med Chem
 
2004
11
1085
1092
[90]
Waring
A.J.
Drake
I.M.
Schorah
C.J.
White
K.L.
Lynch
D.A.
Axon
A.T.
et al
Ascorbic acid and total vitamin C concentrations in plasma, gastric juice, and gastrointestinal mucosa: effects of gastritis and oral supplementation
Gut
 
1996
38
171
176
[91]
Aw
T.Y.
Intestinal glutathione: determinant of mucosal peroxide transport, metabolism, and oxidative susceptibility
Toxicol Appl Pharmacol
 
2005
204
320
328
[92]
Grootveld
M.
Atherton
M.D.
Sheerin
A.N.
Hawkes
J.
Blake
D.R.
Richens
T.E.
et al
In vivo absorption, metabolism, and urinary excretion of alpha,beta-unsaturated aldehydes in experimental animals. Relevance to the development of cardiovascular diseases by the dietary ingestion of thermally stressed polyunsaturate-rich culinary oils
J Clin Invest
 
1998
101
1210
1218
[93]
Kawai
K.
Matsuno
K.
Kasai
H.
Detection of 4-oxo-2-hexenal, a novel mutagenic product of lipid peroxidation, in human diet and cooking vapor
Mutat Res
 
2006
603
186
192
[94]
Gopaul
N.K.
Halliwell
B.
Anggard
E.E.
Measurement of plasma F2-isoprostanes as an index of lipid peroxidation does not appear to be confounded by diet
Free Radic Res
 
2000
33
115
127
[95]
Surh
J.
Kwon
H.
Estimation of daily exposure to 4-hydroxy-2-alkenals in Korean foods containing n-3 and n-6 polyunsaturated fatty acids
Food Addit Contam
 
2005
22
701
708
[96]
Pannala
A.S.
Mani
A.R.
Spencer
J.P.
Skinner
V.
Bruckdorfer
K.R.
Moore
K.P.
et al
The effect of dietary nitrate on salivary, plasma, and urinary nitrate metabolism in humans
Free Radic Biol Med
 
2003
34
576
584
[97]
Zhao
K.
Whiteman
M.
Spencer
J.P.
Halliwell
B.
DNA damage by nitrite and peroxynitrite: protection by dietary phenols
Methods Enzymol
 
2001
335
296
307
[98]
Chamulitrat
W.
Activation of the superoxide-generating NADPH oxidase of intestinal lymphocytes produces highly reactive free radicals from sulfite
Free Radic Biol Med
 
1999
27
411
421
[99]
Lapidot
T.
Granit
R.
Kanner
J.
Lipid hydroperoxidase activity of myoglobin and phenolic antioxidants in simulated gastric fluid
J Agric Food Chem
 
2005
53
3391
3396
[100]
Lee
S.Y.
Munerol
B.
Pollard
S.
Youdim
K.A.
Pannala
A.S.
Kuhnle
G.G.
et al
The reaction of flavanols with nitrous acid protects against N-nitrosamine formation and leads to the formation of nitroso derivatives which inhibit cancer cell growth
Free Radic Biol Med
 
2006
40
323
334
[101]
Babbs
C.F.
Free radicals and the etiology of colon cancer
Free Radic Biol Med
 
1990
8
191
200
[102]
Kato
I.
Dnistrian
A.M.
Schwartz
M.
Toniolo
P.
Koenig
K.
Shore
R.E.
et al
Iron intake, body iron stores and colorectal cancer risk in women: a nested case-control study
Int J Cancer
 
1999
80
693
698
[103]
Arakawa
H.
Maeda
M.
Okubo
S.
Shimamura
T.
Role of hydrogen peroxide in bactericidal action of catechin
Biol Pharm Bull
 
2004
27
277
281
[104]
Mochizuki
M.
Yamazaki
S.
Kano
K.
Ikeda
T.
Kinetic analysis and mechanistic aspects of autoxidation of catechins
Biochim Biophys Acta
 
2002
1569
35
44
[105]
Akagawa
M.
Shigemitsu
T.
Suyama
K.
Production of hydrogen peroxide by polyphenols and polyphenol-rich beverages under quasi-physiological conditions
Biosci Biotechnol Biochem
 
2003
67
2632
2640
[106]
Long
L.H.
Clement
M.V.
Halliwell
B.
Artifacts in cell culture: rapid generation of hydrogen peroxide on addition of (−)-epigallocatechin, (−)-epigallocatechin gallate, (+)-catechin, and quercetin to commonly used cell culture media
Biochem Biophys Res Commun
 
2000
273
50
53
[107]
Halliwell
B.
Oxidative stress in cell culture: an under-appreciated problem?
FEBS Lett
 
2003
540
3
6
[108]
Lapidot
T.
Walker
M.D.
Kanner
J.
Can apple antioxidants inhibit tumor cell proliferation? Generation of H2O2 during interaction of phenolic compounds with cell culture media
J Agric Food Chem
 
2002
50
3156
3160
[109]
Chai
P.C.
Long
L.H.
Halliwell
B.
Contribution of hydrogen peroxide to the cytotoxicity of green tea and red wines
Biochem Biophys Res Commun
 
2003
304
650
654
[110]
Wee
L.M.
Long
L.H.
Whiteman
M.
Halliwell
B.
Factors affecting the ascorbate- and phenolic-dependent generation of hydrogen peroxide in Dulbecco's Modified Eagles Medium
Free Radic Res
 
2003
37
1123
1130
[111]
Skibola
C.F.
Smith
M.T.
Potential health impacts of excessive flavonoid intake
Free Radic Biol Med
 
2000
29
375
383

Author notes

Time for primary review 27 days