Perhaps the category with the most growth promise, polyphenols such as resveratrol, green tea and cocoa nevertheless suffer from being mischaracterised as antioxidants. David A Mark, PhD, reveals the true nature of these healthful compounds
In 2000, the Institute of Medicine (IOM) defined a dietary antioxidant as follows: "A substance in foods that significantly decreases the adverse effects of reactive species, such as reactive oxygen and nitrogen species, on normal physiological function in humans." Using that definition, IOM included vitamin C, vitamin E and selenium as dietary antioxidants. And stopped there.
Normal cellular processes produce free radicals — primarily reactive oxygen and reactive nitrogen molecules — that unless neutralised by endogenous or exogenous antioxidants will react with lipids, proteins and nucleic acids. Cumulative collateral damage has been theorised as contributing to increased risk for many major diseases: cancer, cardiovascular disease, cataracts, immune senescence, chronic inflammation, age-related macular degeneration, neurodegenerative diseases, diabetes and ageing in general. Animal evidence and observational evidence in humans have been supportive, although some large-scale prospective human trials on vitamin C, vitamin E or selenium have been uneven.1,2,3,4
Outside IOM's official definition, claims are made that polyphenols from fruits and other plant sources are potent antioxidants. The emerging consensus among research leaders, however, is that polyphenols are not functioning as systemic antioxidants. Andrew Waterhouse, professor of enology, UC Davis, concludes simply, "Free-radical trapping is not due to dietary polyphenolics."
Circulating antioxidants such as ascorbic acid, uric acid, tocopherols, carotenoids and glutathione are collectively in excess of 1,000 micromolar while the amounts of polyphenols in the blood are on the order of 10-100 nanomolar — that is, at concentrations thousands of times lower. Even the evidence in support of flavonoids being much more potent antioxidants than classic nutrient antioxidants does not compensate for the lower content and transitory nature of circulating polyphenols in human blood, or whatever reaches target tissues. Helmut Sies, a long-time polyphenols researcher at Heinrich-Heine University in Duesseldorf, Germany, addressed the dilemma in a review. He cautioned against the use of "total antioxidant capacity" in blood plasma as a proxy for functionality.5
What are polyphenols doing?
If not acting directly as antioxidants, what are polyphenols doing? Evidence continues to develop for real-life physiological concentrations having molecular rather than unspecific, direct antioxidant effects. Polyphenol bioactives affect vascular function, signaling and anti-inflammation activity. Increased concentrations of nitric oxide (NO) result in vascular relaxation, measurable by changes to flow-mediated dilation. Improved vascular health may also involve reduced platelet and fibrinogen stickinesss, and lower levels of circulating vascular inflammatory molecules. Complex cell signaling either occurs at the cell surface or the phenol compounds reach the nucleus directly, and either way result in up and down regulation of gene expression.
Roger Corder's group at the William Harvey Research Institute used micro-array studies of human aortic endothelial cells to show that food-derived procyanidins changed the expression of dozens of genes. Production of mRNA for endothelin-1 was decreased and Kruppel-like factor 2 increased — both shifts are associated with lower risk of atherosclerosis.6 Helen Kim has published similar work on changes to mouse brain protein expression to demonstrate down regulation of proteins associated with Alzheimer's disease.7 Anti-inflammation hypotheses involve suppression of NFkB and/or cyclo-oxygenases. Finally, down-regulated gene expression may include decreasing the production of endogenous pro-oxidants such as NADPH oxidase, thus manifesting an indirect increase in antioxidant activity.5
Other-functioning theory frees polyphenols from "The Flavonoid Paradox." J Terao of the University of Tokushima, Japan, uses this term at the Fourth International Polyphenols and Health Conference, 2009, to characterise the problem: numerous epidemiological studies suggest that dietary flavonoids are closely related to the prevention of degenerative diseases such as cardiovascular disease and dementia, yet the bioavailability of these compounds appears extremely low and much of what is absorbed appears to be rapidly converted to inactive conjugated metabolites. In vitro research is often misleading because it uses concentrations and compounds that are not achieved in vivo.8 The Flavonoid Paradox is connected to the French Paradox. It postulates that if red-wine consumption is responsible for the lower than expected incidence of cardiovascular disease in France, it is because of the flavonoid content of the wine — but not because those flavonoids are antioxidants.
The Good News
Clinical and epidemiological evidence continues to support cardiovascular benefits in humans. Clinical trials presented at Experimental Biology 2010 added blueberries and strawberries to the list of flavanol foods such as dark chocolate and Concord grape juice with blood pressure-lowering activity.9 Years ago, all of the cognitive evidence was from animal models, plus a bit of epidemiology for red wine reducing risk of dementia.10 Now, human evidence for cognitive benefits begins to accrue. Krikorian reported a small study with older adults drinking Concord grape juice seeing improvements in list learning and trends for other memory improvements.8 Spencer reported similar benefits in humans for blueberries. Spencer postulated changes to protein and lipid kinase signaling cascades and improved vascular function as two possible mechanisms.8
For cancer, a 2009 epidemiological meta-analysis by Tang reported a 24 per cent reduction in risk of lung cancer with high dietary flavonoid intake.11
For weight management, a 2010 meta-analysis by Phung confirmed that green tea catechins have a useful, albeit modest, effect on weight loss — on the order of 3 pounds over 12 weeks.12 This aligns with earlier observations of increased energy expenditure with EGCG consumption. Evidence from recent conferences suggests that some phenols, including resveratrol, will increase the number of mitochondria in adipose and muscle cells. This would support the apparently paradoxical reported decreases in fat mass and increases in muscle mass occurring at the same time.
Cranberry evidence continues to accrue. The proanthocyanidins (PACs) in cranberries are primarily linked by A-type bonds vs B-type for other proanthocyanidin-rich foods. A-linked PACs appear to be more effective in blocking bacterial adherence. The cranberry beverage and supplement clinical evidence is consistent for reducing the risk of urinary tract infections, and promising for oral bacteria as well as other bacteria.
Are polyphenols safe?
A 2005 review by LI Mennen evaluated the risks and safety of polyphenol consumption. Animal models evaluating high concentrations of specific polyphenols have demonstrated carcinogenicity, genotoxicity, thyroid toxicity, estrogenic effects (by isoflavones), antinutritional effects such as interference with protein and iron absorption, and interactions with pharmaceuticals. While the authors go on to state: "It has not been proven that these effects also happen in humans," they caution against the development and marketing of polyphenol supplements without adequate safety assessment. Supplements can be designed to deliver higher polyphenol content than can be reached by polyphenol-rich foods, and hence need greater scrutiny.15
Specific safety studies have been conducted with various polyphenols. For example, one study of pomegranate capsules providing as much as 1,420mg/day containing 870mg of gallic acid equivalents found no serious adverse events in any of the 64 overweight individuals.16
In vitro testing of antioxidant capacity, for which the Oxygen Radical Absorbance Capacity (ORAC) assay is most widely recognised, has become marketing shorthand for claiming 'superfruit' status or a role as an "Antioxidant Superpower." Analytical results are available for a wide list of foods.17 Problems with using ORAC for marketing and comparative marketing purposes start with the revised ORAC fluorescein method giving about double the ORAC value of the old phycoerythins method used in the earlier literature.
Comparisons are also unfair when, based on a "per gram" basis, as this favours spices and dried fruit over fresh fruit and fruit juices. For example, dried Tibetan goji berries (Chinese wolfberries) are claimed as having 10 times the antioxidant content of fresh blueberries.
Some companies are making distinctions between hydrophilic, lipophilic or combined ORAC without there being any physiological basis for the nuances. Finally, starting in 2009, Brunswick Laboratories introduced Total ORAC FN as a patented measurement of antioxidant activity against five free radicals: hydroxyl, peroxyl, peroxynitrite, singlet oxygen and superoxide anion.18 Any claims for "My ORAC is Bigger than Your ORAC" will need to take into account exactly what ORAC methods were used for the products being compared.
Regardless of method, the limitations of in vitro measurement of antioxidant activity does not address limited bioavailability for many of the larger polyphenols, nor the fact that smaller molecules are metabolized so quickly that no trace of the unaltered molecule can be found in the blood, only sulfated, methylated or glucoronidated metabolites.
Claims for relevant biomarkers also have problems. Reporting a higher plasma antioxidant capacity after feeding an antioxidant food or supplement does support the idea that compounds are bioavailable and retain at least some of the purported chemical activity. However, without any connection to a clinically relevant outcome, this is not sufficient to support a health claim.5 The ex vivo assay demonstrating increased resistance of low-density lipoprotein to oxidation is in the same boat.
Are DRIs next?
Are we ready for a Dietary Reference Intake for dietary polyphenols? Not yet. Bioavailability, functionality of specific subsets, dose/response relationships and safety need more research.5 Given that academic researchers continue to shift their focus to beyond simple antioxidant mechanisms, it is time, however, for the major industry marketers of fruit juices, teas, dark chocolates, and polyphenol supplements to support the paradigm shift. This can be accomplished via changed marketing language plus combined support for a conference or series of conferences intended to standardise new terms of measuring content, clinically relevant biomarkers and clinically relevant outcomes.
David A Mark, PhD, is president of dmark consulting LLC, a Boston-area science consulting firm. His 25+ years of industry R&D experience includes functional foods, dietary supplements and medical nutrition products. www.dmarknutrition.com.
Disclosure: Dr Mark has as clients dietary supplement companies which market polyphenol-containing products, but not resveratrol-containing products.
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8. 4th International Conference on Polyphenols and Health, December 7-11, 2009, Harrowgate, UK.
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17. Wu X, et al. Lipophilic and hydrophilic antioxidant capacities of common foods in the United States. J Agric Food Chem 2004;52:4026-37.
18. "Brunswick Labs introduces next generation total ORAC for food and nutrition" Press release, April 20, 2009. www.brunswicklabs.com.