In 2000, the Institute of Medicine defined a dietary antioxidant as “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, the IOM included vitamin C, vitamin E and selenium as dietary antioxidants. And stopped there.
Normal cellular processes produce free radicals—the reactive oxygen and nitrogen molecules mentioned in the IOM definition—which, unless neutralized by antioxidants, interact with fats, proteins and nucleic acids to, in theory, increase risk for many major diseases: cancer, cardiovascular disease, chronic inflammation, neurodegenerative diseases and diabetes. Animal and human studies support this theory, although some large-scale human trials on vitamin C, vitamin E and selenium have been uneven.
Outside IOM’s official definition, some experts claim that polyphenols from fruits and other plant sources—cocoa, resveratrol, green tea, blueberries, strawberries, grapes and cranberries—are potent antioxidants. The emerging consensus among researchers, however, is that polyphenols don’t function the same as vitamins C and E and selenium.
But if they’re not acting directly as antioxidants, what are polyphenols doing? Evidence continues to develop that polyphenols work molecularly, affecting vascular function, cell signaling and anti-inflammation activity.
Benefits of polyphenols
Clinical and epidemiological evidence continues to support the cardiovascular benefits of polyphenols in humans. Clinical trials presented at Experimental Biology 2010, an annual meeting of 13,000 scientists and exhibitors, added blueberries and strawberries to the list of flavanol (a type of polyphenol) foods such as dark chocolate and Concord grape juice that have blood pressure–lowering activity.
Years ago, all the evidence for cognitive benefits from polyphenols was from animal models, plus a few epidemiological studies showing red wine reduces dementia risk. Now, we have human evidence. A small study in which older adults drank Concord grape juice showed improvements in list learning and other memory trends. Another study reported that blueberries had similar benefits in humans.
A 2009 epidemiological meta-analysis reported that high dietary flavonoid intake led to a 24 percent reduction in lung cancer risk.
For weight management, a 2010 meta-analysis confirmed that green tea catechins have a useful, albeit modest, effect on weight loss—3 pounds in 12 weeks. This aligns with earlier observations of increased energy expenditure with consumption of epigallocatechin gallate, the most abundant catechin in green tea. Evidence from recent conferences suggests that some polyphenols, including resveratrol, increase the number of mitochondria (energy suppliers) in fat and muscle cells. This supports the apparently paradoxical finding of decreases in fat mass and increases in muscle mass occurring at the same time.
The proanthocyanidins (a type of polyphenol) in cranberries are primarily linked by A-type bonds versus B-type for other proanthocyanidin-rich foods. A-linked proanthocyandins appear to be more effective in blocking bacterial adherence. Research on cranberry beverages and supplements consistently shows they reduce the risk of urinary-tract infections, and are promising for other types of bacteria, including oral bacteria.
Are polyphenols safe?
A 2005 review evaluated the risks and safety of polyphenol consumption. Animal studies using high concentrations of specific polyphenols showed the polyphenols were carcinogenic and toxic to genes and the thyroid, and also demonstrated estrogenic effects and interfered with protein and iron absorption and interactions with pharmaceuticals. While the authors noted that “... it has not been proven that these effects also happen in humans,” they cautioned against the development and marketing of polyphenol supplements without adequate safety assessment.
On the other hand, one study of 64 overweight people who took as much as 1,420 mg a day of pomegranate capsules containing 870 mg of gallic acid equivalents found no serious adverse events in any of the individuals.
The bottom line is that supplements can be designed to deliver higher polyphenol content than food equivalents, and hence need greater scrutiny.