Health effects of natural phenols and polyphenols
Because of the large structural diversity and extensive metabolism of dietary polyphenols, their fate in vivo and possible health effects remain undetermined as of the early 21st century. Although polyphenols are speculated to be part of the health-promoting effects of consuming fruits and vegetables, no evidence exists to date that dietary polyphenols actually provide health benefits.
Polyphenols have poor bioavailability, indicating that most of what are consumed are extensively metabolized and excreted. Gallic acid and isoflavones may show absorption of about 5%, with amounts of catechins (flavan-3-ols), flavanones, and quercetin glucosides even less. The least well-absorbed phenols are the proanthocyanidins, galloylated tea catechins, and anthocyanins.
A review published in 2013 found insufficient consensus for the hypothesis that the specific intake of food and drink containing flavonoids may play a meaningful role in reducing the risk of cardiovascular diseases. The reviewers stated that research to date had been of poor quality and that large and rigorous clinical trials are needed to define health benefits and to reveal adverse events from excessive polyphenol intake. Currently, lack of knowledge about safety suggests that polyphenol levels should not exceed the intake of a normal diet.
Preliminary research on the association of consuming polyphenol foods, such as olive oil, soy, and pomegranate products, with lower risk of cardiovascular diseases has been low in quality, with little evidence of any possible benefit.
As interpreted by the Linus Pauling Institute and the European Food Safety Authority (EFSA), dietary flavonoids have little or no direct antioxidant food value following digestion. Unlike controlled test tube conditions where antioxidant effects may result when high concentrations of flavonoids are used, the fate of ingested flavonoids in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically-modified metabolites destined for rapid excretion.
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