Health effects of natural phenols and polyphenols

Because of the large structural diversity and extensive metabolism of dietary polyphenols, it is difficult to determine their fate in vivo and assert specific health effects. Although many 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. To what extent effects would apply to entire organisms, and clinical outcomes in human disease in particular, remains an undefined topic in nutrition science and disease prevention.

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 2012 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 disease. The reviewers stated that research to date had been of poor quality and the large and rigorous trials are needed better to study the science, and to investigate possible adverse effects associated with excessive polyphenol intake. Currently, lack of knowledge about safety suggests that polyphenol levels should not exceed that which occurs in a normal diet.

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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