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.
Bioavailability
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, flavanones, and quercetin glucosides even less. The least well-absorbed phenols are the proanthocyanidins, galloylated tea catechins, and anthocyanins.
Cognitive function
Due to their potential beneficial effects on pathways related to cognitive function such as inflammation and vasodilation, several types of polyphenol subclasses including blueberries and resveratrol have been investigated as a potential intervention for cognitive outcomes in both healthy adults and those with chronic diseases.
Cardiovascular health
The European Food Safety Authority found that flavonols in cocoa - at doses exceeding 200 mg per day - could contribute to normal function of the vascular endothelium, and olive oil polyphenols "contribute to the protection of blood lipids from oxidative damage", if consumed daily. Because long-term studies of high polyphenol intake have not been conducted, there is a lack of knowledge about safety or effect on health of intake above levels in a normal diet. A 2017 Cochrane review concluded that there is moderate-quality evidence that flavonol-rich cocoa products decrease blood pressure by only 2 mmHg in the short-term, although longer-term studies are needed.
Antioxidant activity
As interpreted by the Linus Pauling Institute and the European Food Safety Authority, 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, with most of what is absorbed existing as chemically-modified metabolites destined for rapid excretion.
Cancer
, there is little evidence that dietary flavonoids lower the risk of cancer.
