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Flavonoid
Though there is ongoing research into the potential health benefits of individual flavonoids, neither the Food and Drug Administration (FDA) nor the European Food Safety Authority (EFSA) has approved any health claim for flavonoids or approved any flavonoids as pharmaceutical drugs.[19][20][21] Moreover, several companies have been cautioned by the FDA over misleading health claims.[22][23][24][25]
Flavonoids have been shown to have a wide range of biological and pharmacological activities in in vitro studies. Examples include anti-allergic,[26] anti-inflammatory,[26][27] antioxidant,[27] anti-microbial (antibacterial,[28][29] antifungal,[30][31] and antiviral[30][31]), anti-cancer,[27][32] and anti-diarrheal activities.[33] Flavonoids have also been shown to inhibit topoisomerase enzymes[34][35] and to induce DNA mutations in the mixed-lineage leukemia (MLL) gene in in vitro studies.[36] However, in most of the above cases no follow up in vivo or clinical research has been performed, leaving it impossible to say if these activities have any beneficial or detrimental effect on human health. Biological and pharmacological activities which have been investigated in greater depth are described below.
Research at the Linus Pauling Institute and the European Food Safety Authority shows that flavonoids are poorly absorbed in the human body (less than 5%), with most of what is absorbed being quickly metabolized and excreted.[21][37][38] These findings suggest that flavonoids have negligible systemic antioxidant activity, and that the increase in antioxidant capacity of blood seen after consumption of flavonoid-rich foods is not caused directly by flavonoids, but is due to production of uric acid resulting from flavonoid depolymerization và excretion.[39]
Inflammation has been implicated as a possible origin of numerous local and systemic diseases, such as cancer,[40] cardiovascular disorders,[41] diabetes mellitus,[42] and celiac disease.[43]
Preliminary studies indicate that flavonoids may affect anti-inflammatory mechanisms via their ability to inhibit reactive oxygen or nitrogen compounds.[44] Flavonoids have also been proposed to inhibit the pro-inflammatory activity of enzymes involved in free radical production, such as cyclooxygenase, lipoxygenase or inducible nitric oxide synthase,[44][45] and to modify intracellular signaling pathways in immune cells,[44] or in brain cells after a stroke.[46]
Procyanidins, a class of flavonoids, have been shown in preliminary research to have anti-inflammatory mechanisms including modulation of the arachidonic acid pathway, inhibition of gene transcription, expression and activity of inflammatory enzymes, as well as secretion of anti-inflammatory mediators.[47]
Clinical studies investigating the relationship between flavonoid consumption and cancer prevention/development are conflicting for most types of cancer, probably because most studies are retrospective in design and use a small sample size.[48] Two apparent exceptions are gastric carcinoma và smoking-related cancers. Dietary flavonoid intake is associated with reduced gastric carcinoma risk in women,[49] and reduced aerodigestive tract cancer risk in smokers.[50]
Among the most intensively studied of general human disorders possibly affected by dietary flavonoids, preliminary cardiovascular disease research has revealed the following mechanisms under investigation in patients or normal subjects:[51][52][53][54][55]
Listed on the clinical trial registry of the US National Institutes of Health (July 2016) are 48 human studies completed or underway to study the dietary effects of plant flavonoids on cardiovascular diseases.[56]
However, population-based studies have failed to show a strong beneficial effect[57] which might be due to the considerably lower intake in the habitual diet of those investigated.
Flavonoids have been shown to have (a) direct antibacterial activity, (b) synergistic activity with antibiotics, and (c) the ability to suppress bacterial virulence factors in numerous in vitro and a limited number of in vivo studies.[28][58] Noteworthy among the in vivo studies[59][60][61] is the finding that oral quercetin protects guinea pigs against the Group 1 carcinogen Helicobacter pylori.[61] Researchers from the European Prospective Investigation into Cancer and Nutrition have speculated this may be one reason why dietary flavonoid intake is associated with reduced gastric carcinoma risk in European women.[62] Additional in vivo and clinical research is needed to determine if flavonoids could be used as pharmaceutical drugs for the treatment of bacterial infection, or whether dietary flavonoid intake offers any protection against infection.
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WikiPedia: Flavonoid