Quercetin negative effects

From the data as a whole, it appears that flavonoids do not have a negative impact on health, and some positive outcome was found. However, it is very hard to state which flavonoid is responsible for positive effects. No epidemiological data on supplements are available. A long-term study on chronic venous insufficiency (CVI) showed that high dosages of an herbal medicine containing mainly quercetin were effective in the treatment of mild CVI (75). 

The ascorbate transporter SVCTl has been shown to be inhibited by some flavonols such as quercetin but less so by the related catechins. The absence of the ketone at the 4-position of the C ring has a dramatic negative effect on the potency of inhibition, suggesting that a key hydrogen-bonding... 

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