Possible Side Effects of Quercetin

1. At very high doses, antioxidants such as bioflavonoids can actually become pro-oxidants. Combining quercetin with multiple other anti-oxidant and anti-inflammatory products may have a net pro-inflammatory effect in some people. Given the propensity for distal joint pain, it may also be related to uric acid metabolism.
   
   Free Radic Biol Med 1999 Dec;27(11-12):1313-23 Related Articles, Books, LinkOut
   
   Anti- and pro-oxidative effects of flavonoids on metal-induced lipid hydroperoxide-dependent lipid peroxidation in cultured hepatocytes loaded with alpha-linolenic acid.
   
   Sugihara N, Arakawa T, Ohnishi M, Furuno K Faculty of Pharmacy and Pharmaceutical Sciences, Fukuyama University, Hiroshima, Japan.
   
   Lipid hydroperoxide (LOOH)-dependent lipid peroxidation was induced in alpha-linolenic acid (LNA)-loaded hepatocytes by adding Fe, Cu, V, or Cd ions at concentrations from 20 to 500 microM. The effects of structurally related flavonoids at concentrations from 10 to 500 microM on the lipid peroxidation were examined.
   The results with regard to each flavonoid subclass are as follows:
   1. Flavonols such as myricetin, quercetin, fisetin, and kaempferol, but not morin, showed dose-dependent antioxidative activity against metal-induced lipid peroxidation at all metal concentrations. Myricetin, quercetin, and fisetin were the most effective antioxidants, although their efficacies depended on the metal ion. Kaempferol and morin had antioxidative activity equal to the other flavonols in the presence of Cu ions, but were much less effective for the other three metal ions.
   2. Flavones, luteolin, apigenin, and chrysin were antioxidative at low Fe concentrations, but were pro-oxidative at high Fe concentrations. Luteolin exhibited antioxidative activity similar to that of catechol-containing flavonols in the presence of the other three metal ions. Apigenin and chrysin also acted as pro-oxidants with V or with all metal ions, respectively.
   3. Taxifolin, a flavanone, also showed both anti- and prooxidative activity, depending on Fe concentrations, but with other metal showed only antioxidative activity ions.
   4. Epigallocatechin, a flavanol, was antioxidative with all metal ions, and its activity was similar to that of catechol-containing flavonols. The various effects of flavonoids on metal-induced lipid peroxidation in LNA-loaded hepatocytes is discussed with regard to the change in redox potential of flavonoid-metal complexes.