Quercetin is being increasingly researched in view of the fact that diets high in fruits and vegetables are healthy.  Large scale, long-term human dietary studies have found only mild benefits from higher flavonoid consumption.  However, the average human consumption of flavonoids was in the range of 20-25 mg/d in the various studies.  Quercetin made up between 60% and 77% of all flavonoids consumed in the various human dietary studies.  There are some 5,000 different plant flavonoid compounds, but the other significant ones are myricetin, kaempferol, naringenin, and hesperetin

Flavonoids are one of the major anti-oxidant components of tea, onions, apples and some other foods as well as being significant in ginkgo and a number of other plants.  There were 3400 research reports on quercetin listed on PubMed as of October, 2003, although only a few of these are on human beings.  Most are plant content analysis studies, and in vitro studies on the effect of quercetin on cell cultures or animal or human tissues.  However, there are dozens studies showing beneficial effects in animals, especially rats.  Many of these studies used specialized rats that are traditionally used as models for different human diseases.  In these rat studies, the dosages of quercetin used are much higher than that found in the typical human diet.

Quercetin in high doses (250mg) is now available in capsule form.  This is 10 times what the normal human consumes in a day.  It would require consuming 10 pounds of onions to achieve the same intake.  The only human disease studies I have been able to find used quercetin in high doses (500-1000mg/d) to successfully treat chronic prostatitis and interstitial cystitis (a chronic bladder infection or inflammation).  Even these studies are preliminary.  Quercetin appears harmless but it can interfere with the bioavailability of cyclosporin, an important transplant medication.  No long-term toxicological studies are available on high dose quercetin, but I have not found any of the animal or human studies reporting or suggesting any significant harmful effects.

Some of the diseases where quercetin is thought to be of potential benefit are kidney and liver diseases, diabetes, hypertension, Alzheimer's, tardive dyskinesia, asthma, heart disease, cancer, longevity, NSAID and alcohol induced gastric ulcers, and diabetic neuropathy.  In summation, quercetin is promising, but human research has hardly even begun.

Quercetin

Flavonoids Not Help in Zutphen Study: A 5 year follow-up of 738 men with an averare dietary intake 0f 27mg/d of flavonoids (61% for tea and rest from onions, apples, kale) found no association with decreased cancer risk. Dietary flavonoids and cancer risk in the Zutphen Elderly Study. Hertog MG, Feskens EJ, Hollman PC, Katan MB, Kromhout D. Nutr Cancer. 1994;22(2):175-84

Flavonoids Not Significant CVD Help in Nurses’ Study: 38,000+ American nurses with 7yr f/u. Flavonoid intake was not strongly associated with a reduced risk of CVD. The mean flavonoid intake was 24.6 +/- 18.5 mg/d, primarily as quercetin (70.2%). The nonsignificant inverse associations for broccoli, apples, and tea with CVD were not mediated by flavonoids and warrant further study. Harvard. Flavonoid intake and the risk of cardiovascular disease in women. Sesso HD, Gaziano JM, Liu S, Buring JE. Am J Clin Nutr. 2003 Jun;77(6):1400-8

Flavonoids Help Heart, Brain, Cancer, Asthma in Finnish Study: 10,054 men and women were followed.  Those with higher quercetin intakes had lower mortality from ischemic heart disease. The relative risk (RR) between the highest and lowest quartiles was 0.79. The incidence of cerebrovascular disease was lower at higher kaempferol (0.70), naringenin (0.79), and hesperetin (0.80) intakes. Men with higher quercetin intakes had a lower lung cancer incidence (0.42), and men with higher myricetin intakes had a lower prostate cancer risk (0.43). Asthma incidence was lower at higher quercetin (0.76), naringenin (0.69), and hesperetin (0.64) intakes. A trend toward a reduction in risk of type 2 diabetes was associated with higher quercetin (0.81) and myricetin (0.79). Flavonoid intake and risk of chronic diseases. Knekt P, Kumpulainen J, Jarvinen R, Rissanen H, Heliovaara M, Reunanen A, Hakulinen T, Aromaa A. Am J Clin Nutr. 2002 Sep;76(3):560-8

Quercetin Might Help Interstitial Cystitis and Chronic Prostatitis: avmazon.com promotes quercetin 250mg capsules for prostatitis. It claims a UCLA DB PC study was published in Urology by Daniel Shoskes and showed an 82% improvement but this is not true. The Dec. 2002 article was just a review article. Shoskes did report an open trial of 20 patients with interstitial cystitis rx quercetin 500mg bid for 4 weeks with dramatic improvement with an average decrease of 60% in symptoms. Treatment of interstitial cystitis with a quercetin supplement. Katske F, Shoskes DA, Sender M, Poliakin R, Gagliano K, Rajfer J. Tech Urol. 2001 Mar;7(1):44-6; Roughly a dozen in vitro or animal studies report that quercetin might have beneficial effects against prostate cancer including one Harvard study; The chronic prostatitis DB PC study of 30 patients rx 4 weeks with quercetin 500mg bid found that 67% of quercetin vs 20% placebo patients improved at least 25%. The 82% figure comes from an open trial of 17 men rx with quercetin as well as bromelain and papain (Prosta-O), which enhance bioflavonoid absorption, for one month. Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial. Shoskes DA, Zeitlin SI, Shahed A, Rajfer J. Urology. 1999 Dec;54(6):960-3. (Ed: Since DB PC studies almost always find less benefit than open trials, there continues to be no evidence that adding the other two ingredients has any beneficial effect.). Shoskes has also published two studies encouraging prostatic massage for chronic prostatitis, noting that this used to be used 40 or more years ago. He also has a rat study showing that either quercetin or curcumin protects rat kidneys from ischemic damage although the two together did no better than either alone. Transplantation. 1998 Jul 27;66(2):147-52; 90 capsules of quercetin 250mg cost $15.

Helps Mouse Diabetic Neuropathy Pain: 100mg/kg/d but not 50 helped diabetic mice injected with irritant to mimic neuropathy pain. Benefit eliminated by naloxone suggesting action on opioid receptors. Quercetin, a bioflavonoid, attenuates thermal hyperalgesia in a mouse model of diabetic neuropathic pain. Anjaneyulu M, Chopra K., Panjab Univ. Prog Neuropsychopharmacol Biol Psychiatry. 2003 Sep;27(6):1001-5

Protects Mice Kidneys from Oxidant Damage: quercetin, which has antioxidant properties, protects renal tubular epithelial cells from oxidant-induced injury by inhibiting lipid peroxidation. Quercetin significantly inhibited hypoxia-induced functional and structural tubular injury in addition to lipid peroxidation but did not alter hypoxia-induced ATP depletion. Essen. Effect of quercetin on hypoxic injury in freshly isolated rat proximal tubules. Pietruck F, Kuhlmann MK, Lange B, Feldkamp T, Herget-Rosenthal S, Rauen U, Burkhardt G, Kohler H, Philipp T, Kribben A. J Lab Clin Med. 2003 Aug;142(2):106-12

Helps Rat Hypertension: partial or full prevention of most of the effects induced by L-NAME, such as: (1) increases in the left ventricular and kidney weight indices; (2) proteinuria; (3) renal histological lesions, including hyaline arteriopathy and thickening of the vascular wall with moderate decrease of the lumen; (4) increased endothelium-dependent contraction; (5) increased vascular thromboxane B2 (TXB2) synthesis; (6) reduced plasma concentrations of nitrites plus nitrates (NOx); (7) increased plasma and hepatic malondialdehyde (MDA) concentrations; and (8) reduced glutathione peroxidase activity. In most cases these effects were dose dependent, but none of them were observed in normotensive animals. CONCLUSIONS: This study confirms and extends the previous evidence about the antihypertensive effects and end-organ protection of the flavonoid quercetin in animal models of hypertension. Protective effects of the flavonoid quercetin in chronic nitric oxide deficient rats. Duarte J, Jimenez R, O'Valle F, Galisteo M, Perez-Palencia R, Vargas F, Perez-Vizcaino F, Zarzuelo A, Tamargo J. J Hypertens. 2002 Sep;20(9):1843-54; suggest an inhibitory effect of quercetin on the angiotensin-converting enzyme activity, similar to that of captopril. Pharmacology. 2002 Aug;65(4):182-6

Protects Against Toxic Liver Damage & Possibly Liver Fibrosis: The flavonoid quercetin inhibits dimethylnitrosamine-induced liver damage in rats. Lee ES, Lee HE, Shin JY, Yoon S, Moon JO. J Pharm Pharmacol. 2003 Aug;55(8):1169-74

Research for Neurological Protection: Chronic treatment with flavonoids did not alter the locomotor activity in both young and aged mice; however, aged mice showed improvement of performance on Rota-Rod test. The results showed that chronic treatment with flavonoids reverses cognitive deficits in aged and LPS-intoxicated mice which suggests that modulation of cyclooxygenase-2 and inducible nitric synthase by flavonoids may be important in the prevention of memory deficit. Protective effect of flavonoids against aging- and lipopolysaccharide-induced cognitive impairment in mice. Patil CS, Singh VP, Satyanarayan PS, Jain NK, Singh A, Kulkarni SK. Panjob Univ. Pharmacology. 2003 Oct;69(2):59-67

Tardive Dyskinesia in Rats Helped: Chronic neuroleptics leads to abnormal orofacial movements described as vacuous chewing movements (VCMs) in rats, which is widely accepted as one of the animal models of tardive dyskinesia. Oxidative stress and the products of lipid peroxidation are implicated in the pathophysiology of various neurological disorders including tardive dyskinesia. Haloperidol (1.0 mg kg(-1) for 21 days) treatment induced vacuous chewing movements and tongue protrusions in rats. Co-administration of quercetin, a bioflavonoid, dose dependently (25-100 mg kg(-1)) reduced haloperidol-induced vacuous chewing movements and tongue protrusions. Haloperidol treatment induces lipid peroxidation and decreases the glutathione (GSH) levels in the forebrains of rats. The antioxidant defense enzymes, superoxide dismutase (SOD) and catalase were also decreased. Co-administration of quercetin (25-100 mg kg(-1)) significantly reduced the lipid peroxidation and restored the decreased glutathione and reversed the decrease in forebrain SOD and catalase levels. Panjab Univ. Quercetin, a bioflavonoid, attenuates haloperidol-induced orofacial dyskinesia. Naidu PS, Singh A, Kulkarni SK. Neuropharmacology. 2003 Jun;44(8):1100-6

Various Briefs: Quercetin can inhibit the growth of transplantation tumor of breast cancer cell line MCF-7 in nude mice. Sichuan Da Xue Xue Bao Yi Xue Ban. 2003 Jul;34(3):439-42. The peroral use of quercetin before and after administration of diclofenac sodium has been shown to be associated with fewer ulcers in test animals, which fact suggests to us a protective effect of quercetin on their gastric mucosa. Lik Sprava. 2003 Jan-Feb;(1):96-9. Helped a mouse model of chronic fatigue syndrome. Panjab Univ. J Med Food. 2002 Winter;5(4):211-20; Quercetin, 7,8-dihydroxy-4-methyl coumarin, curcumin, resveratrol and vanillin are therefore effective in protection of human single cell DNA from oxidative attack. Pharmazie. 2002 Dec;57(12):852-4; reported that they exert multiple biological effects due to their antioxidant and free radical-scavenging abilities. Although results from different studies have demonstrated that flavonoids can act as pro-oxidant at very high doses, most investigations have reported anti-inflammatory, antiviral, or anti-allergic effects and a protective role in heart diseases, cancer and different pathologies. Nutr Hosp. 2002 Nov-Dec;17(6):271-8; quercetin treatment significantly decreased the number of mast cells and reduced the area of gastric erosions caused by alcohol in rats. Toxicology. 2003 Feb 1;183(1-3):133-42; can decreased cyclosporin bioavailability. Life Sci. 2002 Dec 6;72(3):227-35; Exposure of rats to UVA light leads to oxidative stress, reflected by increased MDA and reduced antioxidant enzyme levels. The administration of quercetin appears to be a useful approach to reduce the damage produced by UVA radiation. J Appl Toxicol. 2002 Sep-Oct;22(5):303-9; Vidalia onions have Quercetin was the major flavonoid (7.70-46.32 mg/100 g fresh weight, FW) present in all varieties, followed by myricetin (2.77-4.13 mg/100 g FW). Minor quantities of kaempferol (1.10-1.98 mg/100 g FW). J Agric Food Chem. 2002 Sep 11;50(19):5338-42;