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Quercetin is another well-known member of the polyphenol family. Considered a flavonoid type of polyphenol, quercetin is found in many fruits and vegetables, including onions, apples, cherries, broccoli, tomatoes, and berries, as well as green tea.
We look to quercetin most often seasonally as a remedy for those with allergic afflictions.1 Quercetin counteracts the allergic response by suppressing antigen-specific immunoglobulin (Ig) E antibody formation, thereby acting at a very early stage in the allergic response.2 3 Additionally, quercetin inhibits the release of histamine and proinflammatory substances implicated in allergic reactions.4 By these and other mechanisms, quercetin may improve contact dermatitis and photosensitivity,5 allergic rhinitis,6 and asthma.7 8
In addition to these anti-allergic effects, quercetin helps to balance the Th1:Th2 immune response, downregulating production of Th2 allergic response-related cytokines, such as interleukin (IL)-4, and increasing interferon (IFN)-γ,9 a key Th1 cytokine involved in the response against viral invaders and the development of immunity to pathogens.10 11 12 13
Quercetin further shows respiratory tract affinity in the protection it offers against oxidative damage and inflammation associated with particulate matter exposure.
Studies have shown numerous mechanisms by which quercetin and other flavonoids can reduce infectivity of a wide variety of respiratory and other viruses,14 including influenza, adenovirus, rhinovirus, and coronaviruses.15 16 17 18 Quercetin further shows respiratory tract affinity in the protection it offers against oxidative damage and inflammation associated with particulate matter exposure.19 Quercetin happens to be found at fairly high levels in St. John’s wort (Hypericum perforatum),20 and may be one of the constituents that contributes to its mood-stabilizing and antiviral effects.21 22 23 24 25
Quercetin helps protect the body against reactive oxygen species, although studies show it also can have pro-oxidant effects.26 27 28 Although we think of pro-oxidants negatively, they are activators of our body’s endogenous antioxidant and protective detoxification enzyme transcription by turning on the Nrf2/ARE pathway. In animals, quercetin supplementation has been observed to increase levels of alpha-tocopherol (a form of vitamin E) while decreasing markers of lipid peroxidation, however variable effects on glutathione levels have been shown.29 30 Providing additional antioxidants along with quercetin such as vitamin C and glutathione may help to negate the pro-oxidant effects and enhance quercetin recycling.31 Indeed, vitamin C appears to enhance the activity of quercetin, in part by stabilizing the quercetin molecule itself.32
In healthy individuals, quercetin supplementation has been shown to significantly reduce the incidence of upper respiratory tract infections. Clinical studies have also shown quercetin has anti-inflammatory and antioxidant effects.33 These effects were seen more clearly in diseases such as sarcoidosis (a condition that primarily affects the lungs) where there is a high level of oxidative stress and inflammation associated with the pathology.34 Quercetin has also been shown to reduce pro-inflammatory cytokine production triggered by lipopolysaccharide (LPS) stimulation of ex vivo blood samples from patients with idiopathic pulmonary fibrosis, suggesting it may be of benefit in this population as well.35
In healthy individuals, quercetin supplementation has been shown to significantly reduce the incidence of upper respiratory tract infection (URTI) in trained male cyclists36 and the number of URTI sick days and symptom severity in physically fit subjects over the age of 40.37 An extensive recent review looks at numerous mechanisms by which the combination of quercetin with vitamin C may be effective as a prophylactic for prevention of highly prevalent respiratory infections.14 Supplementation of quercetin also has been shown in humans to have antiviral potential in individuals with chronic hepatitis C,38 and to have a positive impact on blood pressure,39 cholesterol profiles,40 and other markers of cardiovascular disease risk.41 42 Quercetin is typically well-tolerated and has been demonstrated to be safe in humans when taken acutely up to 5 g/day.38 And clearly, it is worthy of consideration for more than just allergies.