Respiratory Infections

A human study was conducted in 2007 to measure the effects of quercetin on improving immune functions in male cyclists subjected to 3 hours of intense physical exertion a day.12 Excessive exercising is known to increase oxidative stress in the body, and researchers speculate that it can lead to lower immune function and greater risk for infections.70 Although there were no apparent differences in various immune-system measurements between the groups given 1000 mg/day of quercetin and those given a placebo, the cyclists taking the supplement reported significantly less cases of upper respiratory infections—suggesting some direct antiviral effect from the quercetin.12 Other researchers have noted that these effects may have come from other known immune system responses that were not measured during this study.70


Further studies were conducted on mice that seem to confirm this direct effect of quercetin on viral infections. Divided into groups that were given quercetin or those given a placebo, the mice were further separated into sedentary groups or those then subjected to intense exercise. After 4 days of either fatiguing exercise (or rest for the control group), the mice were infected (on the 4th day) with H1N1 influenza virus (the so-called “swine flu”). Both exercise groups (quercetin and non-supplemented) and both sedentary control groups were infected with H1N1 doses that had been shown to induce death in approximately 50% of otherwise healthy mice in preliminary tests. Quercetin supplementation significantly reduced the mortality rates and severity of symptoms in the exercise group, and showed some positive antiviral effect in the sedentary group as well.70

Although the exact mechanisms in this particular virus are not known, lab studies have indicated that quercetin can inhibit infectivity of several respiratory viruses at early stages of exposure by blocking or promoting certain proteins that control pathways which either promote or hinder viral reproduction. In laboratory experiments using quercetin against several adenoviruses quercetin was able to significantly hinder viral replication. The adenovirus subtypes quercetin was tested against were ADV-3, which can lead to severe respiratory infections and even death in children, ADV-8, which is a common cause of pink-eye, and ADV-11, which can cause severe or fatal pneumonia.66,71-73

Other research has suggested that quercetin promotes and enhances immune system activity by multiple mechanisms. One way is by inducing interferon production—proteins expressed in infected cells that then trigger a variety of actions to destroy the infected cells.70

In animal studies with flu-infected mice, quercetin’s antioxidant properties were shown to play a role. Quercetin doses of 1-mg/day significantly decreased production of damaging superoxide free radicals by a type of white blood cell (called macrophages) in the lung, and also led to a significant reduction in other lipid free radicals formed due to virus-induced oxidative stress.74

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