Why would you risk brain infections, psychosis, memory deterioration and mood swings for a 10% improvement when you can get up to a 900% improvement without taking on any of these risks?
- Your immune system is your first-line defense against all types of infections, be they bacterial or viral, so the most effective way to prevent infectious illness — including influenza — is a robust immune system
- Quercetin, a plant flavonol found naturally in apples, plums, red grapes, green tea, elder flower and onions, packs a powerful antiviral punch, inhibiting several strains of influenza, hepatitis B and C and other viruses
- Quercetin also combats inflammation and acts as a natural antihistamine
- Quercetin reduces viral illness and boosts mental performance following extreme physical stress, which might otherwise undermine your immune function and render you more susceptible to infections
- Its antiviral effects can be attributed to three main mechanisms of action: Inhibiting the virus’ ability to infect cells, inhibiting replication of already infected cells and reducing infected cells’ resistance to treatment with antiviral medication
Editor’s Note: This article is a reprint. It was originally published February 26, 2018.
Your immune system is your first-line defense against all types of infections, be they bacterial or viral, so the most effective way to prevent infectious illness — including influenza — is a robust immune system. Your diet and other lifestyle factors are foundational for good immune function, but certain supplements can also be quite helpful.
One such supplement is quercetin,1Fitoterapia 2015 Oct;1[06:25]6-71 an antioxidant flavonol found naturally in apples, plums, red grapes, green tea, elder flower and onions, just to name a few. For a more exhaustive list, see Superfoodly’s ranking of the top 100 quercetin-packed foods.2Superfoodly, 100 Quercetin Foods
Quercetin has been shown to combat inflammation and acts as a natural antihistamine. Elder flower extract, which is rich in quercetin, has been traditionally used as a tonic to boost immunity. It is also widely known to promote lung and bronchial tract health.
Quercetin is also available in supplement form and has been used to ameliorate obesity,3Food Chemistry Volume 179. Pages 305-310. July 15, 2015 Type 2 diabetes,4Biomedicine & Pharmacotherapy Volume 146. February 2022 circulatory dysfunction, chronic inflammation, hay fever and mood disorders.5Fitoterapia 2015 Oct;1[06:25]6-71 A number of studies have also highlighted quercetin’s ability to prevent and treat both the common cold6Semantic Scholar. 2014 and influenza.7Biomolecules. December 24, 2020
Quercetin Significantly Lowers Your Risk for Viral Illness
Research8Newswise. Appalachian State University February 8, 2007 from Appalachian State University in North Carolina published in 2007 found quercetin reduces viral illness and boosts mental performance following extreme physical stress, which might otherwise undermine your immune function and render you more susceptible to infections. The research in question was funded by the U.S. Department of Defense.
To investigate the effects of quercetin on viral illness, 40 cyclists were divided into two groups; half of them received a daily dose of 1,000 mg of quercetin in combination with vitamin C (which enhances plasma quercetin levels9Journal of the American Dietetic Association 2011 Apr;111(4):542-910Journal of Infectious Diseases and Preventive Medicine May 24, 2014; 2: 111) and niacin (to improve absorption) for five weeks while the other half received a placebo.
Three weeks into the trial, the athletes rode a bicycle for three hours a day, three days in a row. Blood and tissue samples were collected before and after exertion. Analysis revealed 45% of the placebo group contracted viral illness after the physical stress, compared to just 5% of the treatment group. According to lead investigator David Nieman:
That’s a highly significant difference. When you have a double-blind, placebo-controlled study and you have those kinds of differences, it can’t be due to chance … These are ground-breaking results because this is the first clinical, double-blind, randomized, placebo-controlled study that has found a natural plant compound to prevent viral illness …
It appears that it takes significant stress to bring out quercetin’s infection-fighting properties. This all happened when athletes were under high oxidative stress, when stress hormones were high, and they were also undergoing muscle damage.
The athletes taking the quercetin supplement maintained their ability to react to an alertness test when exhausted, whereas those who took the placebo became measurably slower. The infection data and vigilance data are our two biggest findings in this study.
Quercetin Protects Against Flu and Other Viral Infections
In another study funded by the U.S. Defense Advanced Research Projects Agency (DARPA), published in 2008, animals treated with quercetin were challenged with a highly pathogenic H1N1 influenza virus. Again, the treatment group had significantly lower morbidity and mortality than the placebo group.
These and similar studies led to the development of a quercetin supplement for military personnel. In 2008, DARPA director Tony Tether provided testimony to the Subcommittee on Terrorism, Unconventional Threats and Capabilities, saying quercetin “will help keep our warfighters healthy during training and deployment.”
The supplement, Q-Force, is also commercially available to the general public. A number of other studies have confirmed quercetin’s effectiveness against influenza, as well as a variety of other viruses, including the following:
- A 1985 study found quercetin inhibits infectivity and replication of herpes simplex virus type 1, polio-virus type 1, parainfluenza virus type 3 and respiratory syncytial virus.11Journal of Medical Virology January 1985 DOI: 10.1002/jmv.1890150110
- A 2010 animal study found that quercetin inhibits both influenza A and B viruses. Two other important discoveries were made. Firstly, the viruses were unable to develop resistance to quercetin, and secondly, when used concomitant with antiviral drugs (amantadine or oseltamivir), the effect was significantly amplified — and it prevented drug-resistance from developing.12Antiviral Research 2010 Nov;88(2):227-35
- A 2004 animal study investigating quercetin’s effect on influenza used a strain of the H3N2 virus. According to the authors:13Experimental Lung Research 2005; 31(5) “In the mice, instillation of influenza virus A/Udorn/317/72(H3N2) intranasally resulted in a significant decrease in the pulmonary concentrations of catalase, reduced glutathione and superoxide dismutase … These effects were observed on the 5th day after viral instillation.Oral supplementation with quercetin simultaneous with viral instillation produced significant increases in the pulmonary concentrations of catalase, reduced glutathione and superoxide dismutase …It is concluded that during influenza virus infection, there is ‘oxidative stress.’ Because quercetin restored the concentrations of many antioxidants, it is proposed that it may be useful as a drug in protecting the lung from the deleterious effects of oxygen derived free radicals released during influenza virus infection.”
- In 2014, researchers noted that quercetin appears to be “a promising treatment for the common cold,” caused by the rhinovirus, adding that “Quercetin has been shown to reduce viral internalization and replication in vitro, and viral load, lung inflammation and airways hyper-responsiveness in vivo.”14Journal of Infectious Diseases and Preventive Medicine May 24, 2014; 2: 111 By attenuating oxidative damage, it also lowers your risk of secondary bacterial infections, which is actually the primary cause of influenza-related deaths. Importantly, quercetin increases mitochondrial biogenesis in skeletal muscle, which suggests part of its antiviral effects are due to enhanced mitochondrial antiviral signaling. According to the authors:“… In vitro studies have demonstrated that quercetin acts as a potent antiviral agent by inhibiting viral replication of several respiratory viruses, including influenza virus, parainfluenza virus, respiratory syncytial virus, adenovirus and rhinovirus. Although the quercetin’s antiviral mechanisms are not well understood, a number of possibilities have been proposed and is summarized in Figure 1.”
- A 2016 animal study found quercetin inhibited mouse hepatitis virus and the dengue virus.15Asian Pacific Journal of Tropical Medicine January 2016; 9(1): 1-7
- Another 2016 study found quercetin offered protection against influenza A virus H1N1 by modulating protein expression. More specifically, the regulation of heat shock proteins, fibronectin 1 and prohibitin was instrumental in reducing viral replication.16Journal of Agricultural and Food Chemistry 2016; 64(21): 4416-4425
- A third study published in 2016 found quercetin inhibited a wide spectrum of influenza strains, including H1N1, H3N2 and H5N1. According to the authors, “This study indicates that quercetin showing inhibitory activity in the early stage of influenza infection provides a future therapeutic option to develop effective, safe and affordable natural products for the treatment and prophylaxis of [influenza A viruses] infections.”17Viruses 2016 Jan; 8(1): 6
Quercetin Inhibits Hepatitis B and C
In summary, quercetin’s powerful antiviral effects can be attributed to three main mechanisms of action:
- Inhibiting the virus’ ability to infect cells
- Inhibiting replication of already infected cells
- Reducing infected cells’ resistance to treatment with antiviral medication
As you can see from the list of studies above, quercetin’s ability to prevent illness is not restricted to influenza. Research shows it’s incredibly effective for boosting general immune health, and studies have shown it can inhibit both hepatitis B18Virologica Sinica August 2015; 30(4): 261-268 and C19Hepatology 2009 Dec;50(6):1756-64 infection. It may even be useful in the treatment of HIV. As noted in a Superfoods Scientific Research article on quercetin:20Superfoods Scientific Research, Quercetin
Hepatitis C is an important cause of liver failure and liver cancers. In August 2009, [a] cell study21Hepatology 2009 Dec;50(6):1756-64 demonstrated that quercetin interfered with the gene signals that enable hepatitis C virus production. The researchers showed … quercetin inhibits hepatitis C viral production in tissue culture, at least partially through its inhibition of heat shock protein expression.
Therapy with quercetin reduced the infectious particle production to nontoxic concentrations of [hepatitis C virus].
Similarly, research published in 2015 found quercetin inhibited hepatitis B virus replication in human liver cells, protecting cells from infection and limiting the spread of infection in already infected samples.22Virologica Sinica August 2015; 30(4): 261-268 As in previous studies, when combined with antiviral drugs, in this case lamivudine, entecavir or adefovir, the antiviral effect was greatly enhanced.
According to the authors, “The results indicate that quercetin inhibited HBV [hepatitis B virus] antigen secretion and genome replication in human hepatoma cell lines, which suggests that quercetin may be a potentially effective anti-HBV agent.”
Quercetin a Far Safer Alternative to Tamiflu
Should you or your child come down with the flu and your doctor or pediatrician recommends Tamiflu, you should know this antiviral drug has been shown to shorten the duration of flu symptoms by less than 17 hours.23BMJ 2014;348:g254524Forbes April 10, 2014 It also does not reduce viral transmission and does not lower your risk of complications from the flu, such as pneumonia.25BMJ 2014;348:g254526The Guardian April 10, 2014 Scientists have also warned that the risks far outweigh the benefits.27Cochrane Database of Systematic Reviews 2014, Issue 4. Art. No.: CD008965
These risks include convulsions, brain infections, psychosis and other neuropsychiatric problems. Tamiflu made recent headlines after a 6-year-old girl started hallucinating and attempted to jump out a second story window.28Newsweek January 15, 2018 Indeed, a number of studies have observed that Tamiflu may cause psychiatric symptoms, including mood swings, suicidal feelings, auditory hallucinations, memory deterioration and insomnia.29Clin Psychopharmacol Neurosci. 2015 Aug; 13(2): 209–211
The drug is particularly risky for children, and more than half of all children taking Tamiflu suffer side effects from the drug.30The Guardian July 31, 200931Eurosurveillance July 2009; 14(30) Considering its risks, and its limited effectiveness, quercetin appears to be a far safer and more effective alternative. Studies have repeatedly found it to be nontoxic, with no adverse side effects.
Vitamin D — Another Potent Immune Booster and Antiviral
Optimizing your vitamin D is another important prevention strategy that will boost your immune function and help prevent infectious diseases of all kinds.
The latest research suggests a vitamin D serum level of 60 to 80 ng/mL is ideal. While conventional health authorities claim getting an annual flu shot is the best way to ward off influenza, the medical literature actually suggests vitamin D optimization may be a far more effective strategy, and the evidence for this goes back at least a decade.
Dr. John Cannell, founder of the Vitamin D Council, was one of the first to introduce the idea that vitamin D deficiency may actually be a causative factor in influenza. His hypothesis32Epidemic Influenza and Vitamin September 15, 2006 was initially published in the journal Epidemiology and Infection in 2006.33Epidemiology and Infection 2006 Dec;134(6):1129-40 It was subsequently followed up with another study published in the Virology Journal in 2008.34Virology Journal 2008; [5:29]
The following year, a large, nationally representative study35Archives of Internal Medicine 2009;169(4):384-390 confirmed that people with the lowest vitamin D levels indeed reported having significantly more colds or cases of the flu. Since then, a number of studies have come to similar conclusions.
Most recently, a scientific review36BMJ 2017; 356:i658337NPR February 16, 2017 published last year concluded that vitamin D supplementation boosts immunity and cuts rates of cold and flu. In all, 25 randomized controlled trials were included in the review, involving nearly 11,000 individuals from more than a dozen countries.
People with significant vitamin D deficiency (blood levels below 10 ng/mL), taking a vitamin D supplement reduced their risk of respiratory infections such as influenza by 50%. People with higher vitamin D levels also benefited, although not as greatly.
Overall, they reduced their risk by about 10%, which the researchers stated was about equal to the effect of flu vaccines. Coincidentally, 10% is the effectiveness rate of this year’s flu vaccine.38New England Journal of Medicine January 4, 2018; 378:7-9 The take-home message here is that vitamin D supplementation far exceeds the flu vaccine in terms of effectiveness, and the more deficient you are, the greater its protective effects.
Aside from vitamin D and quercetin, loading up on vitamins B1 and C may go a long way toward keeping you healthy through the flu season and beyond. Influenza has also been successfully treated with high-dose vitamin C,39Journal of Manipulative and Physiological Therapeutics 1999 Oct;22(8):530-3 and vitamin C also boosts the effectiveness of quercetin. Taking zinc lozenges at the first sign of a cold or flu can also be helpful.
Disclaimer: The entire contents of this website are based upon the opinions of Dr. Mercola, unless otherwise noted. Individual articles are based upon the opinions of the respective author, who retains copyright as marked.
The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of Dr. Mercola and his community. Dr. Mercola encourages you to make your own health care decisions based upon your research and in partnership with a qualified health care professional. The subscription fee being requested is for access to the articles and information posted on this site, and is not being paid for any individual medical advice.
If you are pregnant, nursing, taking medication, or have a medical condition, consult your health care professional before using products based on this content.
I didn’t know about so many benefits of quercetin. I work in a pharmacy, and I may say it is not such a popular supplement. People don’t know much about it or even didn’t hear about it. Those who know about its potency use it mainly for an immune boost before allergy season. I admit I didn’t realize that it could be a flu remedy. Thanks!
Is it safe for kids?
So I guess that is where the saying come from about the Apple a day keeps the doctor away. I didn’t realize that Quercetin was so good for our immune systems. I always take my flu vaccine but maybe this could be more effective in a healthier way. Interesting that it also helps with other diseases too. Though I guess one would need to take a supplement to make sure you are getting enough in your system for it to be effective.
This was a very interesting read. i have always believed that a healthy immune system is important. Taking medicines AFTER getting sick but not protecting and boosting your immune system just doesn’t make sense. I have been taking Vitamin D and Zinc but I didn’t know about Quercetin. It’s found in apples. I wonder if that’s why they say, “An apple a day keeps the doctor away.”
Is there an over the counter supplement to take to consume quercetin?