We really want our immune systems to be working well at this time of year. The worst of COVID-19 may be behind us, but it has not disappeared completely. Add flu and the common cold into the mix, and we need our best defences at the ready.

The immune system needs many vitamins and minerals for it to function optimally, but other compounds in plants, known as flavonoids, have a role to play. One such compound is quercetin.

Quercetin hit the headlines during the pandemic, when quercetin supplementation was found in studies to reduce the severity and longevity of the symptoms of COVID-19. In fact, some medical practitioners were prescribing quercetin to patients alongside pharmaceuticals. So why is it so useful for our immune system?

Antioxidant Power

Quercetin supports immunity via various mechanisms. Firstly, it is an antioxidant. This is useful because infections result in oxidative stress, which can damage body tissues and worsen symptoms. In animal studies, quercetin proved to be a powerful antioxidant and reduced the oxidative stress caused by the flu virus and hence protected the lungs from damage. Chronic inflammation goes hand in hand with oxidative stress and can drive many chronic diseases – quercetin works as an anti-inflammatory.


Quercetin is often used therapeutically to help with symptoms of asthma, hay fever and allergic reactions. All these conditions involve histamine, which causes redness, swelling, and symptoms such as watery eyes and constricted airways. Quercetin inhibits the release of histamine from white blood cells and so reduces the reaction. Consumption of apples (a concentrated source of quercetin) has been linked to a reduction of asthmatic symptoms in children. One study found that women who ate apples during pregnancy bore children with significantly lower rates of asthma and wheezing at 5 years old than the children of pregnant women who did not eat apples. No associations were found with fruit and vegetables in general. This does not, of course, prove that quercetin was responsible for the reduction in symptoms, but it is an interesting finding.

Quercetin has, however, been studied in more depth as an isolated compound in animals. In one rodent study, anaphylactic reactions to peanuts were completely resolved after 4 weeks of being fed quercetin daily. Note that there is no comparable evidence in humans yet, so please do not start experimenting if you or someone close to you has a peanut allergy.


Of particular note during the winter, quercetin is strongly antiviral. Unlike bacteria, viruses are not cellular beings; they exist in our bodies by entering our cells and using them to reproduce. Quercetin stops the viral strains getting into cells and reduces the replication of cells that are already infected. Evidence of quercetin’s antiviral activity began to emerge as long ago as 1949. Since then, it has been found to have activity against viruses including herpes simplex viruses, polio, flu, viral hepatitis and rhinovirus (the common cold).

In terms of COVID-19, more studies are needed, but early results were promising enough for medical practitioners to take note. When quercetin was added to conventional medical treatment (such as paracetamol) for people with mild COVID, they tested negative more quickly and had milder symptoms than those who were not given quercetin. Due to its anti-inflammatory actions, quercetin is also attracting attention as a candidate for calming the “cytokine storm”, a situation where the immune system goes into overdrive, that we know can be such a serious consequence of COVID-19. Because it is an antioxidant and can also reduce lung damage, it is a good all-round support for this virus that can affect the lungs so seriously.

A Team Player

Quercetin works in synergy with other immune-supporting nutrients, including vitamins C, D, E and zinc. For example, zinc can damage SARS-CoV-2’s viral replication mechanisms when zinc is present in cells infected by the virus. Quercetin is a zinc ionophore, which means that it helps zinc enter into cells so that it can carry out its antiviral activity. Vitamin C, as well as being a well-known nutrient itself for supporting immunity, increases how much quercetin we can absorb via our gut.

Dietary Quercetin

Most, if not all, of the studies into the effects of quercetin on the immune system have used it in supplement form. However, because quercetin is so widely distributed throughout the plant kingdom, you can get useful amounts from your diet. Small dietary changes can increase the amount of quercetin you consume. The main food sources of quercetin include:

  • onions, especially red
  • red apples
  • cranberries
  • dark cherries and berries, such as blueberries
  • dark green leafy vegetables
  • cruciferous vegetables such as kale and broccoli
  • citrus fruits
  • capers
  • celery
  • cocoa
  • tea (black, green and chamomile)

Red apples contain much more quercetin than green apples, as the pigment is a sign of quercetin being present. Therefore, when choosing an apple, choose the rosiest for you can find for more quercetin! For the same reason, red onions and spring onions contain more quercetin than brown or white onions. Frying, roasting or baking onions increases the quercetin content. Interestingly, onion skins can contain almost 50 times more quercetin than the flesh. If you really want to increase the quercetin in your diet, you can save your onion skins and put them in a herb bag which you can add to stews and soups while cooking. Just remember to remove the bag before serving!

Eating a diet based on a wide variety of fresh plant foods should ensure that you consume plenty of quercetin. Through the genius of Mother Nature, quercetin in food form will also come packaged with other immune supportive nutrients such as vitamin C. These nutrients will in turn help your immune system to defend you against whatever viruses are going around.

For more information on our seminar/webinar “Supporting the Immune System Naturally” please get in touch at info@vitalhealthnutrition.co.uk 

With thanks to Joan Faria for contributing this blog.

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