Dr Sarah Brewer says: 'Apples and red onions are the main sources of quercetin in an average diet, but they only provide tiny amounts of between 0.13mg and 1.31 mg quercetin per 100g. Significantly higher doses are needed to obtain the immune benefits seen in clinical trials. While quercetin has been available in supplement form for some time, its use among athletes is not widespread despite its reported benefits.’
What does quercetin do?
Quercetin is an antioxidant, stimulates immunity, reduces allergic responses (including the release of histamine) and has antiviral activity. Most quercetin research has focussed on the supplements immune boosting benefits in elite athletes during periods of intensive training.
Quercetin lowers the production of inflammatory chemicals (cytokines such as interleukin IL-4) and inhibits enzymes (eg lipoxygenase) involved in triggering inflammation. Research involving trained cyclists found that when taken alongside Green Tea extracts and 400mg omega-3 fish oil (EPA and DHA) which boost the absorption of quercetin into the body, this combination helped to reduce muscle damage and inflammation after intense exercise.
Prolonged exercise and heavy training can reduce immunity and increase the risk of infections – especially upper respiratory viral colds. In a study involving 40 trained male cyclists, half took 1000 mg quercetin and half took placebo for 3 weeks before, during, and for 2 weeks after a three-day period of intensive cycling activity. Among those taking quercetin, only 1 out of 20 developed a common cold in the two weeks following the intense exercise period, compared with 9 out of 20 in the group taking a placebo. This suggests that quercetin significantly reduced the incidence of stress-related upper respiratory tract infections after intense exercise.
Quercetin can reduce allergic reactions in several different ways, including the suppression of histamine release, and helping to restore normal immune cell responses away from allergic reactions towards fighting infections. It is particularly active within the skin and may help to improve atopic dermatitis, which is an allergic, inflammatory skin condition.
Quercetin is currently under investigation for its ability to reduce inflammation of the cornea and to improve dry eyes.
Getting quercetin from your diet
The richest food sources of quercetin are black elderberries, which contain around 42mg/100g quercetin, and capers which supply 32mg/100g, but these are not often included in the diet in significant amounts. The main food sources of quercetin in a typical western way of eating are apples and onions. Quercetin is also found in broccoli, berries, grapes, tea, tomatoes and dark chocolate. Quercetin is present in some medicinal plants such as St John’s Wort and Ginkgo biloba and may contribute to their health benefits. The dietary intake of all flavonols, including quercetin, is estimated at around 13mg per day.
Quercetin is generally considered safe. Do not take during pregnancy or breast feeding. As quercetin may interact with some prescribed medicines, always check with a doctor or pharmacist before taking if you are receiving medical treatment.
Studies investigating immune benefits have used a daily intake of 1,000mg (1 gram) quercetin, beginning 2 to 4 weeks before undertaking intense exercise. For those training regularly, a daily intake is advisable. To aid quercetin’s absorption it is best taken alongside green tea or omega 3 fatty acids.