Onions, a source of quercetin

Quercetin: benefits, dosage & side-effects

Find out all about quercetin, including what it does, the benefits to taking it and how much you might need

Written by Beth Gibbons on March 17, 2019 Reviewed by Dr Sarah Schenker on March 20, 2019

Overview

Quercetin is a flavonoid, a natural chemical found in plants that has been shown to have a wide number of health benefits, including:1
  • reducing inflammation
  • relieving allergy symptoms
  • preventing infection

What is quercetin and what does it do?

In addition to functioning as a flavonoid, quercetin is also a phytoestrogen. This means that, in women, it can weakly bind to the body’s oestrogen receptors and mimic the effects of the hormone oestrogen on the body.2

Quercetin is available as a stand-alone supplement, in some multivitamin complexes or in combination with other nutrients, for example vitamin C, which has been shown to improve the gut’s absorption of quercetin.3

Which foods are a source of quercetin?

The body doesn’t make quercetin, so you have to get it from plants.4 The best food sources of quercetin include:5
  • onions
  • citrus fruits
  • green leafy vegetables, such as broccoli
  • seeds
  • olive oil
  • red grapes
  • berries
It’s also found in certain herbal remedies, such as St John’s wort and gingko biloba.6

Benefits of quercetin

What does quercetin do in the body?

Researchers are still investigating the effects of quercetin, but it’s thought to have the following effects on the body:

It can protect the body from free radicals – according to a 2008 study by Maastricht University, Netherlands, quercetin acts as a powerful antioxidant, helping to protect cells from damage by free radicals: unstable molecules that can age the body and may lead to serious health conditions.7

It can reduce inflammation – a 2016 Malysian study reported that quercetin hinders the action of inflammatory enzymes that would otherwise activate the immune system and cause inflammation.8 Indeed, an earlier study, published in 2012 by Iran’s Isfahan University of Medical Sciences, found that giving healthy male athletes a combination of quercetin and vitamin C for eight weeks reduced blood test markers for inflammation in the body.9

It may help reduce blood pressure - a meta-analysis of seven trials carried out by the USA’s University of Alabama in 2016 reported that taking 500mg or more of quercetin a day significantly reduced blood pressure levels. How this happens is still not fully understood but one theory is that quercetin may act on cells that control blood vessel contraction and dilation, improving blood flow.10

It may act as a natural antihistamine - quercetin stimulates the immune system and restricts the release of histamine in the body, relieving allergy symptoms in conditions like hay fever, according to a 2016 study published in Molecules.11

It could have anti-bacterial and anti-viral properties – a 2017 study in Microbiology Research found the phytonutrient could prevent the replication of bacteria, including E. Coli. In addition, a 2014 review by the University of Michigan, USA, suggested quercetin also blocked several respiratory viruses from replicating inside the body, including the common cold and flu.12

It can help regulate blood sugar – according to a 2019 review of studies, published in Phytotherapy Research, taking 500mg or more of quercetin daily for at least eight weeks reduced blood glucose levels in people with metabolic syndrome, who have an increased risk of developing heart disease, stroke and diabetes.13

Dosage

How much quercetin is safe to take?

You should be able to get plenty of quercetin from eating a diet rich in fruit and vegetables.14 However, if you prefer to take a supplement, doses of 500-1000mg a day are considered safe.15 Don’t take quercetin if you are pregnant or breast-feeding as there isn’t enough evidence to show it’s safe. Quercetin can interact with certain medications, so speak to your GP before taking quercetin supplements.16

Side-effects

What are the side-effects of taking quercetin?

Quercetin is considered safe to take for healthy people.17 However, it can have side- effects including:18
  • headaches
  • nausea
  • numbness
  • tingling
Shop Supplements Advice is for information only and should not replace medical care. Please check with your GP before trying any remedies

Sources

1. Beth Sissons. Medical News Today. What are the benefits of quercetin? 2. ScienceDirect. Quercetin 3. Kinker B, Comstock AT, Sajjan US. Quercetin: A Promising Treatment for the Common Cold

4. As Source 1

5. Anand David AV, Arulmoli R, Parasuraman S. Overviews of Biological Importance of Quercetin: A Bioactive Flavonoid

6. As Source 1

7. Boots AW, Haenen GR, Bast A. Health effects of quercetin: from antioxidant to nutraceutical

8. As Source 5

9. Askari G, et al. The effect of quercetin supplementation on selected markers of inflammation and oxidative stress 10. Serban M-C, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials 11. Micek J, et al. Quercetin and Its Anti-Allergic Immune Response

12. As Source 3

13. Ostadmohammadi V, et al. Effects of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders: A systematic review and meta-analysis of randomized controlled trials

14. As Source 1
15. As Source 1

16. John P. Cunha. RxList. Quercetin

17. As Source 1

18. As Source 16
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