Quercetin Explained

It’s apple and onion season, which always makes me think of quercetin. Why? Because this bioflavonoid is found in abundance in fresh apples and onions, as well as other fruits and vegetables. I only wish there were fresh apples and onions around in early spring when my seasonal allergies hit hard, given quercetin’s ability to help control allergy symptoms.

What else is quercetin prized for? Let’s take an in-depth look at this natural antioxidant and how it helps support immune function and cardiovascular health, among other things.

What is quercetin

What is quercetin and where can you get it?


Quercetin is a lipid-soluble, natural antioxidant. It is classed as a bioflavonoid, which is a type of plant pigment largely responsible for the bright colours of many fruits and vegetables. These bioflavonoids were once referred to as vitamin P because of their presence in paprika, and due to the fact that they have vitamin-like properties. Indeed, quercetin works especially well alongside vitamin C and other nutrients to support good health.

Quercetin is, as noted, particularly abundant in apples and onions, but it is also found in other fruits, vegetables, herbs and grains. As a lipid-soluble nutrient, though, it can be hard to absorb without the presence of dietary fat (more on this below).

Quercetin is an antioxidant that supports cardiovascular health

Like other bioflavonoids, quercetin possesses strong antioxidant activity, helping to fight oxidative damage and the negative symptoms of aging. This antioxidant activity also helps support cardiovascular health as quercetin is used in herbal medicine as a capillary protectant, meaning it helps to look after small blood vessels.

Quercetin has been associated with a range of activities that may support the health of arteries, including:[1]

  • Inhibiting the oxidation of low-density lipoprotein (LDL) cholesterol
  • Reducing adhesion molecules and inflammatory markers
  • Protecting nitric oxide and endothelial function during oxidative stress
  • Supporting normal platelet activity.

Together, these activities help the body’s blood vessels stay flexible and responsive, dilating and contracting as needed to maintain healthy blood flow.

Quercetin has antiviral activities

Quercetin’s potential antiviral properties have been investigated in numerous studies and preliminary research suggests that the bioflavonoid might inhibit the ability of some influenza viruses to enter cells, which would help to prevent infection.[2] Quercetin’s possible antiviral effects appear to be related to an ability to help prevent a virus from replicating itself and to cause infection. It does this by inhibiting polymerases, proteases, and reverse transcriptase, suppressing DNA gyrase, and binding viral capsid proteins.[3]

There is also evidence that vitamin C and quercetin have a synergistic antiviral action because of their overlapping immunomodulatory properties and because vitamin C can ‘recycle’ quercetin, increasing its efficacy.[5] In return, quercetin and other bioflavonoids help vitamin C to be used effectively by the body and increase the effectiveness of vitamin C by slowing down its breakdown.[4] And, because quercetin is fat-soluble, it helps provide benefits in lipid membranes where vitamin C, which is water-soluble, can’t reach.


Quercetin for allergy control

Quercetin is also well-regarded for its ability to help control allergy symptoms. When my spring dogwood allergies kick in, quercetin is the first thing I reach for. And, if I remember, I start taking quercetin a few weeks in advance of the first dogwood flowers, so my body is ready to keep those histamine levels in check.

A 2016 review of quercetin noted:[5]

“Quercetin is known for its antioxidant activity in [free] radical scavenging and anti-allergic properties characterized by stimulation of immune system, , inhibition of histamine release, [and] decrease in pro-inflammatory cytokines.”

Quercetin can also support the balance of T-helper cells and antigen-specific antibodies, inhibits the production and release of histamine and appears to stabilize the cell membranes of mast cells (influential immune cells involved in the development of allergic responses).[6] These immune modulating and anti-inflammatory actions contribute to quercetin’s usefulness against allergic rhinitis, seasonal allergies, and other allergic responses.[1,7]


Why quercetin absorption matters

The bad news is that quercetin is pretty poorly absorbed in the standard form used in most supplements, largely because it is fat-soluble. This means that the clinical efficacy of quercetin is quite limited, unless care is taken to provide quercetin in an absorbable form.

EMIQ or Enzymatically Modified Isoquercitrin is a highly bioavailable soluble quercetin compound created using a natural enzyme process for superior bioavailability. Natural Factors Bioactive Quercetin EMIQ® reaches peak level in the bloodstream in just 15 minutes for almost instantaneous antioxidant support!

Natural Factors Quercetin LipoMicel Matrix is another great option for absorbable quercetin as it provides an advanced form of the antioxidant with 10 times higher absorption than standard forms. The unique patent-pending LipoMicel technology helps overcome low bioavailability by creating a liquid micelle matrix that disperses quercetin into tiny micro-droplets that enhance absorption. This supplement provides 250 mg of quercetin in each easy-to-swallow liquid softgel and is a great choice for anyone suffering from conditions related to constricted blood vessels or looking for additional immune and antioxidant support.

Natural Factors Quercetin 500 mg provides an even greater dose of concentrated quercetin and is suitable for vegetarians and vegans. And Natural Factors C Extra + Quercetin offers 500 mg of vitamin C and 250 mg of quercetin bioflavonoids in every softgel to provide antioxidant protection against free radicals, help with collagen formation to maintain healthy bones, teeth, gums, cartilage, and skin, and to help protect capillaries while supporting the body’s immune system to keep allergies in check and to fight off viruses. Whether prevention is on your mind, or you are looking for extra antioxidant and immune support, Natural Factors has a form of quercetin sure to meet your needs!





[1] Patel RV, Mistry BM, Shinde SK, et al. Therapeutic potential of quercetin as a cardiovascular agent. Eur J Med Chem. 2018;155, 889-904.

[2] Wu W, Li R, Li X, et al. Quercetin as an Antiviral Agent Inhibits Influenza A Virus (IAV) Entry. Viruses, 2015;8(1), 6.

[3] Colunga Biancatelli R, Berrill M, Catravas JD, & Marik PE. Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19). Frontiers in immunology, 2020;11, 1451.

[4] Colunga Biancatelli RML, Berrill M, Catravas JD, Marik PE. Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19). Front Immunol. 2020;11:1451.

[5] Mlcek J, Jurikova T, Skrovankova S, & Sochor J. Quercetin and Its Anti-Allergic Immune Response. Molecules (Basel, Switzerland), 2016;21(5), 623.

[6] Thornhill SM, & Kelly AM. Natural treatment of perennial allergic rhinitis. Alternative medicine review: a journal of clinical therapeutic, 2000;5(5), 448–454.

[7] Juríková T, Mlček J, Sochor J, Hegedűsová A. Polyphenols and their mechanism of action in allergic immune response. Glob J Allergy, 2015;1, 037–039.