QUERCETIN

Nutritional characteristics

Quercetin is widely distributed in foods but is found exclusively in foods of plant origin. The richest food sources are:

• • • red onion
    • Broccoli
    • tomatoes
    • Asparagus
    • Apple
    • wild berries
    • grape
    • citrus fruits
    • cherries
    • tea
    • coffee
    • Capers

Quercetin is also found in some botanicals, including Ginkgo biloba, Hypericum perforatum , Sambucus canadensis and Sophora Japonica.

Once introduced with food, quercetin is absorbed in the intestine, after which it is metabolized in various organs including the small intestine, colon, liver and kidneys. The metabolites formed are partly distributed in the lungs, liver, kidneys and partly are eliminated in the urine.  

Pharmacological properties

The typical characteristic of quercetin is the presence, in its chemical structure, of different phenolic groups that allow the compound to function as a powerful antioxidant:

• • • It works as a scavenger of oxygen and nitrogen radicals, i.e. it is able to transform these species into non-radical compounds without toxicity
    • activates the transcription factor Nrf2 which promotes the expression of a wide range of antioxidant genes
    • increases the expression of PON2, an enzyme that protects cells from oxidative damage
    • induces the synthesis of glutathione, a powerful antioxidant
    • increases the expression of antioxidant enzymes including superoxide dismutase, catalase, glutathione peroxidase and heme oxygenase

Many in vivo and in vitro studies have shown that querticin, by targeting many intracellular signaling kinases, phosphatases, enzymes and membrane proteins, also possesses potent anti-inflammatory and immunostimulatory capabilities:

• • • inhibits the production of TNF-alpha by macrophages and the consequent inflammatory cascade
    • inhibits the production of enzymes that produce inflammation: cyclooxygenase (COX) and lipoxygenase (LOX)
    • inhibits the release of proinflammatory cytokines
    • inhibits nitric oxide production
    • induces the production of INF-gamma by Th-1 cells and inhibits the production of IL-4 by Th-2 cells

Thanks to its antioxidant and anti-inflammatory properties, quercetin exerts several biological effects:

• • • useful in cases of myocardial cell damage and protects the heart from cardiac dysfunction
    • useful in cases of neurodegenerative diseases but also anxiety and depression
    • reduces the toxicity of certain pharmacological and non-pharmacological agents (e.g. cyclophosphamide, cisplatin, ciprofloxacin, manganese, cadmium, arsenic)
    • useful in cases of idiopathic pulmonary fibrosis and acute lung damage
    • useful in the prevention of type II diabetes
    • useful in cases of osteoarthritis

Other functions of quercetin:

• • • has anti-allergic properties: inhibits the release of histamine from mast cells
    • enhances the relaxation of blood vessels and increases their delay, thus exerting an antihypertensive action

Quercetin helps to inhibit the formation of histamines and prostaglandins by reducing inflammation and at the same time exerts a direct and indirect antioxidant action. It will benefit all those diseases dictated by aging and oxidative stress, such as allergies, insulin resistance, arteriosclerosis, arthritis, Alzheimer's disease, psoriasis. The activity on the cardiovascular system is well known, reducing the possibility of bleeding, thrombi, edema, therefore indicated for those with venous insufficiency, hemorrhoids, night cramps, heaviness and swelling in the legs.  

Bioavailability

Quercetin is poorly bioavailable due to reduced absorption in the intestine (ranging from 17 to 50%), rapid metabolism and excretion. All this leads to a short half-life of quercetin metabolites in the blood.  

Contraindications and side effects

Oral quercetin intake in humans appears to be well tolerated and to date only a very low incidence of adverse effects has been observed. These are represented by a mild headache, stomach pain and nausea if quercetin is taken without food. Animal and human studies have shown that quercetin can interfere with several drugs by increasing their bioavailability (and thus increasing their effects, including toxic ones) or decreasing it (and thus decreasing the pharmacological effect of the drug). The drugs with which it interferes are midazolam, taninolol, cyclosporine, parvastatin, fexofenadine, oral anticoagulants.  

      Bibliography:

• Oxid Med Cell Longev. 2016. Mechanisms of Neuroprotection by Quercetin: Counteracting Oxidative Stress and More. Costa LG, Garrick JM, Roquè PJ, Pellacani C.
• Molecules. 2019 Mar 21. Antioxidant Activities of Quercetin and Its Complexes for Medicinal Application. Xu D, Hu MJ, Wang YQ, Cui YL.
• Nutrients. 2016 Mar 15. Quercetin, Inflammation and Immunity.Li Y, Yao J, Han C, Yang J, Chaudhry MT, Wang S, Liu H, Yin Y.
• Food Chem Toxicol. 2020 Jan. Quercetin as a Lyn kinase inhibitor inhibits IgE-mediated allergic conjunctivitis. Ding Y, Li C, Zhang Y, Ma P, Zhao T, Che D, Cao J, Wang J, Liu R, Zhang T, He L.
• Molecules. 2017 Jan 29. Actions of Quercetin, a Polyphenol, on Blood Pressure. Marunaka Y, Marunaka R, Sun H, Yamamoto T, Kanamura N, Inui T, Taruno A.
• Biomolecules. 2019 May 6. Fisetin and Quercetin: Promising Flavonoids with Chemopreventive Potential.Kashyap D, Garg VK, Tuli HS, Yerer MB, Sak K, Sharma AK, Kumar M, Aggarwal V, Sandhu SS.
• Nutrients. 2019 Sep 25. Dietary Quercetin and Kaempferol: Bioavailability and Potential Cardiovascular-Related Bioactivity in Humans.Dabeek WM, Marra MV.
• Mol Nutr Food Res. 2018 Jan. Safety Aspects of the Use of Quercetin as a Dietary Supplement. Andres S, Pevny S, Ziegenhagen R, Bakhiya N, Schäfer B, Hirsch-Ernst KI, Lampen A.
• Alexander Victor Anand David, Radhakrishnan Arulmoli, and Subramani Parasuraman. Overviews of Biological Importance of Quercetin: A Bioactive Flavonoid. Pharmacogn Rev. 2016 Jul-Dec; 10(20): 84–89.