What is Quercetin?

Ingredient Quercetin

Quercetin is a substance of very high antioxidant power which is credited with remarkable medicinal properties, which in essence translate into delaying the onset of ageing symptoms by cushioning the deterioration of cellular structures. Consequently, we can say that it is a vital nutrient to maintain health.

It belongs to the family of flavonoids, which are a chemical variant of polyphenols. Flavonoids are a very extensive family of substances classified as phytonutrients (given their strictly vegetable origin), of which more than 600 have already been catalogued, belonging to one of these subfamilies: flavonols, isoflavonoids, neoflavonoids and anthocyanins, among those kaempferol, catechins and anthocyanins stand out above the rest.

One of its most unique features is the variegated variety of pigments that are distributed among the different molecules so that we can find from intense blue tones in blueberries to the characteristic red raspberries through the greenish yellow of apples or onions.

As a group, flavonoids in general and quercetin in particular display a great biological activity, primarily in the tissues of plants and, once in the trophic chain, in animals and people after being consumed by them.

In plants, they control the levels of specific substances that regulate the growth and specialization of cells. They also have antifungal and antibacterial action, and it is proven that their pigmenting power attracts bees, which is stimulating for the development of pollination.

But if an attribute is consubstantial to the characteristics of flavonoids is to be part of the select world of antioxidants, whose role in the environment of the smallest unit endowed with a life of its own, the cell, focuses on preventing the triggering of the cascade of formation of oxygen free radicals, highly harmful elements for the integrity of cell membranes and precursors of tissue degeneration and even the appearance of neoplasms or cancers.

Table of contents

Foods rich in Quercetin

Being one of the pigments that confer colour to the different parts of the plant anatomy, it is easy to understand that the presence of quercetin is very common among foods that stand out for any of the varied tones that can bring, such as red wine, onions, apples, green tea, berries, Ginkgo Biloba or St. John's wort. But we have selected some of them that can affect the daily diet with greater contributions.


Both black and green tea contain relatively appreciable amounts of quercetin, we could even mention a form of red tea known as rooibos, although their levels remain in more modest figures. Of all of them, it would be black tea in its decaffeinated version that would head the ranking, followed by green tea and common black. A peculiarity about green is its richness in another antioxidant, the catechin, also belonging to the family of flavonoids and reputed anticancer, tripling the content in this compared to dry black tea.


Perhaps it is the most familiar among the food sources of quercetin, at the same time, leading the list of these. Perhaps it is partly attributable to its abundance in quercetin the popularity of the onion as an emblematically healthy food, traditionally awarded with goodness related to blood circulation and the maintenance of cellular vitality, the great enemy of ageing. Its nutritional properties do not end there, completed with vitamin C, potassium, soluble fibre, iron and calcium, the latter important to increase the mineral density of bone tissue, key in the development of osteoporosis.

We must bear in mind that the biological quality of this bulb in relation to quercetin is not only due to the amount of flavonoid contained in its core, but also to its intrinsic bioavailability, since according to data extracted from a study carried out at the Agricultural University of Wageningen (The Netherlands), it is absorbed by the body three times more efficiently than that contained by apples.

It can be estimated that an onion of about one hundred grams contains between 25 and 50 milligrams of quercetin, an oscillation that is a function of the acidity of the soil, among other factors of less importance.

Only one obstacle separates the onion from excellence as a dietary source of quercetin: once harvested, during storage, there is a 25% reduction in its quercetin content if this is prolonged more than a week before consumption.


There are few fruits that deserve being taken into account when talking about foods rich in quercetin, but among all of them, undoubtedly, the apple stands out, whose consumption raw is always more interesting to these effects.

Its skin is the part that mostly treasures its richness in antioxidants, differing the content between varieties with a clear advantage for the Red Delicious, Red Ida and Northern Spy varieties.

A renowned doctor, known as Dr. Weil, of the Environmental Working Group, appeals to the regular consumption of apples as a nutritional measure to prevent Alzheimer's disease thanks to quercetin.


Generally speaking, it is considered one of the cereals of the highest nutritional quality, and one of the reasons lies in its abundance in quercetin. It is increasingly common to see it integrating diets designed to reduce the caloric level, curbing the excessive appetite for the satiating condition that gives it its high fibrous component and promotes rejuvenation.


The prestige acquired by this allium as one of the most potent natural antibiotics is, in large part, due to the therapeutic virtues of quercetin.

Foods rich in Quercetin

To such extent, experts insist on the nutritional excellence that represents eating at least one raw garlic a day.


They are one of the vegetables that most regularly appears in the lists of foods rich in some substance of high biological value, being quercetin among them. Combined with onion and garlic, they make a wonderful recipe when promoting any of the healthy functions of both ingredients.


According to the United States Department of Agriculture (USDA), one of the world's leading food organisms, capers have one of the highest natural concentrations of this antioxidant. As an interesting suggestion for its consumption, to say that the conserve of its flower buds in brine contains 173 milligrams of quercetin per hundred grams.

Red and purple fruits

Fruits with a red, purple and dark blue colour contain significant levels of quercetin, especially blueberries, containing 15 milligrams per 100g, followed by black plums, with 12.5 milligrams in the same amount.

Chokeberries, currants and cherries, among others, are also very acceptable sources of quercetin.

It is preferable to eat these fruits raw, because, as will be seen later, contact with heat and water is very damaging for this active principle.

Biological functions of Quercetin and its contribution to maintaining health

A balanced and selectively enriched nutrition is one of the best weapons available to protect health. Numerous research studies come to confirm that a regular and abundant consumption of fruits and vegetables highlighted by its quercetin content decreases the risk of being affected by some diseases and, consequently, increases life expectancy.

The scientific literature is full of references to quercetin that position it as one of the natural substances of greatest biological value known. However, the ability of the organism to assimilate this compound reduces its bioavailability somewhat within it.

Benefits and Properties of Quercetin

Before approaching in depth its implications within the organism, we can advance a review of the main aptitudes of this flavonoid:

    • One of its main objectives is the control of deviations in the functioning of the heart and the circulatory system: arteriosclerosis, hypercholesterolemia, thromboembolism and hypertension, fundamentally. A regular supply of quercetin behaves as a balance factor of blood pressure in a natural way, contributing to the stabilization of systolic, diastolic and mean arterial pressures. In relation to this function, it reduces the risk of strokes such as stroke or cerebral infarction. 
    • Anti-inflammatory in clinical conditions linked to the urinary system, such as inflammation of the prostate and interstitial cystitis (affecting the bladder), acting in turn as a palliative for the symptoms caused by the herpes virus. Additionally, we can indicate that, as unproven indications, it could be effective in treating chronic pelvic pain syndrome.
Benefits and Properties of Quercetin
  • Its antispasmodic and anti-inflammatory properties are not depleted in the excretory system but are also manifested, albeit with less intensity, in joints (arthritis) and intestines (colitis and Crohn's disease)
  • Regarding its effects on the respiratory tract, a laboratory study completed in 2013 has attributed a bronchodilator effect to its power to inhibit the enzyme that catalyses the mechanisms of inflammation and narrowing of the bronchi; This, ultimately, results in the relaxation of the smooth muscle fibers of the bronchial wall.
  • Antidiabetic action: it is basically characterized by buffering the typical renal damage caused by hyperglycemia. On the other hand, recent research conducted in experimental animals has revealed that quercetin helps reduce blood glucose levels.
  • It supposes a natural barrier against infectious diseases: all foods rich in quercetin are attributed the nature of natural antibiotics, as they constitute a very remarkable stimulus of the immune system.
  • Antioxidant power: responsible for what may be the feature that covers more functions, it slows down cell aging and, consequently, the progressive deterioration of the organism. Within this conglomerate, its anticarcinogenic action shines through, since it stimulates the process known as apoptosis, which is the death of tumor cells. Let us not forget that the root of the genesis of malignant tumors is the cascade reaction of free radical formation in cell membranes, which is why its antioxidant power helps in the prevention of cancers, especially those of prostate, breast, colon and skin.
  • Helps to fight the unpleasant symptoms of the allergic reactions, as are, sneezing, tearing, urticaria or eczema. In addition, it is a good alternative for asthmatics when facing acute episodes of asthma, as well as those who suffer from hay fever. The scientific explanation is based on the fact that quercetin has a considerable antihistamine effect. Several laboratory tests have shown what is its mechanism of action: acts on mast cells, which are the cells responsible for the release of histamine, the substance that causes the most characteristic symptoms of allergic reactions. Within those cells, it neutralizes the enzymes that catalyze its synthesis and the consequence is that it inhibits its liberation. For this reason, it is widely recommended in natural treatments against these conditions. Within this field, the effectiveness of quercetin against contact dermatitis and photosensitivity, both characterized by redness and quite rebellious to drug treatments, has also been studied. And the results seem to be quite promising.
  • Strengthening of the hair: this flavonoid stimulates the nutrition of the scalp and hair from the inside of the organism, densifying it and preventing the cutaneous flaking that is manifested in the form of dandruff.
  • Regarding stress, the main metabolic change directly related to stress is the increase in the production of cortisol by the adrenal glands. This is the hormone that triggers the defense mechanism that according to the Austrian psychiatrist H. Selye was called the fight or flight syndrome. Although it is considered a physiological reaction, sustained stress can generate a plateau in the graph of blood level of cortisol with harmful consequences, such as protein degradation originated in the muscles. Well, this flavonoid combats these effects by participating in the neutralization of the key enzyme for that release of cortisol occurs.
  • Quercetin is included among the substances recommended to control sleep. A good initiative for those who can not get to sleep can be to enrich their diet with plant products well supplied with this flavonoid, as indicated above, as it is known to help optimize sleep patterns and prevent insomnia. This offers an interesting side effect in reducing weight if such a goal is desired, since not sleeping properly makes the person more susceptible to weight gain.
  • Also noted are its properties in relation to menstruation: among the most frequent problems that the menstrual cycle entails are colic-type pains and spasms. These symptoms can be mitigated on the basis of a diet well supplied with quercetin, not only during the days of the period but in a continuous manner.
  • Increased oxidative stress and inflammatory phenomena occur in athletes subjected to heavy stress loads. The ingestion of quercetin compensates for these negative effects by minimizing the chemical markers of inflammation and the generation of free radicals (closely related to the antioxidant action). In addition, it offers a defensive advantage in this type of people against respiratory infections (common cold and flu, for example), to which they are especially vulnerable after intense physical exercise.
  • Anti-ageing effect. In this regard we must highlight for example that in Russia it is elevated to the category of anti-ageing substance per se, to the point of being known as a Russian rejuvenator. This suggestive effect is derived from the convergence of two pathways: on the one hand, reducing the susceptibility to a good catalogue of chronic and degenerative diseases (among which we can cite Alzheimer's); on the other, by its direct effect, experimentally demonstrated, on life expectancy as a result of producing the activation of a few genes closely linked to this vital characteristic (it is unknown with certainty if the results obtained with lower organisms can be, as such, extrapolated to the human being).
  • Quercetin can be used to support therapy for sarcoidosis, a serious condition that usually involves inflammation of lung tissue, skin, and lymph nodes. And as a catch-all, we can list some other indications, such as gout, pancreatitis, cataracts, duodenal ulcers, some viral infections such as herpes Zoster and chronic fatigue syndrome.

The other side of Quercetin: its possible side effects and contraindications

Among the diverse list of antioxidants, it should be noted as common denominator to almost all of them that their potential health risks are very scarce, but also that, at the same time, some caution is required in their consumption as supplements (of which we will talk about later).

Gastric discomfort, headache, tingling and a certain feeling of dizziness have been reported by some patients.

In this way, it can be stated without hesitation that quercetin is a potentially safe substance consumed orally and for short periods of time (say, one gram dose divided into two daily doses for three months.)

However, with the appearance of any adverse reaction, however the severeness, it is prudent to suspend the intake immediately and consult a physician.

But it can not be said in the same terms if there is an ingestion at significantly excessive doses, in which case the induction of changes in the genetic material (DNA) of lower organisms during observations has been revealed in laboratory tests, according to an article published in the magazine "Internet Journal of Medical Update" a decade ago. And in the field of daily clinical, overdose of quercetin have been identified as a cause of chronic renal failure.

Contraindications and interactions with other substances

In the form of food contributions, quercetin has no associated profiles of individuals of any kind in which its consumption is discouraged.

But in the case of dietary supplements, a cautious approach should be taken in the case of pregnant and lactating women and being treated with some antibiotics (especially ciprofloxacin and erythromycin) or substances related to liver function (for example, nifedipine or herbal products designed to improve the functionality of the liver).

More specifically, it is known that platelet aggregation can become difficult, altering the time of blood coagulation. It can intensify the side effects of some drugs used in chemotherapy, such as bisulfan or cisplatin. And it can also interfere with the action of cyclosporine used to mitigate the effects of the immune system on transplants and thus avoid rejection.

Finally, it is necessary to add that papain and bromelain, protein substances contained in papaya and pineapple, respectively, can increase its intestinal absorption.

Quercetin Supplements and their most important features

In the pharmaceutical industry of many countries Quercetin is a raw material used to elaborate supplements to increase the protective action of the blood vessels against arteriosclerosis, preventing the LDL cholesterol molecule, its main precursor, from oxidation and adhering to the arterial walls. It also becomes part of multiple multivitamin formulations and various natural remedies.

According to “Vitamin Retailer Magazine”, a prestigious publication of the dietary supplementation industry, unconsciously the majority of the population consumes between 5 and 70 mg of quercetin a day through food. On that basis, the specialists of that magazine have argued in favour of an additional intake of 400 mg two or three times a day.

Experts suggest that quercetin should be taken a quarter of an hour or twenty minutes before meals. The market provides various physical presentations, such as tablets, capsules and gels, with the symbolic exception of injectable preparations for intravenous or intramuscular administration.

Apart from these generic questions about its posology, it is convenient to discern between the objectives that can be reached depending on the dose chosen. At the rate of one gram per day, in several doses, it is endowed with interesting anti-inflammatory properties for preventing the synthesis of the so-called NF-kappaB, a protein that plays a decisive role in the coding performed by certain genes on specific substances, cytokines, which they are the main drivers of the phenomena of inflammation in tissues.

However, all sources of quercetin are not equivalent. There is a special type, called quercetin chalcone, which has strong advocates in favour of a more effective absorption in the intestinal tract. On the other hand, the only GRAS (Generally Recognized As Safe by the North American FDA) source is that obtained from the seeds and pods of an indigenous shrub from Brazil listed among the legumes. This form contains 99.5% of quercetin anhydrous while lacking metals (the main contaminating hazard) and its tissue bioavailability is well above average. In general, its assimilation occurs in the duodenum and colon, giving rise to peaks in blood levels after five to eight hours after absorption and remaining in significant figures one day or a day and a half.

A tandem of excellence: Quercetin and Vitamin C

The key to the powerful benefits of a combination of quercetin and vitamin C lies, on the one hand, in the enhancement in the regeneration of quercetin and the increase in the bioavailability of this, and on the other hand, in increasing the absorption of vitamin C and slow down its elimination.

This combination has widely recognized effects on reducing cell injury and inflammation. Regarding the latter, different research trials have found:

  • A decrease close to 50% of a parameter that behaves as a marker of the inflammatory reaction, C-reactive protein.
  • A 60% decrease in another inflammatory marker, interleukin 6.1

In fact, the high levels of the latter and other cytokines can lead to serious health disorders, as severe as cerebrovascular diseases, heart disease and Alzheimer's disease. Not in vain, we have knowledge of a very revealing fact: the predictive capacity of nonspecific mortality that the blood levels of interleukin-6 and C-reactive protein have. A mixture of high figures in both parameters should be taken into consideration as a warning about the existence of a situation of significant risk of death.

From all this, the interest of a combined supplementation regime of quercetin and vitamin C can be extracted as a conclusion

Both the HSN line of supplements and the Vitaminalia brand with its excellent preparations stand out for their great benefits.

Synergy between Resveratrol and Quercetin

Both natural antioxidants integrate a set of what we could call essential nutrients. Used in combination they can become an effective procedure to prevent, by blocking the cascade of oxygen free radicals, prevent cardiovascular and cerebrovascular diseases and strengthen the organism against cancer in a good part of its modalities. Resveratrol can be obtained in the form of capsules and tablets.

Quercetin and Probiotics

We can not finish this exhibition without mentioning another of the interesting synergies that the market offers based on quercetin; its combination with probiotics B. bifidum and L. gasseri (a pair of bacterial strains of the type present in fermented foods such as yogurt or kefir) that combine with the quercetin its power to induce cell death in colon cancer cells. From this, the possibility of introducing this tandem in the therapeutic approaches of colorectal cancer; combining foods of plant origin rich in quercetin with supplements of this antioxidant and probiotics seems to be a good option in this regard.

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