In today’s fast-paced world, our regular diet can hardly meet the nutritional needs of the body. Synthetic and artificially separated nutritional supplements have become the only way to fight chronic inflammatory conditions such as heart diseases and cancer, which are getting more widespread every day.

Quercetin is one such nutritional supplement obtained from plants; in fact, one of the most abundant bioflavonoids found in plants. Research suggests its use as a supplement and drug therapy adjunct for its anti-inflammatory, antihypertensive, anti hypercholesterolemic, anti-obesity, antioxidant, anticancer, and anti-allergic effects.

Its benefits as an anti-inflammatory agent are comparatively recent, and the research and experiments are still ongoing. In this article, we will be looking at the benefits, potential applications, adverse effects, safe dosage, contraindications, in fact, everything you must know about this recent plant-derived supplement.


Understanding Quercetin and Flavonoids

Quercetin belongs to a group of plant compounds called flavanols. Interestingly, Quercetin is a plant pigment. And many pigments like Quercetin impart colors to the fruits, vegetables, and flowers of plants.

Plant pigments and substances have potent antioxidant and anti-inflammatory properties that fight damaging free radicals. Flavonoids are such phytonutrients getting popular every day for their minimum side effects and optimum antioxidant efficacy.

Quercetin is one such flavonoid found in large amounts in onions, parsley, citrus fruits, apples, green tea, olive oil, and grains. Almost all edible plant derivates possess some quantity of Quercetin, broadly speaking, flavonoids.

Dietary intake of quercetin has shown tremendous benefits against cardiovascular conditions, which is the most significant medical problem of this century. Quercetin supplements lower lung cancer risk and experiments have also proved these as highly effective against aggregation of platelets, all types of arthritis, hypertension, atherosclerosis, and allergy symptoms.

Various studies on animal subjects have also revealed the anti-diabetic potential of flavonoids, but their application and efficacy in humans are still subject to research and evidence.


Health Benefits of Quercetin Supplements

Quercetin and related plant flavonoids have the following health benefits:

Relieves Allergy Symptoms

Flavonoid quercetin possesses documented anti-inflammatory properties. That is one mechanism of strengthening the immune system and lung function. But the more pronounced benefit of taking Quercetin against allergy is relieving the debilitating symptoms such as runny nose, irritating and watery eyes, and bouts of consecutive snee

Common allergy symptoms include runny nose, watery eyes, sneezing, headache, swelling of the face and lips, and sinusitis. Test tube research and trials on some animals have suggested that Quercetin can bind the eosinophils and prevent the release of histamine. Histamine is the main culprit involved in the precipitation of an allergic response.

Quercetin also works by in vivo tyrosine kinase inhibition in the lymphocytes and preventing the precipitation of allergic responses.

Quercetin ingestion may help relieve the allergy symptoms, but research does not confirm its efficacy in the prevention of allergy attacks.

Quercetin Reduces Blood Pressure

A systematic review of a randomized controlled trial conducted in 2016 suggests the effects and the role of quercetin intake in reducing blood pressure in hypertensive patients. Dietary flavonoids are getting increasingly popular for their efficacy in preventing, managing, and treating cardiovascular disorders.

One mechanism of lowering blood pressure is reducing and preventing oxidative stress and blood vessel damage. Another effect that is not dependent on the endothelium is by causing vasodilation. Dietary antioxidant flavonol Quercetin and quercetin metabolites have been reported to decrease resistance in the arterioles by causing vasodilation in hypertensive rats.

When a study sample of mice was administered Quercetin for 5 weeks, the researchers observed a decrease in both systolic and diastolic blood pressures.

One important clinical aspect of the blood lowering effect of dietary quercetin glycosides is that it is more pronounced in diabetics and hypertensives. Even in prehypertensive individuals, blood pressure did not show any significant difference after quercetin intake.

This finding makes this supplement safe for consumption by normal, healthy adults. They won’t experience hypotension or unprecedented dilatation of the blood vessels, such as in the kidneys, liver, and brain, which is not beneficial in any way.

Besides, the effective dose for the blood pressure-lowering effect of Quercetin is 500 milligrams or greater per day.

Dietary Flavonoids have Anti-cancer Effects

Dietary flavonoid intake helps prevent and treat cancer in various ways. Flavonoids act as antioxidants in normal conditions and become pro-oxidants for mutated cancerous cells. Quercetin increases the ROS-mediated scavenging activities inside these cells, induces free radicals to injure the membranes, arrests cell survival and growth, induces apoptosis and suppresses invasion and metastasis.

Quercetin has renowned benefits in ovarian cancer treatment. It exerts its anti-inflammatory, ROS-mediated scavenging effects, arrests the cell cycle, and induces apoptosis to kill the ovarian cancer cell growth.

In fact, all reproductive cancers, Ovarian cancer easily develop resistance to conventional cancer therapies. Quercetin can be added as an adjunct to the chemotherapy protocol against ovarian cancer, given its proven anticancer effects.

Quercetin supplementation has been proved effective against prostate cancer. Prostate cancer has a high incidence and prevalence in men above the age of 60 years, and it eventually leads to the death of the individual.

In a randomized placebo-controlled trial, prostate cancer cells treated with Quercetin showed a time- and dose-dependent decrease in survival and viability. At the same time, it didn’t affect the normal prostate epithelial cells.

Quercetin supplementation also turned off anti-apoptotic signals and pathways and caused mitochondrial damage that led to the death of the cancer cells. It also tampered with the expression of the NF-KB pathway, again leading to the death of the cancer cells.

In vitro experiments have suggested the efficacy of Quercetin against oral cancer cell growth. It uses the mechanism of mitochondria-mediated apoptosis in the test tube samples and has the potential to be used in chemotherapy against oral cancers.

The final benefit of the antioxidant properties of Quercetin is that it can reduce the lung cancer risk. In a study conducted in 2020, dietary Quercetin inhibited the growth and proliferation of non-small cell lung cancer (NSCLC).

Quercetin supplementation reduces the risk of cardiovascular disease

Cardiovascular disease is the biggest threat to the western population, the biggest risk factor being their diet and lifestyle. The most important benefit of Quercetin has to be its role in the prevention and treatment of cardiovascular disease.

Firstly, Quercetin reduces systolic blood pressure in hypertensive subjects. This is one of the most important effects of Quercetin, as high blood pressure is the most important cardiovascular risk factor that eventually leads to heart disease.

Secondly, flavonoids Quercetin inhibit LDL oxidation without affecting plasma antioxidant vitamin concentration. The antioxidant effect of quercetin protects the blood vessels against the lipid peroxidation caused by oxidative stress.

Since it has been shown to reduce the level of inflammatory markers in the blood, Quercetin is equally effective against free radicals and reactive oxygen species. LDL oxidation, endothelial damage, lipid deposition, and high blood pressure are all precursors of all kinds of cardiovascular diseases.

Thirdly, Quercetin inhibits platelet aggregation. Aggregation of platelets and thrombus formation is the main culprits in heart disease. Underlying atherosclerosis, such as diabetes mellitus, paves the way for life-endangering heart disease.

And then comes the effects of Quercetin in vasodilation, protection of the nitric oxide mechanism, and endothelial function. Therefore, dietary antioxidant flavonoids or supplemental Quercetin can prevent and protect against coronary heart disease.

Effects of Quercetin on Rheumatoid Arthritis

Rheumatoid type of arthritis is a chronic inflammatory condition that benefits from the effects of Quercetin, which reduce the inflammatory markers in the Body. Quercetin has been shown to reduce the migration and the infiltration of neutrophils to the inflammatory site, which in the case of arthritis are the weight-dependent joint cavities of the Body.

It can relieve the painful signs and symptoms of all types of arthritis by inhibiting the cyclooxygenase (COX-2) levels and suppressing the NFKB levels in the blood. In addition to some cytokines, these substances are involved in the bouts of arthritis inflammation and the associated pain.

A study in 2016 examined the effects of Quercetin on the symptoms and disease progression of rheumatoid arthritis in women. The results of the study showed that the women who took 500 mg of dietary antioxidant flavanol quercetin experienced considerably reduced signs of inflammation such as morning stiffness and compromised exercise capacity.

The same study revealed lower levels of stress and inflammatory markers in the blood, such as the tumor necrosis factor-alpha (TNF-alpha).

Quercetin Supplementation Prevents Infection

Taking Quercetin has been linked to reduced rates and risks of developing bacterial and viral upper respiratory tract infections in middle-and old-aged individuals.

A study on animals revealed the antibacterial and antiviral effects of dietary flavonoid glycosides. Quercetin reduced the proinflammatory cytokines, reactive oxygen species, mucus production, number of respiratory secretions, and airway resistance in virally infected animals.

The recommended dosage of Quercetin has the following reported antiviral effects:

  1. Inhibits polymerase, reverse transcriptase, and protease, which collectively inhibits the growth and survival of the virus
  2. Suppresses DNA gyrase to hinder the viral replication
  3. Binds viral capsid proteins so that they do not enter the host cell

In combination with vitamin C, Quercetin has strong prospects for the prevention and treatment of COVID-19 and other diseases and complications related to the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2).

Journal of Food Production assessed the bacteriostatic and bactericidal effect of Quercetin in vivo. It was seen that Quercetin could damage the cell membranes of Staph. Aureus and E. coli both of which are notorious for causing urinary, skin, and throat infections in humans and animals.

Effect of Quercetin on Diabetes Mellitus

A study conducted in 2019 revealed that taking Quercetin in dosages greater than 500 mg per day can significantly reduce fasting plasma glucose levels (FPG). Another important outcome of the study was a reduction in the plasma insulin concentration, indicating increased insulin sensitivity and improved glycemic control.

Insulin sensitivity is important in the management of metabolic syndrome in patients with diabetes mellitus type 2. There are abundant evidence and study reports available on animal populations that indicate Quercetin as a potential treatment and prevention against the complications of diabetes.

Apart from that, Quercetin has known benefits in reducing chronic inflammation, which can be highly beneficial against the development and progression of diabetes. Despite this, it is not yet included in the anti-diabetic treatments and its efficacy in humans, in this case, is also not well-established.

Health Benefits for Chronic degenerative Brain Disorders

Quercetin reduces illness related to chronic inflammation, and it is neuroprotective too. Both these aspects make Quercetin a potential adjunct to the drug therapy against neurodegenerative diseases.

In a randomized community clinical trial, Quercetin in combination with fish oil reduced oxidative stress in the brain of rats and improved their learning outcomes after taking Quercetin.

Chronic quercetin ingestion has been shown to improve memory, cognitive abilities, and general brain performance in healthy individuals.

Neurodegenerative disorders are characterized by the death and injury to the neuron cells in the brain and peripheral nervous system.

Key events in the pathology and development of Alzheimer’s disease include toxic metal accumulation, inflammatory processes, free radical reactions, loss of protein structure and b-amyloid plaque formation, and oxidative stress. Parkinson’s disease and Huntington’s disease have less understood but related pathologies.

Quercetin helps halt the disease by various other mechanisms: it slows neuronal loss and degradation, alters gene expression, fights off pro-inflammatory cytokines, chelation of toxic metals, antioxidant and anti-inflammatory effect.

On the whole, Quercetin, through these mechanisms, helps alleviate symptoms of Alzheimer’s such as dementia, confusion, the decline in cognitive function, and attention deficit. At the same time, it prevents the development of Alzheimer’s in the elderly who have a genetic predisposition by keeping the inflammatory contributors in check.

Effects of Quercetin on Immunity

The antiallergic effects of Quercetin may give an impression that Quercetin tends to weaken the immunity. It gives the same impression because of its role in preventing graft rejection.

It does stabilize mast cells and prevents them from releasing histamine, which is the main substance involved in an allergic response, even severe anaphylactic reactions. And sure, it does activate and inactivate certain signaling pathways that eventually suppress the immune system enough to prevent graft rejection.

The immune system boosting capabilities of flavonoids are more prominent in the gut and the intestine. Consumption of flavonoids such as Quercetin increases the proportion of gut bacteria coated with the immunoglobulin IgA.

Quercetin has been seen to boost the overall immunity of the body against bacterial and viral infections too. Just recently, researchers have begun considering this phytonutrient in managing the complications of COVID-19.

Quercetin has protective Effects on Blood and Blood Vessels

Quercetin glycosides and their conjugates have shown prominent antioxidant, anti-inflammatory, and protective effects on the blood vessels. Quercetin crosses the endothelial cells to reach the smooth muscle covering of the blood vessels to exert its anti-inflammatory effects.

Monocytes are immune cells notorious for causing blood vessel inflammation. They become macrophages after a few days of their release into the blood. Monocytes and macrophages then engulf fat and lipid to become foam cells. These local accumulations attract more inflammatory cytokines and chemokines and damage the blood vessels.

Quercetin absorbed through the gut reaches the blood and alters the monocyte adhesion to the human aortic endothelial cells. Quercetin manages the risk of heart disease, epithelial ovarian cancer, and thrombus formation by reducing inflammation in normal or diseased people.

These flavonoids also have the ability to inhibit the proliferation of human leukemic mast cells. Human leukemia is one of the most aggressive forms of blood cancer, and it runs in families. Administration of antioxidant and anticancer substances such as bioflavonoids and Quercetin can help mitigate the familial risk of blood cancers.

Improves Early Graft Function

Quercetin and bioflavonoids curcumin have unique roles in preventing graft rejections. A randomized controlled trial assessed the beneficial effects of bioflavonoids and Quercetin in cadaveric renal transplantation.

Individuals treated with bioflavonoids and Quercetin before transplantation or graft insertion considerably improved the outcomes. The chances, risks, and incidence of acute graft rejection and neurotoxicity reduced as the dose increased following transplant surgery.

Besides, it has been observed that the control group who took flavonoids before and after the transplant present with an early onset of function by the transplanted organ or tissue as compared to the placebo.


How to take Quercetin Daily?

Quercetin aglycone is the most effective form of quercetin flavonoids. The optimum recommended dosage of Quercetin for an average, healthy adult is 500 mg per day. If you eat fruits and vegetables daily, especially raw and fresh, you are getting your healthy dose of pure dietary flavonoids Quercetin all right.

But if you intend to use Quercetin for a certain health condition, or ovarian or pancreatic cancer risk, you may need to use the supplements as well. The best effective dosage and duration, in this case, will have to be decided by a healthcare provider.

Usually, dosages between 250 and 1000 mg are prescribed for 12 weeks in addition to the daily fruits and vegetables for quercetin treatment of a condition.


Are there any Side Effects to taking Quercetin?

Quercetin supplement has not yet reported any significant side effects. If you take your daily quercetin dosage from the fruits and vegetables only, you will experience no side effects.

When you take the prescription dosage of Quercetin in the form of supplements, you may experience mild side effects in the beginning. Commonly reported adverse effects in such cases are nausea, headache, and mild stomach upset. These side effects of taking Quercetin subside in a few days, though.


Contraindications to Flavonoid Quercetin

By and large, quercetin dietary supplement has been documented as safe for consumption by all age groups and ethnicities. There are some rare scenarios that need further research and confirmation.

Pregnancy and breastfeeding mothers need to take extra care with the dosage of quercetin supplement. Research suggests that pregnant and breastfeeding women should only take those supplements prescribed and recommended by their physicians.

Since Quercetin supplements may be accompanied by some minerals such as magnesium, calcium, and zinc, they are not recommended for patients with kidney stones. But with natural sources such as vegetables and fruits, this risk is not significant.

Quercetin has no documented drug interactions or in vivo toxicity with herbs or other supplements. But those who are taking drugs dependent on the liver for metabolism, discretion should be observed. Since Quercetin is also metabolized and broken down by the liver, it can affect the rate of clearance of other drugs such as warfarin, cyclosporine, and other antibiotics.


How do you Absorb Quercetin Better?

Daily quercetin supplementation alongside glucose has shown an increase in plasma quercetin concentration in young, healthy adults. This proves the effective absorption of this antioxidant in the human gut. Accompanying the supplement with glucose or some other better water-soluble substance produced better results.

At the same time, the rate of urinary quercetin excretion or clearance is very slow. Quercetin in the plasma has a half-life of 11 to 28 hours.

But Quercetin is not soluble in water. That is why it does not absorb well on its own, especially in its synthetic form. Quercetin supplements hence also contain vitamin C and Bromelain.

Vitamin C is another potent antioxidant and anti-inflammatory substance. And Bromelain is a protein-digesting enzyme found in fruits and vegetables such as pineapple. It is separated and combined with Quercetin to enhance its absorption in the gut.

Quercetin is eliminated by the kidneys in urine, by the lungs in the exhaled air, and by the human intestinal cells in the feces. And the aggregated rate depends upon the individual and the expression of liver enzymes in his plasma.

Some theories imply that fruit juice intake maintains the plasma quercetin concentrations for a longer time, but there is no clinical evidence as yet to support this.

What is the Best Natural Source of Dietary Flavonoid Intake?

Onion as a source of quercetin

The primary dietary sources of Quercetin are fruits and vegetables. Onions are known to be the most abundant sources of Quercetin and flavonoids. Onions contain almost 300 mg of Quercetin per kilogram.

Citrus fruits, apples, blueberries, blackberries, grapes, and parsley are very popular for their flavonoids and quercetin content. Even more popular are olive oil, green tea, and red wine. Quercetin is also known as a popular, predominant flavonoid constituent of an herb called Ginkgo Biloba. Ginkgo biloba is native to China and has immense medicinal benefits.


What does Quercetin do for your Body?

Whether you take quercetin supplements as prescription medicine or for overall well-being, you will experience the following effects and results collectively:

  1. Reduction in the inflammation-causing substances.
  2. Slows down the oxidation of lipids without reducing the carotenoid concentrations
  3. Improves brain function, cognition, and memory
  4. Has neuroprotective properties that prevent and treat neurodegenerative disorders such as Parkinson’s, Alzheimer’s, dementia, and Huntington’s disease
  5. Improves exercise capacity and lung function due to its anti-inflammatory effects.
  6. Helps in the management of cardiovascular disorders by treating high blood pressure, preventing thrombosis and atherosclerosis, and directly causing vasodilation.
  7. Acts as a potent anti-viral and antibacterial agent. Research suggests its efficacy in significantly reducing the risk of respiratory, urinary, and skin infections.
  8. It slows down aging and its signs by reducing chronic inflammation
  9. Shows immediate results against allergy symptoms
  10. What Reduces the risk of cancer in susceptible individuals and helps treat existing tumors too

Quercetin supplementation reduces illness but not immune perturbations induced after intense exercise, as shown by the experiments on rat hepatic stellate cells.


The Bottom Line

The most important mechanism of action of quercetin is to fight and neutralize free radicals. quercetin supplementation has shown great potential against several types of cancers, allergies, cardiovascular diseases, bacterial and viral infections, blood vessel injury caused by free radicals, and diabetes mellitus.

Adverse effects are minimal but dietary flavonoids interact with medications that are emtabolised by the liver, just like quercetin itself. Otherwise, it is completely safe for all age groups to boost their general health or fight particular diseases.

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