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9 Benefits and Side Effects of Quercetin (4 Contraindications To Be Noted)

Quercetin, also known as onion or quercetin, is a member of the flavonoid family with important medicinal properties and an important dietary antioxidant.

It is found in a variety of foods, including fruits, vegetables, tea, wine, and other dietary supplements, and has health benefits.

The reason why quercetin is remarkable is its special antioxidant, anti-inflammatory, anti-allergic effects, one of the best among all phytonutrients, known as natural antihistamine, but also the key to making these fruits and vegetables a superfood.

What are the benefits of quercetin in empirical medicine? Are there any side effects and contraindications? See text analysis for details.

What is quercetin?

Quercetin is classified as a flavonol, one of the six subclasses of flavonoids, which are widely distributed in plants.

The name Quercetin has been used since 1857, derived from the Latin “Quercetum”, meaning oak forest.

Quercetin has been found in research to have a variety of biological properties, including antioxidant, anti-inflammatory, antibacterial, antiviral, free radical scavenging, and immunomodulatory activity, which form the basis of potential benefits for overall health and disease resistance.

This type of substance is a natural pigment evolved by plants over hundreds of millions of years, in addition to giving vegetables and fruits a lot of color, the most important role is like the immune system, which is the key for plants to resist external aggression and prolong life.

Food sources of quercetin

Quercetin is found in a wide variety of foods, including broccoli, asparagus, shallots, green peppers, tomatoes, and red lettuce, and fruits such as apples, strawberries, grapes, blueberries, cranberries, and blackcurrants

Quercetin is also found in medicinal plants, including ginkgo, forsythia sinensis and ginseng, as well as many seeds, nuts, flowers, bark, and leaves, and green tea and wine are also considered rich dietary sources.

What are the proven benefits of quercetin?

1. Quercetin adjunctive treatment of COVID-19 disease

Coronavirus disease 2019 (COVID-19) is a novel viral infection caused by SARS-CoV-2 that was declared a global pandemic by the World Health Organization on March 2020.

Mild COVID-19 can present with high fever, persistent cough, and loss or alteration of smell/taste. However, more severe and critical cases can lead to lung inflammation, low oxygen levels, and acute respiratory distress syndrome.

A randomized, controlled, and open-label study (30-day enrollment in 152 outpatients with COVID-19) noted that quercetin (1000 mg daily dose) in combination with standard care in the early stages of viral infection helped improve clinical outcomes, including: reduced need and length of hospitalization, need for non-invasive oxygen therapy, progression in intensive care units, and death. Note 1

*Conclusion: Quercetin in combination with standard therapy may help improve early symptoms and help reduce the severity of COVID-19 disease for COVID-19 infection, but more large studies are needed due to small sample sizes

2. Quercetin improves aerobic exercise performance

Physical inactivity is a widespread problem worldwide, with about two-thirds of adults not meeting the minimum recommended physical activity.

Exercise training, such as aerobic exercise, not only promotes an individual’s physical health, but may also affect mental well-being. Since physical and mental states are often interdependent, a decline in physical abilities may lead to a decline in self-esteem.

Multiple studies have reported that regular and moderate exercise training can reduce the risk of cancer, heart disease, diabetes, osteoporosis, obesity, anxiety and depression.

A meta-analysis of 7 randomized, double-blind, placebo-controlled trials with 288 participants found that quercetin did not significantly contribute to aerobic exercise performance (measured by endurance performance and VO2max). Note 1

*Conclusion: To date, there is limited evidence that oral quercetin does not improve aerobic exercise performance

3. Quercetin regulates blood pressure

The American Heart Association estimates that 8000 million adults in the United States currently have essential hypertension.

The pathological factors of hypertension are largely due to increased oxidative stress, increased endothelin 1 production, decreased nitric oxide production, and improper fluid control due to overstimulation of the renin-angiotensin system.

Treatment depends on the cause of the disease and includes dietary changes (reducing sodium intake), weight loss, increased physical activity, and pharmacological interventions.

A systematic literature review and meta-analysis (7 studies, 587 participants) noted that quercetin supplementation had a hypotensive effect (mean −3.04/−2.63 mm Hg, systolic/diastolic blood pressure), and was most effective at doses greater than 500 mg per day and more than 8 weeks of use. Note 1

*Conclusion: Quercetin supplementation has the effect of regulating blood pressure, but due to insufficient sample size, heterogeneity, and short duration, large studies are still needed to confirm the possibility of being used as an add-on therapy for hypertension

4. Quercetin helps with weight loss

Overweight and obesity are global public health problems, and as a risk factor, being overweight and obesity increase the risk of cardiovascular disease, type II diabetes, kidney disease, arthritis, musculoskeletal problems, depression, certain cancers, insomnia, and chronic fatigue.

In addition to the well-known factors that contribute to obesity, such as increased intake of high energy density foods and decreased physical activity.

There are also several different factors, such as: gut microbiota, endocrine disruptors, epigenetics, increasing maternal age, mating, sleep debt, iatrogenicity, etc

A meta-analysis (9 randomized controlled trials with 525 participants) found that daily quercetin supplementation had no significant effect on body weight, body mass index, waist circumference, and waist to hip ratio. Note 2

*Conclusion: Up to now, quercetin supplementation has not significantly helped weight loss, and more long-term large studies have further validated

5. Quercetin is beneficial for blood sugar control

Diabetes mellitus is a complex chronic disease associated with high blood sugar or hyperglycemic states that occur when insulin is insulin-secreted, inactive, or both.

Often feeling thirsty, increased urine output, ketonemia, and ketonuria are common symptoms of diabetes and occur due to abnormal carbohydrate, fat, and protein metabolism.

A systematic literature review and meta-analysis (9 randomized controlled trials, 781 patients with metabolic syndrome and related disorders) noted that quercetin supplementation did not affect fasting plasma glucose and the homeostasis model of assessment-estimated insulin resistance) and glycosylated hemoglobin A1c levels. Note 3

However, subgroup analysis found that quercetin supplementation was most effective in reducing fasting blood glucose at a ≥8-week ≥500 mg per day.

In addition, the effect of quercetin insulin lowering concentration was most pronounced in participants aged < 45 years and at a daily dose of ≥500 mg/day.

*Conclusion: For patients with metabolic syndrome and related diseases, quercetin supplementation may bring positive help to blood glucose control

6. Quercetin is beneficial for systemic inflammatory states

Inflammation is the immune system’s response to harmful stimuli, such as pathogens, damaged cells, toxic compounds, or radiation, and works by eliminating harmful stimuli and activating healing processes, and is an important defense mechanism for the body to maintain health.

Inflammation is characterized by redness, swelling, heat, pain, and loss of tissue function, which is caused by the response of local immune, blood vessels, and inflammatory cells to infection or injury, and uncontrolled acute inflammation can become chronic, leading to a variety of chronic inflammatory diseases.

A meta-analysis (7 randomized, double-blind controlled studies with 549 participants) showed that quercetin/onion supplementation had a significant effect on C-reactive protein reduction, especially in subjects with daily doses greater than 500 mg and CRP less than 3 mg/l. Note 4

Another meta-analysis (15 randomized controlled trials) showed that quercetin supplementation had anti-inflammatory effects (reduced C-type reactive protein and interleukin-6) in participants with disease. Note 5

The mechanism behind it may be related to inhibition of nuclear transcription factor kB pathway, inhibition of leukotrienin formation, interference with the AMPK/SIRT1 pathway, and promotion of nitric oxide production.

*Conclusion: Quercetin/onion supplementation has a positive effect on improving systemic inflammation, but due to heterogeneity between studies, more studies are needed to confirm this

7. Quercetin regulates blood lipids

Dyslipidemia is one of the important risk factors for cardiovascular disease, and according to epidemiological data in the United States, 45% of people aged 84 to 29 years without clinical cardiovascular disease have dyslipidemia.

Studies have shown that a 10% reduction in plasma total cholesterol reduces the incidence of coronary artery disease by 5% after 25 years, a 1 mmol/L reduction in LDL cholesterol, and a 21% reduction in cardiovascular mortality and non-fatal myocardial infarction.

A systematic literature review and meta-analysis (16 randomized, placebo-controlled trials involving 1575 participants in metabolic syndrome and related disorders) showed that quercetin supplementation significantly reduced total cholesterol, LDL cholesterol, and C-reactive protein levels. Note 6

In addition, quercetin is not significantly helpful in improving triglycerides, high-density lipoprotein cholesterol (HDL-C), and tumor necrosis factor-α (TNF-α).

*Conclusion: Quercetin may have a positive effect on lipid regulation, but due to the high heterogeneity of the included studies, more large trials are needed to support this

8. Quercetin prevents upper respiratory tract infections

Acute upper respiratory infection is the most common disease in adults, generally refers to acute nasal, sinus, pharynx, middle ear, larynx, epiglottis, airway, bronchial area infection, with the common cold is the most common, generally occurring 2 to 5 times a year.

The common cold can be caused by various pathogenic viruses. Symptoms include mild fever, runny nose, nasal congestion, sneezing, sore throat, cough, and muscle pain.

A randomized, double-blind, placebo-controlled trial (12 weeks, 1002 participants of all ages) found that, overall, supplementation with a combination containing quercetin did not significantly help with the incidence, severity, and symptom scores of upper respiratory tract infections. Note 7

However, an analysis of participants over 40 years of age (who considered themselves healthy) in the same study showed lower upper respiratory tract infection severity and total days in the 1000 mg daily group compared with the placebo group.

*Conclusion: For middle-aged and middle-aged people with good health, quercetin supplementation may be helpful in preventing upper respiratory tract infections, but limited by small sample sizes, more studies are needed to support this

9. Quercetin is beneficial for allergic diseases

Allergic disease is a hypersensitive disease in which the immune system reacts to substances normally considered harmless in the environment, and the prevalence of organ-specific allergic diseases such as allergic rhinitis, asthma, and eczema has skyrocketed in recent decades.

In some cases, allergic diseases are fairly mild, but they can also be a real nuisance and have considerable impact in everyday life, and severe allergic reactions can be life-threatening.

A literature review pointed out that quercetin has anti-allergic mechanisms such as mast cell activation inhibition, antiviral activity, inhibition of histamine release, reduction of pro-inflammatory cytokines, leukotriene production, inhibition of interleukinin IL-4 production, improvement of Th1/Th2 balance, and inhibition of antigen-specific IgE antibody formation, which may be helpful for the treatment or prevention of allergic diseases. Note 8

*Conclusion: Animal and in vitro studies have shown that quercetin is helpful for the prevention and treatment of allergic diseases, but further verification is still needed in human experiments

Are there any side effects of quercetin?

Moderate use (less than 1000 mg daily for 12 weeks) quercetin is safe for most adults in good health, and possible side effects or adverse effects that have been reported include nausea, headache, stomach pain, or tingling in the extremities. Note 9

Safety precautions

1. Do not use for poor liver and kidney function (related safety unknown)

2. Do not use by pregnant and lactating women (related safety unknown)

3. Do not use with quinolone antibiotics, may reduce the efficacy of the drug, related drug names are: Ciprofloxacin, enoxacin, norfloxacin, sparfloxacin, trovafloxacin, grepafloxacin

4. Do not use it with drugs that need to be metabolized by cytochrome P450 enzyme, which may increase the side effects of this drug, and common related drug names are as follows:

Cyclosporin, paclitaxel, rosiglitazone, amiodarone, docetaxel, repaglinide, verapamil

Celecoxib, diclofenac, fluvastatin, glipizide, ibuprofen, irbesartan, losartan, phenytoin, piroxicam, tamoxifen, Tolbutamide, Torsemide, Warfarin

Amitriptyline, Codeine, Flecainide, Haloperidol, Imipramine, Metoprolol, Ondansetron, Paroxetine, Riperidone, Tramadol, Venerfaxine

Lovastatin, Clarithromycin, Diltiazem, Estrogens, Indinavir, Triazolam, Verapamil, Alfentanil, Fentanyl, Losartan, Fluoxetine, Midazolam, Omeprazole, Lansoprazole, Propranolol, Fexofenadine, Citalopram, Sertraline, Ketoconazole, Itraconazole

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