Fruits With Quercetin comes from plants especially onions, apples and strawberries, which means we can get it from real food instead of manufactured dietary supplements. Just one serving of these 10 delicious quercetin fruits provides more than your daily needs. Here are the top sources of quercetin that you can start eating today to ward off cancer, boost immunity and more.
Healthy Foods High in Quercetin
Quercetin is a pigment that adds color to many fruits and vegetables. It’s found mainly in the skins and leaves of plants. Light stimulates the production of quercetin, so an apple at the top of a tree may have more quercetin than one that doesn’t get direct sunlight.
Quercetin may be referred to as a phytochemical, polyphenol, or flavonoid. Phytochemicals are substances produced by plants that may have health benefits for humans. Polyphenols and flavonoids are types of phytochemicals.
Quercetin is a powerful antioxidant. Antioxidants help fight free radicals, which are molecules that contain unpaired electrons. Because electrons naturally want to pair up, free radicals roam around the body, pulling electrons away from other molecules. This process can damage cells and DNA. Quercetin “cleans up” free radicals by pairing with their single electrons so they can no longer cause damage.
Dietary intakes of quercetin in the U.S. have been reported to be around 6-18 milligrams (mg) per day. However, if you’re eating several servings of fruits and vegetables on a daily basis, you’re likely consuming much more quercetin.
Why You Need Quercetin
Research shows that quercetin has many health benefits, including:
Quercetin has been shown to support the cardiovascular system by lowering blood pressure and cholesterol and relaxing blood vessels. Because reduced blood flow can cause erectile dysfunction, flavonoids like quercetin can also improve men’s sexual health.
Improved circulation improves brain health as well. But quercetin can protect the brain in other ways, too. It may reduce inflammation and protect brain cells from toxins. Its antioxidant powers could lower the risk of Alzheimer’s disease and other degenerative diseases of the brain.
Anti- C ancer Effects
When free radicals damage cells in the body, those cells sometimes develop into cancer. Quercetin and other antioxidants reduce the risk of cancer by combating free radicals. A few studies have targeted quercetin particularly. In one, it slowed tumor growth. In another, it lowered the risk of lung cancer. The third was a lab study, which found that quercetin had the ability to attack leukemia cells.
Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their colors.
Flavonoids, such as quercetin, are antioxidants. They scavenge particles in the body known as free radicals which damage cell membranes, tamper with DNA, and even cause cell death. Antioxidants can neutralize free radicals. They may reduce or even help prevent some of the damage free radicals cause. In test tubes, quercetin has strong antioxidant properties. But researchers are not sure whether taking quercetin (and many other antioxidants) has the same effects inside the body.
Quercetin may help protect against heart disease and cancer. Quercetin can also help stabilize the cells that release histamine in the body and thereby have an anti-inflammatory and antihistamine effect.
Allergies, asthma, hay fever and hives
In test tubes, quercetin prevents immune cells from releasing histamines, which are chemicals that cause allergic reactions. As a result, researchers think that quercetin may help reduce symptoms of allergies, including runny nose, watery eyes, hives, and swelling of the face and lips. However, there is no evidence yet that it works in humans.
Test tube, animal, and some population-based studies suggest that the flavonoids quercetin, resveratrol, and catechins (all found in high concentrations in red wine) may help reduce the risk of atherosclerosis, plaque build up in arteries that can lead to heart attack or stroke. These nutrients appear to protect against the damage caused by LDL (bad) cholesterol and may help prevent death from heart disease. However, most human studies have looked at flavonoids in the diet, not as supplements. Animal studies have used extremely large amounts of flavonoids (more than you could get through a supplement). More studies in people are needed to see if flavonoid supplements can be effective.
Test tube studies show that quercetin prevents damage from LDL cholesterol, and population studies show that people who eat diets high in flavonoids have lower cholesterol. One study found that people who took quercetin and an alcohol-free red wine extract (which contains quercetin) had less damage from LDL cholesterol. Another study found that quercetin reduced LDL concentrations in overweight subjects who were at high risk of heart disease. More studies are needed to show whether taking a quercetin supplement will have the same effect.
Studies show that quercetin supplementation reduces blood pressure in people who have hypertension.
Two small studies suggested that people with interstitial cystitis might benefit from consuming flavonoids. People with this condition have bladder pain similar to that from a bladder infection, and often experience an urgent need to urinate. In both studies, those who took a supplement containing quercetin appeared to have fewer symptoms. However, the studies included other flavonoids. So it is not known which flavonoid offers the most benefits. More and better designed studies are needed.
Preliminary evidence indicates that quercetin might reduce symptoms of prostatitis, or inflammation of the prostate. One small study found that men who took quercetin experienced fewer symptoms than men who took placebo. More research is needed.
Rheumatoid arthritis (RA)
There are reports of people with RA who had fewer symptoms when they switched from a typical Western diet to a vegan diet with lots of uncooked berries, fruits, vegetables, nuts, roots, seeds, and sprouts containing quercetin and other antioxidants. But there is no evidence that the positive effects were due directly to antioxidants, and no evidence that quercetin supplements help treat RA.
Scientists have long considered quercetin, and other flavonoids contained in fruits and vegetables, important in cancer prevention. People who eat more fruits and vegetables tend to have lower risk of certain types of cancer. Animal and test tube studies suggest that flavonoids have anti-cancer properties. Quercetin and other flavonoids have been shown in these studies to inhibit the growth of cancer cells from breast, colon, prostate, ovarian, endometrial, and lung tumors. One study even suggests that quercetin is more effective than resveratrol in terms of inhibiting tumor growth. Another found that frequent intake of quercetin-rich foods was associated with lower lung cancer risk. The association was even stronger among subjects who smoked more than 20 cigarettes daily, and a third suggests that quercetin slows tumor growth in the laboratory (in leukemia cells). More research is needed.
Fruits and vegetables are the primary dietary sources of quercetin, particularly citrus fruits, apples, onions, parsley, sage, tea, and red wine. Olive oil, grapes, dark cherries, and dark berries such as blueberries, blackberries, and bilberries are also high in quercetin and other flavonoids.
Quercetin supplements are available as pills or capsules. They are often packaged with bromelain (an enzyme found in pineapple) because both are anti-inflammatories. Other flavonoid-rich extracts include those from grapeseed, bilberry, Ginkgo biloba, and green tea.
There are also water-soluble forms of quercetin available, such as hesperidin-methyl-chalcone (HMC) or quercetin-chalcone.
How to Take It
There is not enough evidence to recommend quercetin for children.
Recommended adult dosages of quercetin vary depending on the condition being treated.
Quercetin is generally considered safe. Side effects may include headache and upset stomach. Preliminary evidence suggests that a byproduct of quercetin can lead to a loss of protein function. Very high doses of quercetin may damage the kidneys. You should take periodic breaks from taking quercetin.
Pregnant women, breastfeeding women, and people with kidney disease should avoid quercetin.
At doses greater than 1 g per day, there have been reports of damage to the kidneys.
If you are being treated with any of the following medications, you should not use quercetin supplements without talking to your health care provider first.
There is some concern that quercetin may reduce the effectiveness of certain antibiotics. Speak with your doctor.
Anticoagulants (blood thinners)
Quercetin may enhance the effect of these drugs, increasing your risk for bleeding. Anticoagulants include:
- Warfarin (Coumadin)
- Clopidogrel (Plavix)
Test tube and animal studies suggest that quercetin may enhance the effects of doxorubicin and cisplatin, which are two chemotherapy medications used to treat cancer. In addition, some doctors believe taking antioxidants at the same time as chemotherapy can be harmful, while others believe it can be helpful. In one study, combining quercetin with the anti-tumor drug doxorubicin, increased the drug’s beneficial effects on breast cancer cells. In another, taking quercetin alongside cisplatin reduced the medicines’ therapeutic effects in ovarian cancer cells. Talk to your oncologist before taking any supplements if you are undergoing chemotherapy.
Quercetin may cause these drugs to stay in the body longer.
Quercetin may interfere with the body’s absorption of this drug, which is used to suppress the immune system.
Concomitant use may increase the risks of digoxin.
Concomitant use may reduce the effectiveness of fluoroquinolones.
Medications changed by the liver
Since quercetin affects the liver, concomitant use with medications that are changed by the liver may alter how the body metabolizes these medications. Speak with your physician.
Quercetin In Food As Compared To Supplementation
Flavonoids are continuing to receive an ample amount of attention for their biological activities. Four types of flavonoids have been researched for clinical use and include: polyphenols derived from tea, quercetin and its diverse molecular cousins, citrus bioflavonoids, and proanthocyanidins found in grapes and certain pine species. Due to this attention, quercetin, one of the most abundant flavonoids found in food, has become perhaps the most studied flavonoid today.
Benefits of Quercetin
Quercetin has multiple influences on immune system function, aside from its most commonly known mast cell stabilizing activity. Transcription factors involved in immune function are controlled by cytokines and quercetin appears to play a role. Cytokines are a major part of the immune system and function as mediators within the immune system and hematopoiesis, which is the process of creating new blood cells.
In vitro evidence demonstrates that quercetin downregulates cytokine gene expression and production in peripheral blood mononuclear cells, particularly NFkappaB expression and production. Quercetin can block angiogenesis in vitro and other signaling pathways including insulin-like growth factor, components of MAPK, and quercetin’s free radical scavenging activity is attributed to several actions.
Quercetin’s ability to scavenge and bind transition metal ions affects lipid peroxidation.Free radical scavenging and reducing inducible nitric oxide synthase activity provides a supportive effect in circulation.* Quercetin additionally supports vitamin C absorption.A double-blind, cross-over study evaluating the effect of 150 mg of quercetin daily demonstrated support of cardiovascular health.In macrophages, quercetin is metabolized to the active aglycone form by the enzyme ß-glucuronidase which leads to reduced expression of lipopolysaccharide (LPS)-induced cyclooxygenase (COX)-2.
Food Sources of Quercetin
Quercetin can be consumed through foods naturally containing quercetin or through supplementation. Quercetin primarily enters the diet as glycoside conjugates.8 One of the most abundant glycosidic forms present in plants is quercetin-3-glucoside (isoquercitrin), which is hydrolyzed in the small intestine and rapidly absorbed.
Foods rich in isoquercitrin include leafy vegetables, broccoli, red onions, peppers, apples, grapes, black tea, green tea, red wine, and some fruit juices. The amount of quercetin received from food is primarily dependent on an individual’s dietary habits. Research has found a typical Western Diet provides approximately 0 to 30 mg of quercetin per day, but a diet rich in fruits and vegetables was estimated to provide more.It is also important to note, that the food content of quercetin reflects variations in soil quality, time of harvest, and storage conditions.
Quercetin 3-O-glucodise (Isoquercitrin) Content in Food
*Created from Phenol-Explorer, Database on polyphenol content in foods.9
It is common practice to evaluate dietary supplements in terms of food equivalence. In some cases it is not reasonable to consume food to reach a therapeutic or evidence-informed level found in dietary supplements while other times, it is a worthwhile exercise. If a supplement provides 40 mg of quercetin as isoquercitrin per day, an individual would need to consume the equivalent amount of quercetin from food, by eating approximately 8½ cups of fruits or vegetables that have a quercetin content of 2 mg per 100 grams per day. Although this is within the recommended daily intake for servings of fruits and vegetables, the average consumption in the United States is much lower. Identifying foods rich in quercetin and analyzing the diet are a couple of simple ways to estimate daily consumption of quercetin and determine any supplementation considerations.
Be mindful of the power of flavonoids and give special attention to quercetin and its applications. While a number of dietary assessment programs exist that evaluate everything from Calories, macronutrients, and micronutrients, these can be overwhelming for patients in many cases. Using an online database in relation to a specific food constituent can be an excellent exercise. Phenol-Explorer is one such online database. After evaluating a three-day food diary and noting, in this case, quercetin consumption can provide insight into whether or not lifestyle or dietary changes are required.
Quercetin, a bioflavonoid available in many over-the-counter products, may have the anti-inflammatory effects of other members of this class of compounds found in fruits, vegetables, and some spices.Katske et al. (2001) administered 500 mg twice daily to 22 IC/BPS patients for 4 weeks. All but one patient had some improvement in the O’Leary-Sant symptom and problem scores and in a global assessment score. Further larger studies with placebo controls are necessary to determine efficacy.
Quercetin is a natural flavonoid found abundantly in vegetables and fruits. There is growing evidence suggesting that quercetin has therapeutic potential for the prevention and treatment of different diseases, including cardiovascular disease, cancer, and neurodegenerative disease. Mechanistically, quercetin has been shown to exert antioxidant, anti-inflammatory, and anticancer activities in a number of cellular and animal models, as well as in humans through modulating the signaling pathways and gene expression involved in these processes. This chapter focuses on experimental studies supporting the anticancer, cardioprotective, and neuroprotective effects of quercetin.
Quercetin is a dietary polyphenolic compound with potentially beneficial effects on health. Most research has focused on the antioxidant properties of quercetin, its effects on several enzyme systems, and effects on biological pathways involved in carcinogenesis, inflammation and cardiovascular diseases. Upon absorption in the small intestine, quercetin is metabolized immediately by enzymes in the epithelial cells and further metabolized by the liver. Even if the bioavailability of quercetin is now relatively well documented, data is still lacking on the association of this flavonoid in the diet with respective to absorption and metabolism. As there is increasing evidence that there is little to no free quercetin in the plasma, the effects of the glucoconjugates and not free quercetin must be further investigated. Most of the studies presented focus on the aglycone form of quercetin, so the effects of the conjugates are still unkown, although, free quercetin may be readily absorbed and not detectable. Therefore, especially in the intestinal epithelial cells where there is evidence of deglycosylation, studying the effect of free quercetin may be of greater value. Understanding the mechanism of action of quercetin and the determination of its free, and conjugated forms of quercetin in plasma and urine prior to or after supplementation seems to be an important aspect as its various biological actions seem to be dose-dependant in many of these studies we have cited in this chapter. The effects of quercetin concentration are varied with low doses (0–10 μM) resulting in chemoprevention, mid ranges (10–200 μM) resulting in mixed effects, and higher concentration (>200 μM) in pro-oxidant or potential direct therapeutic properties. From the studies presented, these lower concentrations appear to be achievable by diet, while the therapeutic concentrations might require supplementation or intravenous administration and result in little or no side effects.
The Allergic Patient
Quercetin is a bioflavonoid obtained from diverse sources, including apples, buckwheat, onions, and citrus fruits. Most data supporting the role of quercetin in attenuating allergic reactivity have been obtained from in vitro studies, as well as from animal models of allergic disease. In vitro studies have shown that quercetin stabilizes the membranes of mast cells and reduces the release of preformed histamine.In animal models, quercetin is able to suppress anaphylactic responses in sensitized rats,and it inhibits asthmatic inflammation in guinea pigs and rats.
Quercetin must be used as a preventative—taken before allergen exposure. Thus the activity of quercetin is similar to that of cromolyn, a drug that is often prescribed for allergy and asthma prophylaxis (see later). Quercetin also inhibits the production of enzymes responsible for manufacturing the potent leukotrienes. Practitioners usually recommend that quercetin be used regularly during an individual’s entire allergy season, or year-round for those with perennial allergies.
Quercetin is similar to cromolyn in its mechanism of action. Both are basophil and mast cell stabilizers.
The dose of quercetin is usually 400–600 mg of a coated tablet one to three times daily between meals (adjust dose for clinical response). Quercetin is not soluble in water, however, so it is a poorly absorbed nutrient. Bromelain, a protein-digesting enzyme extracted from pineapples, increases the absorption of quercetin, as does vitamin C. Therefore quercetin is typically sold blended with one or both additives
Mechanism of the Anticancer Effect of Phytochemicals
Quercetin is contained in abundance in apples, honey, raspberries, onions, red grapes, cherries, citrus fruits, and green leafy vegetables . Among vegetables and fruits, quercetin content is highest in onions. The bulb color and type seems to be a determining factor for quercetin concentration in onions.
Preparation and storage of food can affect quercetin content in it. Fried or boiled foods contain lower quercetin content with boiling being the main causative for reduction in quercetin level due to thermal degradation and leaching action of boiling water. Long-term storage of foods was found to change their quercetin content. While onions lose their quercetin content by up to 33% in the first 12 days of storage, quercetin level in strawberries has been shown to increase by approximately 32% when stored at − 20 °C for 9 months. Apart from storage and preparation, the conditions of the growth of plants were found to be a factor that influence quercetin levels in them. This is clear from studies that indicate a higher quercetin content in plants exposed to greater amount of UV-radiation is the cause of which is hypothesized to be a defence mechanism against UV-exposure.
Effects of Quercetin and Its Combinations on Health
Quercetin is a polyphenol substance abundant in plants and used for medical and nonmedical purposes from ancient times. Nowadays medical usage of quercetin is justified by its antioxidant, antiinflammatory, antiplatelet, antiapoptotic, nepho-, gastro-, angio-, cardio- and chondroprotective properties.This chapter provides brief characteristics of these qualities of quercetin, based on the data of scientific literature, and then continues with a concise summary of preclinical and clinical trials, performed by the authors, of the quercetin/diclofenac and quercetin/glucosamine combinations. The study of these pharmaceutical compositions showed that combining quercetin with the mentioned substances not only allows enhancing pharmacokinetic properties of quercetin but also boosts particular effects of its multidirectional pharmacodynamics and, as a result, broadens the possibilities for medical application of this substance in treatment of heart, kidney, and joint diseases. The obtained data determine expediency of implementation of the new, highly effective combinations of quercetin into medical practice.
Polyphenolics Evoke Healing Responses: Clinical Evidence and Role of Predictive Biomarkers
Quercetins are naturally occurring flavonoids that function as active dietary antioxidants. These flavonoids are ubiquitous in foods, including vegetables such as onions, garlic, and ginger; fruit such as apples; and in tea and wine. All quercetins however are not equal. Certain forms of quercetin such as quercetin rutinoside (rutin) are poorly absorbed by the body and are more likely to be irritating or allergenic. Another example is quercetin chalcone, a special hesperidin, which has an exceptionally short half-life and is therefore not effective unless it is taken every hour or so.
While quercetin dihydrate is insoluble in water, in physiologic or biological salt solutions it is easily available, especially to first-responder phagocytic and dendritic cells. Quercetin dihydrate therefore seems to have the apparent best bioavailability followed by glycosides, aglycone, and finally rutinoside. The chemical description of quercetin dihydrate is 3,3′4′, 5,7-pentahydroxy flavone (Fig. 29.2).