Quercetin, Are we getting enough?

Quercetin, Are we getting enough?

Quercetin is a water-soluble plant pigment classed as a bioflavonoid. Flavonoids are a large and diverse group of bioactive compounds, known as phytonutrients or phytochemicals that come under the banner of Polyphenols. This particular group of plant bioactive compounds have been identified as possessing huge benefits to human health, best known for their antioxidant and anti-inflammatory benefits. Flavonoids are present in practically every fruit and vegetable and are the largest group of phytonutrients; there are more than 6,000 different identified types of flavonoid and they are the polyphenol most consumed in the human diet.

Flavonols are one of the more common types of flavonoid and quercetin is one of the most commonly consumed flavonols. It’s a specific type of flavonol found in a range of green leafy vegetables, berries, fruits, leaves, beans and grains. Other types of flavonol regarded as having health benefits but less commonly consumed in our diet are Kaempferol, Myricetin and Isorhamnetin.

Chemical Structure

Its chemical name is 3,3',4',5,7-pentapentahydroxyflavone, its natural state is a yellow crystalline powder. Molecularly speaking quercetin is a flavonoid, consisting of a six-member ring condensed with an a-pyrone benzene ring which in the 2-position carries a phenyl ring as a substituent. It has the basic flavonol structure of hydroxylation on the 3 carbon of the central ring, with two other hydroxylations on the outer ring.

History of Quercetin

In cultures across the globe, traditional medicine has for centuries adopted the use of plants rich in flavonoids. By comparison, in the Western world of medicine, the health benefits of plant bioactive compounds such as flavonoids is a relatively recent discovery. Bioflavonoids and their huge range of health benefits specifically caught the attention of scientists when they were discovered by Nobel Prize laureate, Albert Szent Gyorgyi in the 1930’s.1 Quercetin itself has gained in interest and use over the past 30 years, particularly in the field of sports nutrition where scientists have been keen to research the capabilities of quercetin in terms of physical endurance and performance.2 The name quercetin is thought to be accounted to
Joseph Quercetanus Duchesne, a medieval scientist with a particular interest in Iatrochemistry, Alchemy, Medicine and Pharmacology.3 The name quercetin is also thought to derive from the fact that it’s present as the glyco-side Quercitrin in the bark of Quercus tinctoria (American Oak).4 And a further connection to this is that quercetin is a derivative of the latin word, quercetum, meaning oak grove or oak forest.5

Health Benefits and Uses

Quercetin and bioflavonoids in general are regarded as possessing properties beneficial to a wide range of health conditions and capable of reducing the risk of fatal diseases. Quercetin has been researched in recent years in particular, for its potential ability to decrease the risk of cancer; inflammatory; and cardiovascular diseases in humans.

  • Anti-inflammatory
  • Antiviral
  • Antioxidant; Protecting cells from damage by fending off free radicals which harm the health of cells.6
  • Anticancer and chemopreventive; reducing the risk of cancerous cells and the growth rate of tumours.7
  • Respiratory health.
  • Treatment of Pancreatitis; due to anti-inflammatory capabilities.
  • Anti-allergy and antihistamine properties; used to treat hay fever, hives and other allergies and conditions.8
  • Treatment of skin conditions like dermatitis and photosensitivity.
  • Neuroactive capabilities similar to that of caffeine.9
  • Cardiovascular health; protects against heart disease, reducing the risk of a heart attack or stroke. Promotes blood flow; lowers cholesterol; and protects against LDL cholesterol oxidation.10
  • Blood pressure; helps maintain healthy stable blood pressure.
  • Protects body against stress; the body releases cortisol when we are stressed. Too much cortisol can cause harm to muscle tissue, which can cause protein to breakdown in the body. Quercetin can counteract this process by blocking the enzyme which is needed for cortisol to be produced.11
  • Sports performance; improves mental and physical performance and enhances stamina. Quercetin could also lessen the risk of infection when undertaking extreme physical exercise.12
  • Prostate Health.13
  • Treatment of Fibromyalgia.14
  • Treatment of Gout; due to anti-inflammatory action.15
  • Supports general good health; helps to bolster immune system.16

Natural sources of Quercetin

Natural sources of quercetin are vast and include green tea; red grapes; red onion; chili peppers; red kidney beans; capers; watercress; sweet potato (particularly the leaves) citrus fruit; red wine; cocoa powder; celery; plums; fennel leaves; broccoli; kale; tomatoes; lovage leaves; dock leaves (eg sorrel) apples; apricots, blueberries; raspberries; cherries, cranberries; some green algae; ginkgo biloba; St. John’s Wort and buckwheat.

Food Chart: Quercetin mg per 100g 17

Food Source

mg/100 g.

Apple, raw, without skin


Apple, raw, with skin


Apricot, raw


Blueberries, raw


Broccoli, cooked


Broccoli, raw






Celery, raw


Cherries, raw


Chives, raw


Coriander, raw


Cranberries, raw


Dock leaves, raw (like sorrel)




Green Tea, brewed


Hot Green Chili Peppers, raw


Hot Wax Yellow Peppers, raw


Kale, raw


Lemons, raw, without peel


Lovage leaves, raw


Plums, raw


Red Grapes


Red Onion, raw


Sweet Potato


Sweet Potato, leaves, raw


Tomatoes, cherry, raw


Tomato juice, canned


Tomato puree, canned


Watercress, raw


Are we getting enough Quercetin?

Making sure we’re regularly consuming a wide variety of flavonoids is a great start to supporting our overall every-day health. Quercetin is present in so many fruits and vegetables that if we stick to at least our five a day intake we can rest easy that we’re going some way to covering our quercetin quota. For example, a meal containing red kidney beans, onion, broccoli, kale, tomatoes, sweet potato, hot yellow peppers, coriander and iceberg lettuce; followed by a mixed fruit salad of apples; apricots, blueberries; raspberries; cherries and cranberries; washed down with a mug of green tea or a glass of orange juice or red wine, is absolutely crammed full of quercetin!

However, in a study conducted in Japan in 2015 results showed participants were consuming an average and median quercetin intake of 16.2mg and 15.5mg day, respectively.18 Sources of quercetin were mainly onions, green tea, green peppers, asparagus, red leaf lettuce and tomatoes. This is a relatively low consumption of quercetin when compared to the recommended dose of 1g a day or maximum advised dose of between 3-4g per day.

Even when we are consuming plenty of fruits and vegetables, factors such as heat, light and oxygen can all bring about passive oxidation which can change the chemical compounds and alter the molecular nature of the bioactive compounds present in the plant. This will affect the nutritional content or medicinal properties available. For example, much of the concentration of quercetin is lost when vegetables, such as kale, sweet potatoes or broccoli are boiled. This is clearly illustrated in the previous table; cooked broccoli has a quercetin content of 1.06mg per 100g compared to the much higher content of quercetin found in raw broccoli of 3.21 mg per 100g. In a similar way, fruit such as blueberries, lemons or apricots, for example, will lose much of their nutritional value and their flavonol content and the presence of quercetin will diminish if kept at room temperature.

Quercetin powder.

The benefit of taking quercetin in powder form is that it’s very versatile and can be used as a nutritious addition to beverages or food.19 It’s also easy to monitor how much quercetin we’re getting when using a quercetin supplement. In addition to quercetin powder, try taking a combination of other flavonoid-rich supplements such as Ginkgo biloba, Green tea, Grape seed or Bilberry. Combining flavonoid-rich supplements can have a synergistic relationship where the combined health effects are greater than using a single supplement alone.

Taking Papain and Bromelain may help the body to better absorb quercetin in the intestine.20

Papain and Bromelain Tablets can also work in synergy by boosting the overall anti-inflammatory effects and other health-giving properties of quercetin.21


The bioavailability of a substance is determined by how successfully it’s absorbed, distributed and kept in the body and also how well the body can excrete the substance. After consuming a 250 or 500 mg dose, quercetin takes between 20-30 minutes to enter the system. Maximum concentration is reached approximately two to three hours after consumption. Research has shown quercetin has a long half-life in the human body and can remain present for around about 16-17 hours but will have left the body completely within 24 hours of ingestion.22

Safe Consumption

It’s important to ensure quercetin is consumed safely. The recommended dose of quercetin as a general health supplement is 500 milligrams twice a day.23 Some research indicates, however, that levels of up to 3 or 4 grams a day is safe for human consumption.24

Too much quercetin can overwork the kidneys. Research has shown quercetin could also interfere with aspirin, blood thinners and corticosteroids.25 Specifically avoid the use of quercetin if you’re taking quinolone antibiotics as quercetin could reduce their effectiveness. Also don’t take quercetin if you’re taking CYP3A4 or CYP2C19 substrate drugs because quercetin could increase the risk of side effects in these drugs.26

Quality Quercetin

Although the name quercetin may have been an unfamiliar one, we have, no doubt, been consuming quercetin-rich foods daily in the fruits and vegetables we include in our diets, which are also packed with other goodness, such as essential vitamins and other salubrious bioactive compounds. Consuming a wide variety of fruits and vegetables containing quercetin and including in our daily meals more beans, roots, leaves, grains and berries rich in quercetin will help ensure we’re getting our quercetin quota. Broadening dietary horizons to include foraged foods such as sorrel, a type of dock chockablock with quercetin (and also rich in Vitamin C, Vitamin A, Vitamin B6, iron, magnesium, potassium and calcium) could be a novel and healthy way to up our quercetin levels.27 Interestingly, sorrel is one of the food sources most rich in quercetin, containing 86mg per 100g. Extreme care should be taken, however, when identifying wild plants for consumption and the golden rule is; unless you’re absolutely 100% certain it’s safe and edible; don’t risk it. Including a range of herbs in our daily diets is another great way to up our quercetin intake. Lovage, an old English herb used less frequently now, is jam packed with quercetin, containing a whopping 170mg/100g. In general, mindful eating and dietary management is the way forward. When we ensure we’re getting our daily fill of a varied range of polyphenols, and perhaps choosing to boost this with a combination of appropriate supplements such as Quercetin powder, Green Tea powder and Papain and Bromelain Tablets, we can relax safe in the knowledge that we’ve got our bioactive compounds covered.


1 Quercetin: a versatile flavonoid: http://www.akspublication.com/paper05_jul-dec2007.htm

6 The role of quercetin, flavonols and flavones in modulating inflammatory cell function:


14 Natural Health Research Institute:http://www.naturalhealthresearch.org/quercetin

18 Estimated Daily Intake and Seasonal Food Sources of Quercetin in Japan: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4425148/

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