What is it?

Description

Quercetin (derived from the Latin word quercetum, meaning “oak forest”) is an abundant yellow bioactive flavonoid found in many plant-based foods. (3)(65) In Western diets, daily intakes have been estimated to range between 3 to 40 mg, though intake may be as much as 250 mg in individuals with high fruit and vegetable consumption. (2) Apples, berries, broccoli, cabbage, cauliflower, grapes, nuts, onions, bell pepper, red wine, black and green tea, and tomatoes all contain quercetin, but cooking these foods can substantially reduce their quercetin content. (10) Many popularly used botanicals, including Ginkgo biloba, St. John’s wort, milk thistle, or elderberry, also contain small amounts of quercetin. (45)

Quercetin possesses antioxidative, anti-inflammatory, anti-carcinogenic, anti-microbial, and immunomodulating properties. (2)(64)(65)  Though clinical studies in humans are limited, quercetin may have applications in allergic diseases, (24) bone health, (63) cancer, (46)(61) cardiovascular disease, (18)(39) diabetes, (6)(53) neurocognitive disorders (1)(27)(59)  obesity, (8) viral infection, (30) and wound healing. (43) It is possible that, to date, the clinical use of quercetin has been limited by its low bioavailability; however, as new formulations are developed, improved therapeutic benefits may be achieved. (3)

Main uses

Antioxidation
Blood pressure and endothelial function
Muscle fatigue in resistance exercise
Allergies
Pain

Formulations

Form
Characteristics
Quercetin (aglycone/anhydrous)
Supplements typically contain quercetin in its isolated aglycone or anhydrous form by removing the glucose and water molecules from quercetin that are naturally present in plants and herbs.
This is the purified, lipophilic form of quercetin. (11)
Providing quercetin in a lecithin formulation (Phytosome ®) increased the bioavailability of quercetin aglycone by up to 20x in humans. (49)
Quercetin dihydrate
Supplements may use quercetin dihydrate, which attaches two water molecules to isolated quercetin. By weight, this form contains ~90% quercetin.
An individual would require 16.6 mg of quercetin dihydrate to achieve an equivalent bioavailability to 1 mg from red onion in humans. (54)
The provision of 155 mg of quercetin glycones (from various sugar moieties) from onion skin extract was approximately 5x more bioavailable than 134 mg of quercetin aglycone from quercetin dihydrate in humans. (7)
Enzymatically modified isoquercitrin (EMIQ ®)
EMIQ increases the water solubility of quercetin.
EMIQ was more bioavailable than natural quercetin glucosides in humans. (33)

Dosing & administration

Adverse effects

Quercetin is considered to be well-tolerated, and mild adverse events are rarely reported.  However, published safety information from studies that provide doses higher than 1,000 mg for longer than 12 weeks is not widely available. (2) Many analyses do not show a difference in adverse effects between quercetin and placebo groups or indicate that no adverse events were reported in individual trials. (23)(38)(52)(57)

Pharmacokinetics

Absorption

  • If ingested in plant-based food, quercetin glycosides are hydrolyzed by β-glucosidases to the lipophilic quercetin aglycone form before passive intestinal transport. (2)(3)
  • Quercetin glycosides that are not hydrolyzed may also be uptaken by the sodium/glucose cotransporter-1. (2)
  • Overall, the absorption of quercetin is considered to be low but highly variable between individuals. Absorption may improve in the presence of the sugar moiety, non-digestible fiber, dietary fat, when quercetin aglycone is consumed as part of a food component (vs. purified), or in various nano-formulations. (20)

Distribution

  • In animal models, the highest levels were found in the liver, small intestine, lungs, testes, and kidneys. (2)

Metabolism

  • Regardless of the ingested form, quercetin is conjugated by phase II enzymes (glucuronidated, sulfated, or methylated) prior to circulation in plasma. (2)(20)

Excretion

  • Quercetin is eliminated via the feces and urine, mainly as 3-hydroxyphenylacetic acid, benzoic acid, and hippuric acid. (20)
  • Quercetin’s half-life is between 3.5 to 7.5 hours (32)(49)
References
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