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Quercetin is a Bioflavonoids found in fruits and vegetables, but highest levels are found in apples and onions.
Like many other bioflavonoids, Quercetin has anti-oxidant, anti-artherogenic, and anti-carcinogenic properties. Quercetin is also neuroactive, with some of the same abilities as Caffeine but less potent.
There is a divide between the effects seen in quercetin in in vitro (cell cultured) studies and in vivo (in living) studies, with cell studies showing great results that are not that amazing in humans or animals. This is mostly due to quercetin having low oral bioavailability (low percentage of the compound is absorbed and put to use), but could also be due to in vitro studies using a form of quercetin called 'quercetin aglycone' whereas this particular form is never found in the blood, even after ingested, as it it gets changed in the liver.
Many studies also note a high range of differences between people who ingest the same amount of quercetin, suggesting a large degree of variability is possible with supplementation.
Quercetin has GRAS (Generally Recognized As Safe) status, and no side-effects have yet been noted in doses of a few grams a day in either humans or animals.
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Apple extract, 3,4,5,7-pentahydroxylflavone
Quercetin can block adenosine receptors. In the brain this would induce wakefulness (although transport is an issue) and in systemic may be a fat burning augmenter.
Quercetin is highly water-soluble
Quercetin is yellow colored
Dosages of quercetin used are in the range of 12.5 to 25mg per kg body weight, which translates to a range of 1,136-2,272mg daily consumption of quercetin when in isolation.
It is suggested to supplement with other Bioflavonoids such as Resveratrol, genistein, or Green Tea Catechins to increase the potency synergistically and theoretically get the benefits at a reduced level of intake.
When looking for quercetin, the form of dihydrate has the apparent best bioavailability followed by glycosides, aglycone, and finally rutinoside.
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The Human Effect Matrix looks at human studies (excluding animal/petri-dish studies) to tell you what effect Quercetin has in your body, and how strong these effects are.
|Grade||Level of Evidence|
|A||Robust research conducted with repeated double blind clinical trials|
|B||Multiple studies where at least two are double-blind and placebo controlled|
|C||Single double blind study or multiple cohort studies|
|D||Uncontrolled or observational studies only|
|Level of Evidence ||Effect||Change||Magnitude of Effect Size ||Scientific Consensus||Comments|
No significant influence on sleep quality
No significant influence on fatigue nor vitality in otherwise healthy persons
|C||Anaerobic Running Capacity|
No significant influence on anaerobic exercise capacity when preloaded
Mixed influence on inflammation, but does not appear to at all be practically significant
An increase in the amount of intestinal permeability induced by training in the heat has been noted with quercetin supplementation, which is an adverse event; the influence... show
|C||Exercise-induced Stress Response|
The exercise-induced increase in HSP70 expression is abolished with quercetin preloading
May reduce oxidative biomarkers in serum
No acute alterations in blood pressure following Quercetin supplementation
No significant influence on metabolic rate following acute Quercetin supplementation
No significant alterations in fat oxidation noted with quercetin supplementation
An increase in total cholesterol has been noted, but mostly attributed to HDL
A decrease in LDL-C has been noted in persons with high blood lipids
An increase in HDL-C has been noted following quercetin supplementation
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Quercetin is one of the most prominent bioflavonoid compounds in plants, and is found in the food products:
Dietary supplements (fruits, leaves, or herbal compounds) may also confer a Quercetin content either as free Quercetin or one of its glycosides; popular or particularly good sources of Quercetin include:
Quercetin possesses the basic flavonol backbone (hydroxylation on the 3 carbon of the central ring) with two other hydroxylations on the outer ring. Removal of one of these hydroxy groups creates Kaempferol, which is the backbone for the active metabolite of Horny Goat Weed, Icariin. Substitution of the removed group with a methoxy group creates the metabolite isorhamnetin. A glycoside of quercetin, called quercetin 3-O-beta-rutinode, is more commonly referred to as rutin.
After oral ingestion of quercetin, it is taken up from the gut into the liver. The conjugate of quercetin influences its absorption rates. At least intestinally, quercetin glycosides (food source) were found to have a 52+/-15% uptake, quercetin rutinoside (tea) has a 17+/-15% uptake, and supplemental quercetin aglycone had a 24+/-9% uptake.
One pharmacokinetic study in humans following consumption of 500mg Quercetin (as aglycone) noted that the delivery of Quercetin chews had a Cmax of 1051.9+/-393.1ug/mL at Tmax of 3.66 hours, with the Cmax and Tmax of Food bar format and juice suspension reaching 698.1+/-189.5μg/L (in 2.3h) and 354.4+/-87.6μg/L (4.7h), respectively. This study had all forms using QU995, and was unable to conclude any significant differences between groups due to higher variability (just different average).
Appears to per se have a moderate to low bioavailability, depending on the source
Due to enhanced lymphatic release of Quercetin following administration of Long-Chain Fatty acids (LCFAs), it is thought that the formation of micelles from LCFAs can enhance the apparent bioavailability of Quercetin.
Quercetin is a potent inhibitor of intestinal sulfurotransferases, and has some activity on hepatic sulfurotransferases as well. This mechanism may increase bioavailability of compounds that undergo extensive intestinal metabolism via this method, like Resveratrol.
Interacts with intestinal conjugation enzymes, which may predispose Quercetin to nutrient-nutrient interactions
After the liver, quercetin exists in the blood solely as quercetin glucuronides. Regardless of initial source, all forms of quercetin undergo hydrolysis and get glucuronidated in the liver before being released into systemic circulation.
Quercetin has been measured in the blood 15-30 minutes after ingestion of 250-500mg, with peak levels 120-180 minutes after ingestion and returns to baseline levels in humans 24 hours after ingestion. Additional ingestion of quercetin dose-dependently increases plasma quercetin glucoronides. Accumulation of quercetin has been noted in many tissues including kidneys, colon, liver, brain, lung and muscle.
Systemically, resting quercetin levels are highly variable. The previous study noted resting quercetin levels (from food intake) ranging from 30-163uM/L. After supplementation of 50,100,and 150mg quercetin dihydrate serum ranges increased to 95.9-255, 164-497, and 240-1292 respectively. These concentrations are much larger than neural concentrations.
However, those are general ideas as quercetin bioavailability seems contradictory in many studies, possibly due to large differences between individuals and quercetin supplementation
Quercetin is a highly polar (water-soluble) compound, but seems to be able to cross models of the blood brain barrier. Mixed onion flavanoids (of which Quercetin comprises a large amount) appear to have around a 60% efficacy in crossing the BBB.
In vitro studies at 25-100uM show that quercetin is able to protect PC-12 neurons from oxidative stress induces from toxins and peroxides as well as inhibiting formation of beta-amyloid pigmentation. Protective effects against some ROS has been reported as low as 0.5uM of quercetin-3-glucuronide, which may be the only effects notable with basic quercetin supplementation due to low oral bioavailability and brain concentrations.
On the other side of things, quercetin can be a potential neurotoxic substance in supraphysiological levels. Quercetin has been noted in some studied to initially protect the neuron and later act as a toxin in in vitro studies. Quercetin seems to, in pure in vitro neuronal cultures, to induce toxicity at a concentration of 1-10uM but is protected to a degree in vivo by metabolization by glial cells around neurons. Inducing supraphysiological concentrations of quercetin in the range of 30uM-100uM in cultures with glia cells shows no signs of toxicity and increased survivial of neurons, however the effectiveness of the quercetin per unit is reduced by the protecting metabolization.
In regards to neuroinflammation, quercetin is able to act as an anti-inflammatory in the brain (and thus protectant of Alzheimers and Parkinsons, of which inflammation is an exacerbating factor) by increasing heme-oxygenase-1 expression which suppresses nitric oxide release induced from an inflammation response at concentrations as low as 10uM. Suppression of other pro-inflammatory markers, TNF-alpha and IL-1alpha, occurred with quercetin (and resveratrol) at concentrations as low as 0.1uM.
When fed to rats at doses between 10-40mg/kg bodyweight, Quercetin does not seem to influence movement or stimulation any more than control.
One animal model noted that Quercetin, dosed at 10,20, or 40mg/kg bodyweight taken orally, was able to reduce learning as assessed by the Y-Maze and Morris Water maze task yet no dose-dependence was noted. Only one other study has focused on cognition in healthy animals, and it was found that Quercetin was capable of deteriorating anterograde cognitive functions as assessed by the inhibitory avoidance task. The lowest dose used (10mg/kg), after body surface area conversions, correlated to 48.6mg Quercetin in an adult male human.
In these rats, lowered levels of phosphorylated CREB were noted. This protein (CREB) is activated when short-term memories are translated to long term memories via creating proteins and these proteins appear to be crucial to long-term memory storage. These may be downstream to a reduction in Akt phorphorylation also noted which appears to be a regulator of CREB. CREB phorphorylation was decreased by 28%, 37%, and 35% at 10,20,40mg/kg bodyweight and Akt by 29% (20mg/kg) and 53% (40mg/kg). The decreased phosphorylation of CREB paralleled that of CaMKII much more than it did Akt, and all results were recorded 1 hour after consumption. The authors hypothesized that Quercetin affects acquisitional memory.
Possible that Quercetin could adversely affect memory in healthy humans, but insufficient studies have been conducted
Effects on exercise performance look promising on a molecular level, but studies done on an applicative level are mixed.
Quercetin has been found to increase cardiovascular performance in untrained men compared to a placebo dosed at 500mg twice a day and mice studies note positive effects on exercise performance and mitochondrial biogenesis at doses between 12.5 and 25mg/kg BW. Quercetin has also been noted to increase exercise output in trained cyclists at 300mg, but was studied in conjunction with Green Tea Catechins (300mg) and caffeine(45mg) along many vitamins.
In the above study, the catechins and anti-oxidants alone increased performance but the addition of quercetin added to said increase. This may be due to possible synergism between many Bioflavonoids compounds (of which Green Tea Catechins are a subset).
On the negative side of things, quercetin failed to increase performance in repeated sprint intervals when dosed at 1000mg/day (in 2 doses) of quercetin-3-glucoside and did not increase performance time trials on an erg bike at an acute dose of 2g
So although the effects of Quercetin on exercise performance look promising, there is not enough conclusive evidence to state whether supplementation will increase performance.
Quercetin appears to be an inhibitor of the Heat Shock Response, a response to heat exposure that results in activation of heat-shock and heat-response proteins that can have wide-reaching effects such increasing intestinal permeability. Specifically, Quercetin has shown inhibition at the level of phosporylation and trimerization in the cytosol and downstream effects on promoter binding and results of genetic signalling (mRNA expression and protein accumulation). Through these effects, it may mitigate the anti-inflammatory effects of Heat-Shock Protein 70 (HSP70). In humans, 30mg/kg quercetin a day (averaged to 2,000mg Quercetin daily) taken with exercise was shown to increase urinary lactulose on day 1, and increase both lactulose and serum endotoxin on day 7 after heat acclimatization should have occurred. These results suggest impairment of intestinal permability acutely, and prevention of beneficial adaptations to heat over continual heat exposure associated with 2g Quercetin supplementation.
Quercetin, at 20mg/kg bodyweight, can prevent testicular damage from Dioxins and thus prevent a decline in testosterone levels; the mechanism seems to be through being an anti-oxidant present in the testes as it is the same mechanism by which quercetin protects the kidneys from Dioxins. Quercetin may also protect against physical injuries, as evidenced by rotating rat testicles 720 degrees clockwise.
Quercetin can increase aromatase activity 4x at a concentration of 100uM, but possesses inhibitory actions at a lower dosage (0.026uM). It shows some suppressive effects on mRNA transcription of aromatase in the corpus luteum and in a seemingly dose dependent manner, with 10uM being more potent than 100nM. Quercetin shows synergism with Apigenin in this regard. In intestinal cells, they do not influence mRNA levels but induce aromatase activity.
Using onion juice, a good source of Quercetin, testosterone levels can increase in rats after 4g/kg bodyweight daily for 20 days.
The biochemistry seems to be in line with an estrogen modulator; having the ability to regulate estrogen and androgen levels depending on its concentration.
A glycoside is a term used to refer to a molecule connected to sugar molecules. Glycosides tend to exist in plants as a storage form, and upon human consumption they can either be hydrolyzed into the molecule and sugars (two separate things to make note of) or remain bound together. For example, Cyanidin is a molecule while Cyanidin-3-O-Glucoside is a glycoside thereof that has some unique properties and can be detected in the blood after oral ingestion
Glycoside is a term that does not discriminate the sugar in concern, whereas the term glucoside may be used to refer to the same thing if the sugar is glucose; looking at the following list, Isoquercetin is both a glycoside (bound to sugar) and a glucoside (the sugar is glucose) but Rutin is only a glycoside and not a glucoside. The molecule with no sugars attached can be referred to an aglycone (without sugar) or aglucone (without glucose)
Quercetin-3-O-Rutinoside is more commonly called Rutin, and consists of a Quercetin molecule bound to the sugar rutinose; rutinose is a disaccharide of rhamnose and glucose (6-O-L-rhamnosyl-D-glucose). It can be found in a variety of plants alongside Quercetin, but is in high amounts in Ziziphus Jujuba leaves.
Quercetin-3-O-Rhamnoside is a glycone where Quercetin is attached to the sugar Rhamnose, found in high levels in Irvingia Gabonensis
Quercetin-3-Glucoside is a Quercetin molecule with a lone glucose sugar bound to the 3 carbon, and has the common name of Isoquercetin; it commonly co-exists with Quercetin in food products.
Quercetin 3-O-β-D-glucoside is structurally similar to Isoquercetin, but with modifications on the glucose moiety.
Another glucoside of Quercetin, where the glucose is attached to the 4' carbon rather than the common 3 carbon; this conjugate is sometimes referred to as Spiraeoside, and is found in the herb Filipendula ulmaria as well as common onions.
Quercetin bound to a galactose molecule (one of the two constituents of lactose) results in a glycoside known as Hyperin, with other names included Hyperoside; this glycoside is in relatively high levels in the leaves of the plant bearing Chinese Hawthorn
Quercetin-3,6-Malonylglucoside (Q3MG) is a monoglucoside structure with a malonyl attachment on the glucose moiety, found in high concentrations in the leaves of Morus Alba (260mg/100g) which exceeds that of onions, thought to be one of the best sources of Q3MG at 60-100mg/100g; Morus Alba leaf extracts (tea) is commonly consumed as an anti-diabetic therapy, although this may be more related to non-quercetin structures (the iminosugars, with 1-deoxynojirimycin being particularly important to Morus Alba).
Quercetin-3-O-robinobioside is a glycoside where Quercetin is attached to Robinose, Robinose being a rhamnose sugar attached to a galactose sugar and Robinoside being interchangeable with 6″-O-α-rhamnopyranosyl-β-galactopyranoside. This glycsoide is found in high levels in the leaves of Boerhaavia Diffusa.
Quercetin Rhamnohexoside is a glycoside found in high levels in the leaves of Gynostemma Pentaphyllum, alongside Quercetin Dirhamnohexoside (an additional Rhamnose sugar).
The following molecules are variants on Quercetin, where the structure is slightly modified but not due to the addition of a sugar molecule to the structure
Pentamethylquercetin is a molecule where the Quercetin molecule has been methylated five additional times, and constitutes up to 0.391% of the dry weight of the leaves of Kaempferia Parviflora
3-O-Methylquercetin is a structure where the 3-carbon (where many glycosides attach) is methylated, and appears to be the main bioactive in Rhamnus Nakaharai
Yerba Mate is a form of tea with a Caffeine content and a relatively unique blend of polyphenolic compounds. The saponin content of Yerba Mate synergistically works with Quercetin to suppress inflammation via NO and PGE(2).
(Common misspellings for Quercetin include querctin, qurcetin, quercetn)
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