Quercetin – Benefits and Side Effects

Introduction

A romantic vacation to France wouldn’t be complete without visiting the typical tourist sites, including the Eiffel Tower, the Notre-Dame Cathedral, and the Riviera. However, what most tourists come back remembering is the food. Soufflés and pastries litter bakery windows, as do assortments of breads, cheeses, and, of course, wine. And, as delicious as these foods are, they don’t fit the typical definition of health foods.

Yet, at the same time, France has a remarkably low rate of death by coronary heart disease, despite their high cholesterol and saturated fat diets. This has been dubbed the “French Paradox,” though it seems to be true for the entire Mediterranean region, and there are a number of hypothesis to explain it.

One of them is the abundance of red wine that French citizens drink, which seems to correlate with overall good health, though the mechanism isn’t completely understood. One of the compounds in the wine that may account for the effect is quercetin, a molecule found in plants and fungi which prevents hormones from moving to places they don’t belong.
What does quercetin do, exactly? How can it help humans? Where can we find it in our diet? And are there any risks in taking it as a supplement?

What is Quercetin?

Quercetin is perhaps the best studied member of the class of organic compounds known as flavonoids, which are responsible for giving plants their bright colors and attracting pollinators. However, to understand the main purpose of quercetin, one needs to start with the plant hormone auxin. Auxin regulates the growth and development of plants. Though auxin can be found throughout a given plant, the concentration in an area is what provides instructions for how the plant should grow there. Quercetin ensures that these auxins remain where they belong and aren’t directed elsewhere. So quercetin’s main job is to prevent auxins from being transported in specific directions — this is why it’s referred to as a polar auxin transport inhibitor.

Though this is its main purpose in plants, quercetin and other flavonoids have properties that may make them beneficial to humans. Chief among these is their ability to prevent oxidative damage of cells, which classifies them as an antioxidant. We consume quercetin every day, assuming we’re eating a balanced diet with plenty of varied fruits and vegetables and, though research is ongoing, there are numerous studies to suggest that there are many benefits to ensuring we get enough of it.

Quercetin in Foods

Quercetin is found in all kinds of plant-based food, particularly vegetables. Onions are a particularly dense with the compound, having about 300 mg/kg, as is kale, with about 110 mg/kg. Broccoli and lettuce also have relatively high quercetin contents. Fruits don’t often have as much quercetin as these vegetables do, but we may consume more in a given serving. That is to say that although onions do have a lot of quercetin, we generally don’t eat very much onion at any given time. Apples may only have 50 mg/kg of quercetin, but we’re much more likely to eat an entire apple than we are to eat an entire onion. Based on the sizes of these foods and their quercetin density, it’s likely that a given meal may provide someone with perhaps a few milligrams of quercetin.

That’s about the same amount that one would find in a single glass of red wine. White wine does not have very much quercetin at all — the amount would barely be detectable in a single glass — and neither do beer or milk. Tomato juice, lemon juice, and grape juice have roughly the same amount by volume as wine does, though other fruit juices have significantly less.

However, if one wants the highest concentration of quercetin, look to tea infusions, which can have as much as 25 mg/ml. Again, though, generally a serving of wine or juice is larger than that of tea, so, in the end, we get roughly the same amount whether we’re drinking red wine, grape juice, or tea.

All in all, while the specific number varies, studies have concluded that the average person consumes anywhere from 25 to 50 milligrams of quercetin per day.

Benefits of Quercetin

Many of the benefits of quercetin seem to relate to its antioxidative properties. While oxygen is often seen as our most precious resource and we can’t live more than a few minutes without it, it’s also a very dangerous and reactive gas. It reacts, for instance, with LDL (“bad” cholesterol) to create fatty deposits in our arteries. Antioxidants such as quercetin may help prevent this process from occurring, which may partially account for the French Paradox and explain the link between quercetin and cardiovascular health.

Overall, research supports the notion that quercetin reduces blood pressure in animals as well as humans and studies have even identified particular genes that make an individual more likely to experience positive effects from the supplement. There aren’t any definitive explanations for how quercetin seems to help in this regard, though it’s been observed that daily supplementation can improve the health of a thin structure on the inside of vascular walls called the endothelium, which is responsible for dilating and constricting veins. Its dysfunction commonly leads to a number of cardiovascular diseases, so this may be one of the means by which quercetin improves cardiovascular health.

Studies in dogs and monkeys have found that quercetin can prevent blood platelets from accumulating into a clot. This would suggest that quercetin could decrease the risk of stroke. However, a large-scale study from 2000, looking at the diets of 10,000 Finish citizens didn’t find a direct link between quercetin consumption and reduced risk of stroke. However, the same study did find that apples, a food high in quercetin, did seem to prevent strokes. As the sources of quercetin can affect the absorption of the compound, it’s not completely impossible for the quercetin to still play a role here.

There is some preliminary evidence that quercetin may extend the lives of rats with leukemia. There’s a theoretical possibility that, by acting as an antioxidant, supplemental quercetin can prevent DNA damage and, perhaps, lower the risk of some forms of cancer — including lung cancer — though the current studies don’t indicate that this is a strong effect, if it even exists at all. It is known that quercetin can block the effects of certain carcinogens, though the amount matters here as there’s also the possibility it can also enhance their effects if the dosage is too high. Overall, the evidence seems to be leaning towards quercetin being more beneficial than harmful when it comes to cancer.

Some of the benefits of quercetin aren’t well understood. While it’s known that quercetin blocks certain inflammatory enzymes from acting, it’s not entirely clear how it does so. The end result, however, is that quercetin has anti-inflammatory effects.

Quercetin also has antiviral properties. In several studies, it was shown that the compound surrounds certain viruses, including herpes simplex type I, and can prevent their RNA from duplicating. However, most of these effects were only demonstrated by observing viruses directly in a laboratory setting, as opposed to in a host, where the viruses or the quercetin may act differently. This has led to some research looking into the use of quercetin and other flavonoids in preventing HIV, which, although theoretically possible, hasn’t been demonstrated to be effective at this point in time.

Overall, the strongest data for the benefits of quercetin seems to be in relation to cardiovascular health, though the particular mechanisms are not completely understood at this time.

Side Effects of Quercetin

Quercetin, when taken orally, typically enters the body in a form that dissolves in water and, while different studies have found different results, most conclude that it isn’t well-absorbed by the small intestine. The absorption that takes place seems to happen in the large intestine, as evidenced by the absence of absorption in subjects who don’t have one. Even in the large intestine, however, quercetin isn’t particularly well absorbed: no more than a few percent of the amount we consume makes it into our body unaltered. As a result, it’s unlikely someone could ingest enough to reach toxic levels. Still, in the interest of safety, several studies have looked at potential risks related to quercetin.

One way to test how the body absorbs a given substance is to attach a radioactive element to it and detect where the isotope ends up. Using this method on rats, scientists found that consumed quercetin mainly ended up in the large intestine, unabsorbed. A small amount of the radioactive substance, carbon-14 in this case, ended up in urine, indicating that the body had absorbed the quercetin, though most of it had been broken apart before that point.

This poor absorption indicates that it’s unlikely orally administered quercetin can have severe risks. However, it appears that quercetin is safe even when directly injected into the bloodstream in high amounts. Such an experiment was done with rabbits in a study from the 1950s and no toxic effects were noted, at least in the short term. Likewise, other studies have shown no deviations from controls in several parameters, including body and organ weight, though one detected decreases in hemoglobin and hematocrit levels without providing numbers to indicate if the difference was statistically different or not.

In the long term, some studies have found some possibilities of quercetin being carcinogenic, with rats developing brain tumors after being given quercetin supplementation, though it was only the male rats in this study that developed the tumors, which suggests that either there’s a hormonal link to the effect or that it was just a statistical anomaly. This is somewhat confounding as other studies have found that quercetin reduces risks of cancers and others still found no link. It’s possible that quercetin is toxic to cancer cells and certain types of normal cells, but not others. More research is clearly needed here.

Another study where researchers gave very high doses to rats found that it didn’t have any effect on either male or female fertility. However, a laboratory study found that it’s possible that quercetin affects a particular gene in a way that could lead to infant leukemia. Though there’s no hard data linking the supplement with the disease, and rat studies looking at fetal development found no negative effects from dietary quercetin. However, in the interest of safety, pregnant women or women who may become pregnant should consult a doctor before adding a quercetin supplement to their daily regimen.

Studies specifically looking at humans have generally had low sample sizes, though none of them have reported any negative outcomes from quercetin taken orally. A study looking at cancer patients given intravenous injection of the compound found that subjects experienced increased pain at the injection site, difficulty breathing, vomiting, and signs some mild kidney damage when given doses at levels of roughly 100 times the average daily intake. All of these symptoms were short-lived.

Other studies looking at populations as a whole found no negative correlation between quercetin intake and cancers, though one found that higher levels of quercetin correlated with lower incidences of cancer, especially lung cancer. It should be noted that these population studies can’t determine causation and there may be other factors at play. For instance, a diet high in quercetin may indicate that the person generally eats well. Someone who eats well is generally concerned with their health and won’t smoke. As a result, they’d be less likely to develop lung cancer.

Overall, the data supports the idea that quercetin is safe for most people in normal doses, however certain members of the population should exercise care. Those taking prescription drugs should consult their doctors before adding quercetin supplements to their regiments. Quercetin can prevent some of these drugs from working by blocking the enzyme that allows our body to metabolize them. Additionally, while it was noted that quercetin could prevent the risk of stroke by preventing platelets from accumulating, this same effect could cause excess bleeding when the supplement is used alongside blood thinners.

Quercetin Supplementation

While the FDA doesn’t have specific recommendations for supplemental quercetin dosages, capsules contain around 500 mg apiece, which should be a safe daily dose. Quercetin can be purchased by itself or in capsules where it’s combined with other compounds such as bromelain — an enzyme found in pineapples that is also thought to have anti-inflammatory effects and help the absorption of quercetin — and vitamin C — which, like quercetin, can function as an antioxidant.

There are currently no known risks of interactions between these three compounds, though quercetin can react with some medications and antibiotics, so consult a doctor before beginning supplementation.

Summary

Quercetin is a compound found in many plants, which they use to help regulate their growth. We can get it from many fruit and vegetable sources as well as from red wine and tea. There are many potential benefits to quercetin including the possible reduced risk of stroke and cancer, though many of these aren’t completely confirmed. It may be at least partially responsible for the French Paradox, which notes that French citizens have low incidences of cardiovascular disease, despite consuming diets high in saturated fats.

For most people and in typical doses, quercetin is safe, owing in part to the fact that our body doesn’t absorb it particularly well, though studies have suggested possible risks, including some types of cancer. Those who are taking prescription medication and/or blood thinners should consult a doctor before beginning supplementation, as should women who are pregnant or are planning on becoming pregnant in the near future.

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