Many of us have heard about polyphenols. They are naturally-occurring substances in plants that make up their immune system.
They protect the plant from the environment and damaging pathogens. We can divide polyphenols into two groups. They can be flavonoids or non-flavonoids, and flavonoids can be subdivided, too.
Quercetin is a flavonoid from the flavonol subclass. It can be found in many vegetables and fruits, but also tea. In this article, we are going to explore the most interesting details about this substance, what it does to the human body, and how to use it for your advantage.
What Is Quercetin?
Quercetin is a common flavonoid found in plants. We can find it in many variations, and the majority also have pigment functions. It contributes to giving vivid color fruits have and protect them from harm.
It has a very strong antioxidant activity and avoids UV ray damage to the plants.
Humans can also seize the defense potential of quercetin. It exhibits a powerful antioxidant potential. As such, it is beneficial for heart disease, diabetes, and even prostate cancer.
Chemically, quercetin is very similar to other flavonoids. It has two aromatic rings and one heterocyclic ring. The most common form is called rutin, and it is glycosylated.
In other words, there’s an added glycosyl group. This group is made up of sugar, which can be glucose, routine, or rhamnose. When it has a glycosyl group, quercetin becomes more soluble in water (1).
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What are the health benefits of quercetin?
Similar to the majority of flavonoids, quercetin has impressive antioxidant potential. This property is usually behind the majority of its health benefits. However, it is also anti-inflammatory and antihypertensive.
Quercetin has anti-obesity properties and may lower lipid levels in the blood. However, all of these health effects depend on the dose and form of quercetin you’re using. For example, the glycosylated variants have more anti-inflammatory activity (2).
The most important health benefits are as follows:
Antioxidant properties
In many cases, our endogenous antioxidants can handle free radicals. But when there’s an imbalance, we get something called oxidative stress. It is a condition where too many reactive species (ROS) are produced.
The body makes efforts, but can’t neutralize ROS production by itself. Free radicals cause alterations to the structure of molecules and tissues. As they continue doing so, they cause dysfunction and chronic disease. Quercetin is one of the most important dietary antioxidants.
It scavenges free radicals and clears them out by sacrificing its own structure. But then, other antioxidants can transform quercetin back to its original form. That’s why quercetin works better with adequate levels of vitamin C or glutathione (3).
Additionally, quercetin stimulates the expression of antioxidant enzymes. It activates a signaling pathway that promotes and supports endogenous antioxidants. Moreover, it is known to chelate iron, reducing the oxidative damage from iron overload (4).
Prostate health benefits
The effects of quercetin in prostate health are especially relevant in patients with prostate cancer. However, it can be used for chronic prostatitis, too. Quercetin can modulate the progression of cancer by a combination of anti-proliferative effects.
Quercetin promotes apoptosis in various cell lines and slows down the progression of prostate cancer. It is also an anti-inflammatory and does not allow cancer to use chronic inflammation to its benefit.
Additionally, quercetin works inside the cell, antagonizing aberrant signaling. Mutated cells are forced to die or inhibits their aggressive behavior. Thus, many researchers have described it as a potential chemotherapy agent (5).
Allergy and asthma relief
The antioxidant properties of quercetin may also slow down the pathogenesis of asthma. Clinical trials show that a higher intake of quercetin reduces the incidence of asthma. And it appears that the best source for this particular effect is apples as opposed to an onion, red wine, and tea.
Besides the effect of quercetin in oxidative stress, it has been found to modulate histamine and mast cells. It is a natural antihistamine. This substance is essential to trigger allergy when it is overproduced by the body. It also modulates the immune system, suppressing eosinophil activation.
These white blood cells are also associated with inflammation and allergies. As such, quercetin has an important potential to relieve asthma and allergic rhinitis (6). This anti-inflammatory activity may also work for patients with rheumatoid arthritis.
Reduction of cardiovascular risk
Flavonols like quercetin are antihypertensive. They reduce blood pressure by dilating the blood vessels. Moreover, it appears quercetin has a role in the renin-angiotensin-aldosterone system.
As such, it can be used as a natural antihypertensive agent. And you don’t usually need very large doses to start experiencing changes. According to studies, even low consumption of quercetin-rich foods can lower cardiovascular risk (7).
Quercetin improves vascular function in the epithelial cells of blood vessels. As such, it reduces or slows down the formation of an atherosclerotic plaque. It inhibits oxidation of low-density lipoprotein (LDL). LDL oxidation is fundamental to cause atherosclerosis. Moreover, it regulates cholesterol metabolism and improves circulating levels of fatty acids (8).
Side treatment of type 2 diabetes
Quercetin works as a treatment for patients with diabetes in various ways. For example, it inhibits the enzyme aldose reductase. This enzyme converts glucose into sorbitol. By blocking the enzyme, quercetin prevents the accumulation of polyols in the lens. Thus, it reduces the risk of cataracts in diabetic patients (1).
On the other hand, quercetin is also useful in promoting insulin sensitivity. It helps to activate GLUT-4. It increases second messengers in the cell in response to insulin.
Thus, it reduces insulin resistance. It may also increase the number of insulin receptors in the target cells. Moreover, it also helps to reduce the fast absorption of carbohydrates by the intestines (9).
Other potential benefits
Besides the health benefits described above, other studies have explored others. For example:
Reduction of cancer risk
Similar to prostate cancer, quercetin is known to be useful for other types of cancer. It stimulates apoptosis in many cell lines. It inhibits the cell cycle and suppresses tumor growth. Moreover, studies show that quercetin reduces the risk of metastasis.
It deprives the tumor of blood flow by reducing angiogenesis. In other words, it doesn’t allow cancer cells to form new blood vessels. The tumor keeps growing, and not enough blood vessels are formed.
Thus, it doesn’t have the nutrients it requires to keep on growing. For these and other effects, quercetin is potential chemotherapy for cancer (10).
Anti-Alzheimer effect and other neurologic effects
The anti-inflammatory and antioxidant properties of quercetin may also improve our neurological function. It has a neuroprotective potential derived from its antioxidant effect. Moreover, in Alzheimer’s disease, it prevents the formation of amyloid plaques.
It improves the form and function of the mitochondria in Alzheimer patients, improving cellular function. It is what makes coffee a neuroprotective substance. And the protection is also for patients with Parkinson’s disease and other neurodegenerative diseases. Quercetin also induces the activation of GABA receptors, improving sleep (1).
Anti-obesity potential
Quercetin inhibits the expression of fatty acid synthase. As such, it inhibits the creation of new fatty acids. Even on a high-fat diet. It also suppresses the formation of adipose tissue. In preadipocytes, quercetin can slow down cell proliferation and may even cause apoptosis.
In the clinical practice, patients with obesity reduce their cholesterol levels and inflammatory profile (measured by C-Reactive Protein, CRP). It also raises their HDL levels, which is known as good cholesterol (11).
In animal studies, quercetin works to reduce weight in obese mice. However, in humans, it is more difficult to replicate these findings. Humans need a combination of strategies along with quercetin to reduce weight. Still, it is a useful tool to consider, and it does have a significant role in reducing the health burden of obesity.
Food sources and dosages
Quercetin is one of the most widely available flavonols in nature. Actually, up to 75% of flavonols we consume come from quercetin variants. It is found in many fruits and vegetables.
The ones with a higher concentration are apples, onions, and wine. Other plants such as pepper and coriander are also significant sources of quercetin. It is found in pepper, fennel, dill, radish, and green tea.
In most cases, quercetin content is higher in hot peppers, red lettuce, cocoa powder, onions, and cranberry. The higher quercetin content in foods can be found in capers. However, the polyphenol contents in these foods depend on several factors. Harvesting conditions, growing conditions, and variants of each plant species can have different quercetin concentrations (12).
The typical dose of dietary quercetin varies between 5 and 80 mg. It depends on diet and location. For example, in the United States, the diet includes approximately 13 mg of bioflavonoids.
One-third of them will be quercetin and quercetin variants. In China, a mean of 4.73 mg of quercetin is consumed every day in the diet. In individuals who consume plenty of tea and vegetables, they can reach 250 mg a day. However, in the majority of cases, it is very uncommon to reach such levels.
The recommended dose, if you want to obtain the full benefits of quercetin, is a bit higher. In the majority of clinical trials, we tend to see doses of 500 mg a day. Some people may even use 1000 mg a day.
Quercetin supplements
There are many food sources of quercetin. However, the concentration in food is enough to protect plants, not humans. If we want to seize the health effects of quercetin, we need a very high dose.
According to studies, you need 10-60 times the daily dose of quercetin in the diet. That is why quercetin supplements are superior to dietary quercetin in this particular (7).
If you’re looking for the best quercetin supplements, there are a few things you might want to consider:
- Quercetin variants: As mentioned above, there are various types of quercetin. For example, it can be glycosylated or not. Similarly, oral quercetin can be absorbed faster in some variants. The variant with the highest bioavailability is quercetin dihydrate.
However, you might want to go for quercetin glycoside if you’re primarily looking for anti-inflammatory properties.
- Organic ingredients: It is useful to ask if you’re dealing with organic ingredients. Some supplements are made with GMO ingredients and may not be the best choice. If possible, look for a non-GMO label.
- Purity: In some cases, the concentration advertised in the label is not true. Thus, try to look for a reliable seller. Do not buy the supplement through third-party agencies. If possible, buy directly with the manufacturer.
- Additives and fillers: Try to find supplements with no fillers and additives. If you find additives listed in the ingredients, look them up. Make sure they are not allergens, especially if you suffer from atopy.
Safety and side effects
Safety concerns about quercetin are considered in many studies. But the majority of them do not report any side effects. Still, it is recommended not to use excessive doses that may trigger unexpected adverse events. Studies have evaluated quercetin at a dose of 500 mg twice a day.
A total of 1000 mg of quercetin daily has no side effects when it is taken for 12 weeks. No study has evaluated this concentration for a longer time, and it appears to be very safe.
Long-term use and higher doses have not been thoroughly assessed. Therefore, we recommend not exceeding these doses in oral supplementation.
Among the side effects after oral administration of quercetin, we have the following (13):
- Headache
- Tingling in the arms and legs
Other studies have evaluated the effect of quercetin administered intravenously. These studies used doses of 722 mg and lower. Higher doses are not recommended. The side effects of intravenous administration of quercetin may include the following (13):
- Facial flushing
- Nausea and vomiting
- Sweating
- Difficulty to breath
- Kidney damage with excessive doses
There is not enough information to evaluate the safety of quercetin for pregnant or breast-feeding mothers. Even though no adverse events are still reported, it is better to avoid using these supplements in such circumstances.
Since high doses of quercetin may cause kidney damage, it is recommended to stay away if you have kidney problems.
Also, if you’re taking other medications, consider that quercetin can interact with the following (14):
- Quinolone antibiotics: This group of antibiotics includes ciprofloxacin and levofloxacin. Quercetin may reduce the effectiveness of these antibiotics.
- Cyclosporin, amiodarone, verapamil: These medications are changed in the liver. Quercetin might interact with their metabolism. It increases the effects and may cause unexpected side effects when taken along with these medications.
- Other drugs: The list of medications changed by the liver is very large. Thus, ask your doctor if you use drugs for chronic disease. Other drugs that may interact with quercetin include medications moved by cell pumps—for example, itraconazole, digoxin, cimetidine, and ranitidine.
Conclusion
Quercetin is a naturally-occurring substance in fruits and vegetables. It protects them from environmental damage and does the same for humans. However, we need higher doses than those found in dietary sources.
Supplements of quercetin are beneficial for prostate cancer, other types of cancer, allergies, chronic inflammatory conditions, and neurodegenerative diseases. It has a big therapeutic potential with little to no side effects.
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