Quercetin: Benefits, Side Effects, and Research
Found in many fruits and vegetables, quercetin may have some potential in the context of aging. Here, we take a look at this natural antioxidant and popular dietary supplement.
What is quercetin?
Quercetin is a natural flavonoid, which makes it part of a large family of water-soluble plant compounds. Flavonoids are not created by the body and are an important part of a healthy diet.
Albert Szent-Györgyi discovered flavonoids in 1936 while researching ways to treat scurvy. He won the Nobel Prize in 1937 for his “discoveries in connection with the biological combustion processes with special reference to vitamin C and the catalysis of fumaric acid”.
We have much to thank him for, especially his discovery of the flavonoids, particularly quercetin. Flavonoids were once known as vitamin P for permeability. This was because some flavonoids can influence the permeability of blood vessel walls.
What is quercetin used for?
Traditionally, people take quercetin because they believe it is effective in managing various issues, such as supporting the brain and immune system, slowing down skin aging, improving NAD+ levels, removing senescent cells as a senolytic, reducing blood pressure, and alleviating allergies.
What does quercetin do?
Quercetin is one of the most common antioxidants found in the diet and plays a key role in reducing damage to cells from free radicals. It may also have broader anti-inflammatory properties, as it reduces the activity of nuclear factor kappa B (NF-κB), a pro-inflammatory molecule. It also appears to reduce the inflammatory response of macrophages. Some research suggests that it is also a senolytic when combined with other drugs and may destroy senescent cells, which are another reason we age.
Foods rich in quercetin
Quercetin is present in a variety of fruits and vegetables.
|Food||Quercetin (mg per 100 g)|
|Raw hot/yellow peppers||50.73|
|Raw red onions||39.21|
|Raw hot/green peppers||14.7|
|Raw red leaf lettuce||7.61|
|Raw white onions||6.17|
|Golden delicious apples||3.69|
|Raw sweet cherries||2.29|
Due to the variable nature of dietary sources, many people opt to use quercetin supplements to ensure that they are getting enough. A typical dietary supplement contains 500 mg of quercetin, which is many times higher than is typically present in the average diet.
Isoquercetin (also known as isoquercitrin) is related to quercetin and has better bioavailability . It is harder to get than regular quercetin and may be more expensive, though smaller amounts are required, as it is around six times more potent.
Potential quercetin benefits
Some data suggests that quercetin intake may have a beneficial impact on health, and we have taken a look at some studies and potential benefits of quercetin.
Quercetin is an antioxidant
Quercetin has shown antioxidant properties by neutralizing and scavenging reactive oxygen and nitrogen species [2-3]. Our bodies produce free radicals as a byproduct of our mitochondria making energy, which is likely to be one of the reasons we age.
It also has anti-bacterial [4-5], anti-inflammatory , and anti-carcinogenic [7-12] properties.
Quercetin supports efficient protein production
Quercetin has the ability to enhance proteolysis and maintain proteostasis [13-14]. Proteostasis ensures that proteins are created and folded correctly before use in the cell. The loss of proteostasis leads to misfolded proteins and is another reason we age.
Quercetin appears to support the brain and immune system
Animal studies with aged mice showed that quercetin improved general and spatial awareness . The same mice also had an increased level of exploratory behavior, which is typically seen in younger mice.
Another study showed an improvement of the immune cells of artificially aged progeric mice . While these mice are not an exact emulation of aging, they do provide useful data for studying aging.
Quercetin may slow down skin aging
Topical treatment increases the hydration and elasticity of the skin, reducing wrinkles . The skin is also easy to access, unlike internal organs, making it an ideal target for anti-aging studies using flavonoids.
Quercetin has also shown efficacy in treating skin disorders such as dermatitis [18-20].
Quercetin may improve NAD+ levels
This compound reduces inflammation and increases NAD+ levels by reducing inflammatory factors such as CD38 [21-23]. NAD+ is in every cell, interacts with sirtuins to regulate metabolism, and plays a role in health and longevity. CD38 increases with age and consumes NAD+, making our metabolism increasingly dysfunctional.
Quercetin as a senolytic
As we age, increasing amounts of dysfunctional, non-dividing senescent cells accumulate. These damaged cells are normally removed by the immune system, but as we age, this system declines and more and more of these cells build up. Senescent cell accumulation is another reason we age.
Senescent cells secrete a cocktail of pro-inflammatory signals known as the senescence-associated secretory phenotype (SASP). The SASP generates chronic levels of inflammation and an altered cell-to-cell signaling environment. This persistent inflammation, called “inflammaging”, is believed to be one of the drivers of aging [25-26].
Senolytics are compounds that can induce senescent cells to enter apoptosis. Inducing this cellular self destruct could be a potential way of dealing with accumulated senescent cells.
Quercetin reduces inflammation by inhibiting some elements of the SASP. It can also directly induce apoptosis when used in combination with other drugs to improve vascular health [27-28].
Quercetin may reduce blood pressure
This flavonoid has shown promise in reducing blood pressure, with some positive results for people with hypertension . A detailed meta-analysis on quercetin and blood pressure was also conducted .
Quercetin may help with allergies
Quercetin has been shown to halt immune cells from releasing histamines, chemicals that trigger allergic reactions. This antihistamine effect may reduce the symptoms of allergies, such as runny nose, watery eyes, hives, and facial swelling.
Safety of quercetin and its side effects
No serious adverse effects from quercetin use have been reported. In fact, studies have shown that it can be taken safely at 500 mg twice daily for 12 weeks. Longer-term use and higher doses remain an unknown at this time. Some people may experience headaches, stomach aches, or tingling sensations if taking more than 1000 mg of quercetin in a day. As with all supplements, if you do experience adverse effects, you should cease taking it and consult your doctor.
This article is only a very brief summary. It is not intended as an exhaustive guide and is based on the interpretation of research data, which is speculative by nature. This article is not a substitute for consulting your physician about which supplements may or may not be right for you. We do not endorse supplement use or any product or supplement vendor, and all discussion here is for scientific interest.
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