What Is Quercetin and What are its Benefits & Side Effects?

by Ahmed Zayed, MD on July 9, 2020
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From the many different flavonoids, it is the quercetin that is known as the master flavonoid. You will quickly learn, quercetin is truly one of the best flavonoids out there. Being able to prevent, but also treat, numerous health issues, thus, improving the overall quality of your diet and life, quercetin is a favorite among many. 

Today we are here to explain why that is. If you are interested to learn more about the natural quercetin, but also the many quercetin supplements found on the market today, then you came to the right place. Let’s start off by explaining what quercetin really is and what it is capable of doing. 

What is quercetin?

Quercetin is a pigment and a part of a group called flavonoids. You probably have heard about the popular flavonoids, being the plant-based antioxidant pigments that they are with their multiple benefits for our health (1). Quercetin, like the rest of the flavonoids, can be found in veggies, fruits, grains, tea, and even wine. 

It is flavonoids that give the plants their characteristic color. Flavonoids belong to the class of polyphenols. Researchers have been quite interested in polyphenols in the last couple of years. Science shows that polyphenols can prevent a long line of chronic diseases, including cancer and heart disease, among others. 

It is thanks to its strong antioxidant properties that quercetin and the rest of the flavonoids can bring upon all of the health benefits that we will discuss shortly (2). After having learned the importance of quercetin, researchers have developed quercetin supplements. 

There are still some divided opinions when it comes to the daily use of quercetin supplements due to the lack of scientific research. We are going to discuss the use of such supplements, but first, let’s review the many beneficial effects of quercetin. 

The many health benefits of quercetin

There are many potential health benefits that the daily use of quercetin has to offer. Here are some of the greatest health benefits caused by quercetin.

 

  • Reduced number of free radicals 

 

Quercetin is best known as a strong antioxidant flavonoid. Quercetin is said to be an even more powerful flavonoid than Vitamin C, Vitamin E, and beta carotene. If you did not know, antioxidants are working towards protecting the body by eliminating free radicals. Free radicals are unstable molecules that have been scientifically proven to contribute to the development of various chronic diseases (3). 

 

  • Reduced risk of several types of cancer

 

Quercetin has supposed anti-cancer properties, ones that can shield you from a number of different cancer types. Over the past few years, there have been both in vitro and animal studies that have demonstrated the power of quercetin to prevent cancer. 

A 2015 report has shown that quercetin has successfully stopped the growth of prostate cancer cells in rats and mice (4). Then another study done in 2018, demonstrated similar results, but this time, however, it was an in vitro study (5). Other studies have gotten similar results while using quercetin against liver, lung, colon, ovarian, and other types of cancer (6, 7). 

 

  • Relieved inflammation

 

Having high levels of free radicals in the body can influence the genes that promote inflammation, hence leading to an increased inflammatory response. The presence of chronic inflammation in the body is being linked to numerous health issues. The list includes heart disease, kidney disease, and certain cancer types, among others. 

Luckily, the use of quercetin can help you reduce the number of free radicals in the body, thus relieving the present inflammation as well. An 8-week study showed that taking 500 mg quercetin per day has helped individuals with rheumatoid arthritis to relieve their symptoms otherwise caused by the present inflammation in their joints (8). Knowing this, quercetin is also highly recommended to anyone who is struggling with arthritis. 

 

  • Prevention of infections

 

Anti-inflammatory, anticancer, and antioxidative properties are not the only ones that can be found in quercetin. This powerful flavonoid also carries strong antibacterial properties as well. These properties are helpful against any type of bacteria, which means that quercetin can efficiently prevent several bacterial infections, including stomach, skin, respiratory, and urinary infections. 

 

  • Relieved allergy symptoms

 

Quercetin is also thought of as a strong antihistamine since it possesses the ability to prevent the release of histamine, which is otherwise known as the inflammatory chemical involved in the characteristic allergy symptoms (9). Studies that have been performed in both animal subjects and in vitro have shown that quercetin can help fight allergic rhinitis (10).

  • Reduced risk of heart disease

Obesity is one of the major risk factors for heart disease, but also heart attack. On the other hand, eating a diet rich in fruits and veggies is one of the best natural ways to prevent heart disease. This is one of how flavonoids are helping to prevent heart disease. 

But flavonoids, including quercetin, are also capable of lowering high blood pressure, which is yet another big risk factor that contributes to the development of heart disease. According to a study done by the American Heart and Stroke Association, taking quercetin supplements is an effective way to reduce high blood pressure (11).

And yet, the benefits do not stop there. Quercetin is also able to reduce the high LDL “bad” cholesterol levels. Having high cholesterol levels exposes you to the risk of heart disease and heart attack, among other health issues. A 2016 study has shown that only 150 mg quercetin per day was enough to cause a significant decrease in the LDL cholesterol levels (12).

 

  • Improved blood sugar control

 

Diabetes is one of the major chronic diseases of the 21st century. Millions of people each year are losing their lives because of the complications that this disease can lead to. If you too are at the risk of developing diabetes, we recommend trying out quercetin. Quercetin has been able to reduce the fasting blood sugar levels and protect against further complications due to diabetes, according to several studies done over the years (13, 14).

 

  • Reduced risk of brain disorders

 

Oxidative stress has been found guilty of causing neurological disorders such as Parkinson’s disease and Alzheimer’s disease. Since quercetin has been scientifically proven to be able to reduce the free radicals and oxidative stress in the body, its antioxidant properties are considered helpful in the prevention of these brain disorders. 

 

  • Improved physical performance

 

Quercetin may also be beneficial for anyone who is looking to improve their athletic performance. One review of a total of eleven studies has shown that quercetin may improve athletic performance (15). 

Another study done on professional athletes has revealed that the participants have experienced an improvement in their lean body mass, total body water, metabolic rate, and total energy expenditure after having used quercetin daily (16). Perhaps taking a quercetin supplement or simply eating a red apple, orange, or red grapes before and/or after your workout can help you gain such beneficial effects as well.

The best food sources of quercetin

As we mentioned earlier, quercetin is naturally found in many tasty fruits and veggies, among other food sources. Quercetin is particularly present in the outer layer of the peel of these foods. Some foods are richer in this flavonoid than others. And so, here is a list of the biggest natural sources of quercetin. 

  • Onions, both white and red;
  • Cooked asparagus;
  • Capers;
  • Eggplant;
  • Peppers, especially yellow and green;
  • Cherries;
  • Celery;
  • Tomatoes;
  • Red grapes;
  • Citrus fruits;
  • Broccoli;
  • Kale;
  • Shallots;
  • Buckwheat;
  • Red apples;
  • Red leaf lettuce;
  • All types of berries;
  • Green and black tea;
  • Red wine, etc. (17)

Some herbal remedies such as ginkgo biloba, American elder, and St John’s wort contain quercetin as well. It is worth mentioning that the conditions in which the food has been grown play a great role in the amount of quercetin found in those very same foods. 

For example, a study published in the Journal of Agricultural and Food Chemistry has shown that organically grown tomatoes contain up to 79% more quercetin as compared to the tomatoes that have been conventionally grown (18). Another study has shown a similar difference in the amount of quercetin found in organically and conventionally grown onions (19). However, such difference has not been seen speaking of organic and conventionally grown bell peppers (20). 

The use of quercetin supplements

By introducing the foods and drinks which we mentioned earlier, you can satisfy your needs for quercetin daily. However, lately, a variety of quercetin supplements came available on the market. Various brands are offering their version of quercetin supplements meant for daily use. Such supplements can be purchased online or in retail stores. They can also be found in different forms, such as tablets, capsules, and powders, depending on the brand. 

Keep in mind that quercetin supplements, like most supplements in general, have not been tested for safety by the Foods and Drug Administration (FDA). This means that they are yet to be regulated as there is a large possibility for the actual content of the product to be different from what is shown on its label. Despite the fact that there have been some scientific studies that investigated the effectiveness of quercetin supplements, it is still unclear whether or not these supplements are safe to be used long-term. This, of course, is not the case with the natural quercetin found in foods and drinks. 

The body struggles to absorb quercetin properly when it is present on its own due to its low bioavailability. That is why the majority of quercetin supplements have also included Vitamin C and/or digestive enzymes, such as bromelain, to help increase the absorption. 

Daily dosage & safety of quercetin

For most quercetin supplements, the daily dosage ranges anywhere between 500 to 1000 mg per day, with 500 mg per day being the most commonly used dosage. As a part of scientific studies, quercetin has been used in amounts of 1000 mg twice a day for a period of 12 weeks (21). The safety of its long-term use is yet to be determined.

Possible side-effects caused by quercetin supplements, including headaches, tingling in the arms and legs, nausea, and possible kidney damage. Due to the lack of further safety data, the exact number of side-effects caused by quercetin supplements is hard to be determined. 

It is unknown if quercetin supplements can interact with other prescription and over-the-counter medications, as well as other dietary supplements. Knowing that it is always a good idea to consult a medical professional before you use any quercetin supplement. The same rule applies to any woman who is pregnant or breastfeeding and anyone who has been diagnosed with a chronic disease. 

As for quercetin found in food – its daily use has been marked as safe and, as such, is highly preferred over any quercetin supplement. So far, there have been no side-effects whatsoever, caused by quercetin, which is naturally found in many foods and/or drinks that we mentioned before. The naturally found quercetin is also not considered harmful or dangerous during pregnancy or breastfeeding unless a medical professional has said otherwise. 

Conclusion

Quercetin is a natural flavonoid found in many of our favorite fruits and veggies. It is also found in red wine, giving us yet another reason to treat ourselves with a nice, cool glass of red wine. Enriched with many powerful properties – from antioxidant and anti-inflammatory to antibacterial properties and more, quercetin is able to help you resolve many health issues that you may be struggling with.

This powerful flavonoid is recommended to anyone who is experiencing high blood pressure, diabetes, arthritis, and even allergic rhinitis. Do not make the mistake of missing out on this wondrous flavonoid. Rethink your diet and start introducing more of these quercetin-rich foods to your daily diet today!

References

  1. https://pubmed.ncbi.nlm.nih.gov/25272572/
  2. https://pubmed.ncbi.nlm.nih.gov/18417116/
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249911/
  4. https://www.spandidos-publications.com/or/33/6/2659
  5. https://wjso.biomedcentral.com/articles/10.1186/s12957-018-1400-z
  6. https://pubmed.ncbi.nlm.nih.gov/28677813/
  7. https://pubmed.ncbi.nlm.nih.gov/28264020/
  8. https://pubmed.ncbi.nlm.nih.gov/27710596/
  9. https://pubmed.ncbi.nlm.nih.gov/18187018/ 
  10. https://www.mdpi.com/1420-3049/21/5/623
  11. https://www.ahajournals.org/doi/pdf/10.1161/JAHA.115.002713
  12. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5214562/
  13. https://pubmed.ncbi.nlm.nih.gov/30848564/
  14. https://pubmed.ncbi.nlm.nih.gov/30551359/
  15. https://pubmed.ncbi.nlm.nih.gov/21606866/
  16. https://pubmed.ncbi.nlm.nih.gov/23412140/
  17. https://www.ars.usda.gov/ARSUserFiles/80400525/Data/Flav/Flav_R03-1.pdf
  18. https://pubmed.ncbi.nlm.nih.gov/17590007/
  19. https://pubmed.ncbi.nlm.nih.gov/9414116/
  20. https://pubmed.ncbi.nlm.nih.gov/17032035/
  21. https://pubmed.ncbi.nlm.nih.gov/20517329/

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