Quercetin and isoquercetin have a wealth of health benefits and it has been used in traditional medicine for centuries. Today we are going to take a look at some of the potential health benefits of this polyphenol.
Polyphenols like quercetin have a range of health benefits and when included as part of your personal health strategy could help you to remain healthy and independent as you age.
For a healthy heart
A human clinical trial in 2013 showed that quercetin taken daily reduces systolic blood pressure (1) Quercetin supplementation reduced systolic blood pressure significantly although the researchers noticed no effect on other cardiovascular risk factors and inflammatory biomarkers.
Another human study with quercetin showed that it was able to improve cardiovascular health in male smokers, who are at higher risk of cardiovascular disease (2). Daily quercetin supplementation improved blood lipid profiles, glucose, and blood pressure, suggesting that quercetin has a beneficial role as a preventive measure against cardiovascular risk.
Helps the Immune system
A clinical trial showed that the administration of quercetin reduced vulnerability to upper respiratory tract infections (URTI) in professional cyclics (3). Upper respiratory tract infections are caused by an acute infection involving the upper respiratory tract including the nose, sinuses, pharynx or larynx. The majority of these infections are viral in nature and although sometime the cause is bacterial. Quercetin was given for 3 weeks before, during, and for 2 weeks after a 3 day period in which subjects cycled for 3 hours a day. Quercetin did not alter exercise-induced changes in several measures of immune function, but it significantly reduced URTI incidence in the cyclists during the 2 week period after intense exercise.
A systematic review and meta-analysis examined the effect of quercetin on exercise capacity (4). They concluded that on average, quercetin provides a statistically significant benefit in human endurance exercise capacity although that increase was relatively small.
Another study suggests that 500mg of quercetin taken twice per day was able to improve endurance without exercise training (5). The data suggests that as little as 7 days of quercetin supplementation can increase endurance without exercise training in untrained participants.
Reduces blood pressure
Quercetin is shown to decrease blood pressure and reduce the impact of hypertension (6). These 2007 study was the first to show that quercetin supplementation reduces blood pressure in hypertensive subjects. Unlike animal-based studies, there was no reduction in systemic markers of oxidative stress observed by the researchers.
A study showed that quercetin improves the function of the vascular smooth muscle cells and endothelial cells which are responsible for controlling blood pressure (7).
Might protect the liver
In a rodent study supplementation with quercetin prior to receiving toxic levels of ethanol, protected the rats liver against oxidative stress (8). Quercetin was able to neutralize the products of lipid peroxidation and boosted the creation of antioxidant glutathione in rats and protected the liver. Similar results were observed in a mouse study using ethanol, again quercetin protected the liver (9).
Another study saw rats exposed to a diet contaminated with aflatoxin (10). Aflatoxin is a powerful hepatotoxic and hepatocarcinogenic mycotoxin produced by the Aspergillus flavus group of fungi. The researchers fed rats a diet contaminated with this toxin to evaluate the protective role of quercetin against cytotoxicity, DNA damage and oxidative stress levels. The researchers concluded that quercetin produced a protective effect on the liver against aflatoxin induced toxicity and oxidative stress damage to liver tissue in a dose dependent manner.
In a 2015 study researchers showed that quercetin was also able to mitigate acetaminophen-induced liver damage by reducing levels of reactive oxygen and reactive nitrogen species (11).
And in another 2015 study a team of researchers explored the potential of quercetin for treating obesity-induced hepatic steatosis or commonly known as fatty liver disease (12). The researchers suggest that quercetin stimulates the hepatic mitochondrial oxidative metabolism by inducing HO-1 via the Nrf-2 pathway. This suggests that quercetin might be useful in protecting against obesity-induced fatty liver disease.
A human clinical trial showed that quercetin improved the health of blood vessels by increasing the level of nitric oxide and reducing levels of endothelin-1 in healthy men (13). In the trial a randomized, placebo-controlled, crossover trial in 12 healthy men was conducted to compare the acute effects of the oral administration of 200 mg quercetin and epicatechin (a flavonol found in dark chocolate). The researchers concluded that dietary flavonoids, such as quercetin and epicatechin, can augment the nitric oxide status and reduce endothelin-1 concentrations and may as a result improve endothelial function.
It might protect the Kidneys
In a rat study quercetin was able to reduce the damage done to the kidneys when toxic substances are administered (14). The researchers treated rats with Cisplatin a chemotherapy drug to induce kidney damage. In animals given Cisplatin there was an increase in plasma creatinine levels, tubular cell necrosis and an increase in proinflammatory NF-kB. They observed that in animals given quercetin had less damage associated changes and lower levels of NF-kB compared to control animals.
In another study researchers used an iron chelate, ferric nitrilotriacetate (Fe-NTA) to induce tubular necrosis (15). Within an hour of injection with Fe-NTA, a marked deterioration of renal architecture and renal function was observed. Fe-NTA induced a significant levels of oxidative stress and a reduction in activities of renal catalase, superoxide dismutase and glutathione reductase. Pretreatment of animals with quercetin 30 minutes before Fe-NTA administration had a marked reduction of renal dysfunction and oxidative stress.
Might help protect your eyes
This review examines the influence of quercetin and how it reduces the risk of cataracts (16). We can see from the data that quercetin reduces the risk of cataracts by targeting a number of pathways such as, oxidative stress, non-enzymatic glycation and epithelial cell signalling.
Quercetin was found in a study to have a protective effect on retinal cells (17). Retinal cells are located at the back of the eye and function as light receptors and play a key role in colour, focus and brightness of your vision. Quercetin was found in the study to have a protective effect improving cell survival when exposed to oxidative stress.
This protective effect against oxidative stress damage in retinal cells is confirmed in a 2010 study by the National Eye Institute (18). They found that quercetin was a powerful antioxidant and prevented inflammation and oxidative stress in retinal cells.
There are a many potential health benefits from quercetin, isoquercetin and these are just a few of them. With various beneficial effects it is worth considering making this supplement a regular part of your health regimen.
(1) Zahedi, M., Ghiasvand, R., Feizi, A., Asgari, G., & Darvishi, L. (2013). Does quercetin improve cardiovascular risk factors and inflammatory biomarkers in women with type 2 diabetes: a double-blind randomized controlled clinical trial. International journal of preventive medicine, 4(7).
(2) Lee, K. H., Park, E., Lee, H. J., Kim, M. O., Cha, Y. J., Kim, J. M., … & Shin, M. J. (2011). Effects of daily quercetin-rich supplementation on cardiometabolic risks in male smokers. Nutrition research and practice, 5(1), 28-33.
(3) Nieman, D. C., Henson, D. A., Gross, S. J., Jenkins, D. P., Davis, J. M., Murphy, E. A., … & McAnulty, L. S. (2007). Quercetin reduces illness but not immune perturbations after intensive exercise. Medicine and science in sports and exercise, 39(9), 1561.
(4) Kressler, J., Millard-Stafford, M., & Warren, G. L. (2011). Quercetin and endurance exercise capacity: a systematic review and meta-analysis. Med Sci Sports Exerc, 43(12), 2396-404.
(5) Davis, J. M., Carlstedt, C. J., Chen, S., Carmichael, M. D., & Murphy, E. A. (2010). The dietary flavonoid quercetin increases VO2max and endurance capacity. International journal of sport nutrition and exercise metabolism, 20(1), 56-62.
(6) Edwards, R. L., Lyon, T., Litwin, S. E., Rabovsky, A., Symons, J. D., & Jalili, T. (2007). Quercetin reduces blood pressure in hypertensive subjects. The Journal of nutrition, 137(11), 2405-2411.
(7) Larson, A. J., Symons, J. D., & Jalili, T. (2010). Quercetin: A treatment for hypertension?—A review of efficacy and mechanisms. Pharmaceuticals, 3(1), 237-250.
(8) Chen, X. (2010). Protective effects of quercetin on liver injury induced by ethanol. Pharmacognosy magazine, 6(22), 135.
(9) Molina, M. F., Sanchez-Reus, I., Iglesias, I., & Benedi, J. (2003). Quercetin, a flavonoid antioxidant, prevents and protects against ethanol-induced oxidative stress in mouse liver. Biological and Pharmaceutical Bulletin, 26(10), 1398-1402.
(10) El-Nekeety, A. A., Abdel-Azeim, S. H., Hassan, A. M., Hassan, N. S., Aly, S. E., & Abdel-Wahhab, M. A. (2014). Quercetin inhibits the cytotoxicity and oxidative stress in liver of rats fed aflatoxin-contaminated diet. Toxicology Reports, 1, 319-329.
(11) El-Shafey, M. M., Abd-Allah, G. M., Mohamadin, A. M., Harisa, G. I., & Mariee, A. D. (2015). Quercetin protects against acetaminophen-induced hepatorenal toxicity by reducing reactive oxygen and nitrogen species. Pathophysiology, 22(1), 49-55.
(12) Kim, C. S., Kwon, Y., Choe, S. Y., Hong, S. M., Yoo, H., Goto, T., … & Yu, R. (2015). Quercetin reduces obesity-induced hepatosteatosis by enhancing mitochondrial oxidative metabolism via heme oxygenase-1. Nutrition & metabolism, 12(1), 33.
(13) Loke, W. M., Hodgson, J. M., Proudfoot, J. M., McKinley, A. J., Puddey, I. B., & Croft, K. D. (2008). Pure dietary flavonoids quercetin and (−)-epicatechin augment nitric oxide products and reduce endothelin-1 acutely in healthy men. The American journal of clinical nutrition, 88(4), 1018-1025.
(14) Francescato, H. I. S. D. C., Coimbra, T. M., Costa, R. S., & Bianchi, M. D. L. (2004). Protective effect of quercetin on the evolution of cisplatin-induced acute tubular necrosis. Kidney and Blood Pressure Research, 27(3), 148-158.
(15) Singh, D., Chander, V., & Chopra, K. (2005). Quercetin, a Bioflavonoid, Attenuates Ferric Nitrilotriacetate‐Induced Oxidative Renal Injury in Rats. Drug and chemical toxicology, 27(2), 145-156.
(16) Stefek, M., & Karasu, C. (2011). Eye lens in aging and diabetes: effect of quercetin. Rejuvenation research, 14(5), 525-534.
(17) Nakayama, M., Aihara, M., Chen, Y. N., Araie, M., Tomita-Yokotani, K., & Iwashina, T. (2011). Neuroprotective effects of flavonoids on hypoxia-, glutamate-, and oxidative stress–induced retinal ganglion cell death.
(18) Cao, X., Liu, M., Tuo, J., Shen, D., & Chan, C. C. (2010). The effects of quercetin in cultured human RPE cells under oxidative stress and in Ccl2/Cx3cr1 double deficient mice. Experimental eye research, 91(1), 15-25.