Quercetin's therapeutic uses

DEFINITION: Natural plant product used to treat specific health conditions.

• PRINCIPAL PROPOSED USES: None
• OTHER PROPOSED USES: Allergies (hay fever), antiviral, asthma, cancer prevention, eczema, heart disease prevention, high blood pressure, hives, interstitial cystitis, prostatitis (chronic pelvic pain syndrome), stroke prevention

Overview

Quercetin belongs to a class of water-soluble plant coloring substances called bioflavonoids. Bioflavonoids have strong antioxidant effects when they are studied in the test tube, and this is the basis for some of the health claims attached to them. However, growing evidence suggests that bioflavonoids do not, in fact, act as antioxidants in human beings. Nonetheless, as widely available plant substances, they are considered possible seminutrients, substances that are not essential for life but might help promote optimal health.

Requirements and Sources

Quercetin is not an essential nutrient. It is found primarily in the peels of plant-based foods but also in red wine, grapefruit, onions, apples, cherries, and black tea and, in lesser amounts, in leafy green vegetables and beans. The soil used to grow these foods impacts their quercetin content. However, to get a therapeutic dosage, one needs to take a supplement. Quercetin supplements are available in pill and tablet form.

Therapeutic Dosages

A typical dosage is 200 to 400 milligrams (mg) three times daily. A special type of quercetin, quercetin chalcone, is claimed to be absorbed better, but there is little reliable evidence to prove this.

Therapeutic Uses

Quercetin is widely marketed as a treatment for allergic conditions such as asthma, hay fever, eczema, and hives. These proposed uses are based on test-tube research showing that quercetin prevents certain immune cells from releasing histamine, the chemical that triggers an allergic reaction. Quercetin may also block other substances involved with allergies. However, scientists have found no direct evidence that the ingestion of quercetin supplements will reduce allergy symptoms.

Another proposed use of quercetin has some meaningful supporting evidence: the treatment of prostatitis. This condition is an inflammation or infection of the prostate gland. Prostatitis, sometimes called chronic pelvic pain syndrome, causes chronic pain and difficulty with urination. Conventional treatment for this condition is often unsatisfactory. One small double-blind, placebo-controlled study has found preliminary evidence that quercetin might be useful for treating prostatitis. Another small, double-blind, placebo-controlled trial found that a supplement containing quercetin reduced symptoms of interstitial cystitis.

As noted, it has been suggested that quercetin’s antioxidant properties might help prevent heart disease and strokes. However, evidence that it works is highly incomplete, and other powerful antioxidants, such as vitamin E and beta-carotene, have been ineffective in preventing these conditions. There is limited evidence, however, from a single small, double-blind trial that quercetin might have a separate effect of lowering blood pressure when it is high.

Test-tube studies and animal research have suggested that, like other flavonoids, quercetin might have cancer-preventive properties. In laboratory research, quercetin has successfully destroyed colon, prostate, breast, skin, and lung cancer cells. Additionally, in a study involving patients who smoked, a higher intake of quercetin correlated with a lower risk of developing lung cancer. However, most human studies fail to replicate the success demonstrated in the lab.

An animal study found that quercetin might protect rodents with diabetes from forming cataracts. Another intriguing finding from test-tube research is that quercetin seems to prevent a wide range of viruses from infecting cells and reproducing once they are inside cells. One study found that quercetin produced this effect against herpes simplex, poliovirus, and various respiratory viruses, including influenza. However, such studies have been too indirect to determine whether humans taking quercetin supplements can hope for benefits against diseases caused by those viruses.

Scientific Evidence

Prostatitis. A one-month, double-blind, placebo-controlled trial of thirty men with chronic pelvic pain (prostatitis) tested the potential effectiveness of quercetin. Participants received either a placebo or 500 mg of the supplement twice daily. The results showed that those who received quercetin experienced a statistically significant improvement in symptoms (such as pain); those given placebo did not improve.

While these are promising results, the study was small and cannot be regarded as definitive. Furthermore, the researchers failed to provide the usual statistical evaluation required for such studies (a statistical analysis that directly compares the results in the treatment group against those in the placebo group). Thus, further study will be necessary to discover whether quercetin is actually effective for prostatitis.

Interstitial cystitis. People with interstitial cystitis experience pain and discomfort in the bladder that is reminiscent of a bladder infection but without the actual presence of such an infection. In a six-week double-blind, placebo-controlled study, twenty people received either a placebo or a supplement containing quercetin and other bioflavonoids. The results appeared to indicate better results in the quercetin group. However, this study has been presented only as an abstract, and it is not clear from the write-up whether the results were statistically meaningful. Studies on the impact of quercetin on interstitial cystitis have been small, and further research is needed.

COVID-19. A small study reported in 2022 on the therapeutic efficacy of a combination of quercetin, remdesivir, and favipiravir in 60 patients hospitalized with severe COVID-19. The control group received one of the antivirals while the intervention group received an antiviral and 1000mg of quercetin daily for one week. The results were promising, but given the small study size, further research was warranted.

Safety Issues

Quercetin appears to be quite safe. However, concerns have been raised that, under some circumstances, it might raise cancer risk. Quercetin “fails” a standard laboratory test called the Ames test, which is designed to identify chemicals that might be carcinogenic. Nonetheless, a bad showing on the Ames test does not definitely mean a chemical causes cancer. Most other evidence suggests that quercetin does not cause cancer and may, in fact, help prevent cancer. Still, one highly preliminary study suggests that quercetin combined with other bioflavonoids in the diet of pregnant women might increase the risk of infant leukemia. On this basis, pregnant women should probably avoid quercetin supplements. Maximum safe dosages for young children, nursing women, and people with serious liver or kidney disease have not been established.

Quercetin can interfere with several medications, including antibiotics, anticoagulants, chemotherapy, corticosteroids, and immunosuppressants.

Evidence suggests that using quercetin supplements can elevate urine and blood levels of homovanillic acid. While this itself should be harmless, lab tests for homovanillic acid are used to diagnose a rare and dangerous condition called neuroblastoma; for this reason, the use of quercetin supplements could cause a false positive diagnosis of this condition.

Bibliography

David, Alexander V., et al. “Overviews of Biological Importance of Quercetin: A Bioactive Flavonoid.” Pharmacognosy Reviews, vol. 10, no. 20, 2016, pp. 84-89. doi.org/10.4103/0973-7847.194044. Accessed 19 Nov. 2024.

Edwards, R. L., et al. “Quercetin Reduces Blood Pressure in Hypertensive Subjects.” Journal of Nutrition, vol. 137, 2007, pp. 2405-11.

Lotito, S. B., and B. Frei. “Consumption of Flavonoid-Rich Foods and Increased Plasma Antioxidant Capacity in Humans: Cause, Consequence, or Epiphenomenon?” Free Radical Biology and Medicine, vol. 41, 2006, pp. 1727-46.

Raman, Ryan. "What Is Quercetin?" Healthline, 16 July 2024, www.healthline.com/nutrition/quercetin. Accessed 20 Sept. 2024.

Shohan, Mojtaba, et al. "The Therapeutic Efficacy of Quercetin in Combination with Antiviral Drugs in Hospitalized COVID-19 Patients: A Randomized Controlled Trial." European Journal of Pharmacology, vol. 914, 2022, doi.org/10.1016/j.ejphar.2021.174615. Accessed 19 Nov. 2024.

Weldin, J., et al. “Quercetin, an Over-the-Counter Supplement, Causes Neuroblastoma-like Elevation of Plasma Homovanillic Acid.” Pediatric and Developmental Pathology, vol. 5, 2004, pp. 547-551.