What quercetin is: Quercetin is a flavonoid (more specifically, a flavonol). Among the flavonoids, quercetin is one of the most frequently studied for its potential health benefits. Fruits and vegetables are rich in quercetin; it is also found in black and green tea and red wine. Capers, surprisingly, are by far the richest source. Quercetin is an antioxidant, like vitamin C and many other compounds, and has anti-inflammatory effects. It is found in FRS, a widely advertised sports supplement and beverage that was promoted by Lance Armstrong until 2012.
Claims, purported benefits: Helps prevent or treat heart disease, diabetes, hypertension, allergic reactions, asthma, arthritis, cancer and prostate problems. Improves athletic performance, bone health and the immune response, prevents fatigue, reduces recovery time and boosts energy.
What the studies show: Findings about quercetin, while sometimes encouraging, have been inconsistent. Researchers at the Linus Pauling Institute in Oregon have found quercetin to be a powerful antioxidant in the test tube, but could not establish any measurable benefits in human subjects. Though potential anticancer effects of flavonoids, including quercetin, have been seen in lab studies, it is difficult to assess which flavonoids are best. They may work better in groups, not as single compounds. So far, there’s no evidence that quercetin on its own has an anti-cancer effect in humans.
Research on quercetin supplements for cardiovascular conditions and risk factors has yielded mixed results. Some small studies have found that quercetin can lower blood pressure modestly in people with hypertension, though it’s unclear if this has any clinical significance. A 2010 German study of overweight people with metabolic syndrome, published in the Journal of Nutrition, found that quercetin supplements reduced blood pressure slightly as well as oxidation of LDL (“bad”) cholesterol— but only in people with a certain genetic variant that increases the risk of cardiovascular disease. And a 2011 Korean study of male smokers found that quercetin reduced LDL cholesterol moderately, raised HDL cholesterol and lowered blood pressure and blood sugar slightly.
Exercise performance. The evidence has been mixed. In small studies done at Appalachian State University in North Carolina, high doses of quercetin reduced colds and boosted alertness in athletes. But, the researchers said, the claims made specifically for FRS are based more on speculation than on science. Only one study on the actual product has been published. It found that two cans of FRS a day for six weeks improved performance in a small group of elite cyclists—though a formulation without quercetin resulted in some improvements, too.
A well-designed 2009 study from the University of Georgia, which looked at 30 young men who did cycling tests before and after taking high doses of quercetin or a placebo, found that the quercetin was no better than the placebo at enhancing performance, boosting energy, or reducing fatigue. And in 2011, a large randomized controlled study from Appalachian State University found that high doses of quercetin did not improve markers of inflammation.
Side effects: Quercetin inhibits an important enzyme (CYP3A4) involved in the metabolism of many common medications, which can alter blood levels and effects of the drugs. It’s well known that grapefruit (which contains quercetin) can cause such interactions. This could be especially risky for people taking digoxin (for the heart). So if you take medication, check with your doctor or pharmacist about possible interactions with quercetin.
Bottom line: If you eat lots of fruits and vegetables, you’ll get a fair amount of quercetin. Skip the supplements— the research is too limited. It’s unclear how “bioavailable” the quercetin in most supplements is, compared to dietary quercetin. In addition, no one knows what adverse effects high doses might have, especially when taken over the long term.