Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their color.
Flavonoids such as quercetin are antioxidants -- they scavenge damaging particles in the body known as free radicals, which damage cell membranes, tamper with DNA, and even cause cell death. Antioxidants can neutralize free radicals and may reduce or even help prevent some of the damage they cause. They also help keep LDL ("bad") cholesterol from being damaged, which scientists think may contribute to heart disease. In test tubes, quercetin has strong antioxidant properties, but researchers aren't sure whether taking quercetin (and many other antioxidants) has the same effect inside the body.
Quercetin acts like an antihistamine and an anti-inflammatory, and may help protect against heart disease and cancer.
Allergies, Asthma, Hay Fever and Hives
In test tubes, quercetin prevents immune cells from releasing histamines, chemicals that cause allergic reactions. On that basis, researchers think that quercetin may help reduce symptoms of allergies including runny nose, watery eyes, hives, and swelling of the face and lips. However, there is no evidence yet that it works in humans.
Heart Disease
Test tube, animal, and some population-based studies suggest that the flavonoids quercetin, resveratrol, and catechins (all found in high concentration in red wine) may help reduce the risk of atherosclerosis (plaque build up in arteries that can lead to heart attack or stroke). These nutrients appear to protect against the damage caused by LDL ("bad") cholesterol and may help prevent death from heart disease. However, most human studies have looked at flavonoids in the diet, not as a supplements. Animal studies have used extremely large amounts of flavonoids (more than you could get through a supplement). Additional studies in people are needed to see if flavonoid supplements can be effective.
High Cholesterol
Test tubes studies have shown that quercetin prevents damage to LDL cholesterol. Population studies, meanwhile, show that people who eat diets high in flavonoids have lower cholesterol. One study showed that people who took quercetin and an alcohol-free red wine extract (which contains quercetin) had less damage to LDL cholesterol. More studies are needed, however, to show whether taking a quercetin supplement will have the same effect.
Interstitial Cystitis
Two small studies suggested that people with interstitial cystitis might benefit from flavonoids. People with this condition have bladder pain, similar to a bladder infection, and often experience an urgent need to urinate. In both studies, those who took a supplement containing quercetin appeared to have fewer symptoms. However, the studies included other flavonoids, so it isn't known which one might have the most beneficial effect. More and better-designed studies are needed.
Prostatitis
Some preliminary evidence indicates that quercetin might reduce symptoms of prostatitis (inflammation of the prostate). One small study showed that men who took quercetin had a reduction in symptoms compared to men who took placebo. The study was small, however, and the results need to be confirmed by other studies.
Rheumatoid Arthritis
There are reports of people with rheumatoid arthritis who had fewer symptoms when they switched from a typical Western diet to a vegan diet with lots of uncooked berries, fruits, vegetables, nuts, roots, seeds, and sprouts containing antioxidants, including quercetin. But there is no evidence that the positive effects were due directly to antioxidants, and no evidence that quercetin supplements will help treat RA.
Cancer
Quercetin and other flavonoids contained in fruits and vegetables have long been considered important in possibly helping prevent cancer. People who eat more fruits and vegetables tend to have lower risk of some types of cancer. And animal and test-tube studies suggest that flavonoids do indeed have anti-cancer properties. Quercetin and other flavonoids have been shown in these studies to inhibit the growth of cancer cells from breast, colon, prostate, ovarian, endometrial, and lung tumors. However, there haven't been enough good quality studies of flavonoids in humans to know if what works in the lab will also work in people.
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