Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their colours.
Flavonoids, such as quercetin, are antioxidants. They scavenge 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. They may reduce or even help prevent some of the damage free radicals cause. In test tubes, quercetin has strong antioxidant properties. But researchers are not sure whether taking quercetin (and many other antioxidants) has the same effects on the body.
Quercetin may help protect against heart disease and cancer. Quercetin can also help stabilize the cells that release histamine in the body and thereby have an anti-inflammatory and antihistamine effect.
Allergies, asthma, hay fever and hives
In test tubes, quercetin prevents immune cells from releasing histamines, which are chemicals that cause allergic reactions. As a result, 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 concentrations in red wine) may help reduce the risk of atherosclerosis, a 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 supplements. Animal studies have used extremely large amounts of flavonoids (more than you could get through a supplement). More studies in people are needed to see if flavonoid supplements can be effective.
High cholesterol
Test tube studies show that quercetin prevents damage to LDL cholesterol, and population studies show that people who eat diets high in flavonoids have lower cholesterol. One study found that people who took quercetin and an alcohol-free red wine extract (which contains quercetin) had less damage to LDL cholesterol. Another study found that quercetin reduced LDL concentrations in overweight subjects who were at high risk of heart disease. More studies are needed to show whether taking a quercetin supplement will have the same effect.
Hypertension
Studies show that quercetin supplementation reduces blood pressure in people who have hypertension.
Interstitial cystitis
Two small studies suggested that people with interstitial cystitis might benefit from consuming flavonoids. People with this condition have bladder pain similar to that from 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 is not known which flavonoid offers the most benefits. More and better-designed studies are needed.
Prostatitis
Preliminary evidence indicates that quercetin might reduce symptoms of prostatitis, or inflammation of the prostate. One small study found that men who took quercetin experienced fewer symptoms than men who took a placebo. More research is needed.
Rheumatoid arthritis (RA)
There are reports of people with RA 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 quercetin and other antioxidants. But there is no evidence that the positive effects were due directly to antioxidants and no evidence that quercetin supplements help treat RA.
Cancer
Scientists have long considered quercetin, and other flavonoids contained in fruits and vegetables, important in cancer prevention. People who eat more fruits and vegetables tend to have a lower risk of certain types of cancer. Animal and test-tube studies suggest that flavonoids 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 tumours. One study even suggests that quercetin is more effective than resveratrol in terms of inhibiting tumour growth. Another found that frequent intake of quercetin-rich foods was associated with lower lung cancer risk. The association was even stronger among subjects who smoked more than 20 cigarettes daily, and a third suggests that quercetin slows tumour growth in the laboratory (in leukemia cells). More research is needed.
