Quercetin is a flavonoid (plant pigment) commonly found in fruits and vegetables, especially onions, citrus, and apples. Other sources include dark berries, grapes, and olive oil. Green tea and red wine have also been pointed out as having notable amounts of quercetin.
That's all great. But, what does it do for us? Well, perhaps the primary benefit of quercetin is that it possesses potent antioxidants. Antioxidants fight against free radicals –- chemically reactive compounds that damage cell membranes and DNA and also cause cell death. In fact, many of the benefits you'll often hear attributed to antioxidants refer to the effects associated with quercetin. Let's take a look at what some of them are.
Benefits of Quercetin
1. Supports Normal Respiratory Health
When your respiratory system is irritated, redness and swelling can result from the release of histamines, and quercetin has been reported to have an antihistamine effect. Lab tests have shown quercetin influences intracellular enzymes and may help inhibit histamine release.  This can often provide relief for watery eyes, runny nose, and swelling in the face.
Another study compared the effectiveness of quercetin against contact dermatitis and photosensitivity, two conditions that do not respond well to a conventional approach. Researchers found that quercetin is effective at inhibiting redness and is easy to administer. What's more, quercetin promoted significant improvement in both conditions. 
2. Supports Cardiovascular Health
Studies have shown that the consumption of flavonoids, specifically quercetin, offer a two fold benefit (at least) in promoting overall cardiovascular health. First, it encourages blood flow.  Secondly, researchers have observed that quercetin's antioxidant action protects against LDL cholesterol oxidation. This may be beneficial because oxidation causes LDL cholesterol to stick to artery walls. 
3. Promotes Balanced Blood Pressure
In addition to supporting cardiovascular health, quercetin naturally promotes balanced blood pressure. A randomized, double-blind, placebo controlled, crossover study evaluated the effect of quercetin supplementation and reported that the participants experienced a stabilization in systolic, diastolic and average arterial pressure. 
4. Offers Protection Against Stress
When your body is stressed, it produces cortisol. Cortisol is a hormone that produces that "fight or flight" response. Although this is normal, when your stress levels are high and ongoing, cortisol can damage muscle tissue, leading to protein breakdown in the body. Quercetin can fight these effects during times of extended stress as it suppresses the enzyme necessary for cortisol release. 
5. Potential for Upper Respiratory Conditions?
Although the jury is still out, numerous animal studies and lab models have suggested that quercetin may offer a bronchial dilating effect. A 2013 study determined that it inhibited an enzyme that breaks down signaling proteins which produce swelling and airway narrowing. Quercetin caused a relaxation of the airway smooth muscle, leading researchers to suggest it may offer therapeutic solutions for persons suffering from upper respiratory conditions.  Hopefully more research will continue to explore this possibility.
6. Offers Nutritional Support for Overall Health
Listen, putting good nutrition into your body is one of the best measures you can take to encourage good health. It's not a guarantee, and some people will still get sick, but it's a good foundation and fundamental approach. Why should quercetin be part of your approach? Because research has shown that people who consume more fruits and vegetables have a lower risk of some diseases and lab studies have shown that quercetin has a positive benefit against some cancers.      Is it a cure? No. Is it something you should check into? I think so.
Supplementing With Quercetin
Although quercetin offers many benefits and it is all-natural, there are a few health considerations. Large amounts can stress the kidneys, it may also interact with blood thinners, corticosteroids, and aspirin. If you're taking any of these, it's probably best to check with your healthcare provider before adding quercetin to the list – especially if you're taking it in supplement form. Most people, however, should be able to enjoy dietary quercetin (fruits and vegetables) without a concern.
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†Results may vary. Information and statements made are for education purposes and are not intended to replace the advice of your doctor. If you have a severe medical condition or health concern, see your physician.