What Is Quercetin?

Quercetin is a plant chemical, or flavonoid, that's found naturally in foods including apples, onions, teas, berries, and red wine. It's also found in some herbs such as ginkgo biloba and St. John's wort.

Quercetin acts as an antioxidant, neutralizing free radicals—the chemical by-products that harm cell membranes and damage DNA. Available as a dietary supplement, quercetin also possesses antihistamine (allergy-relieving) and anti-inflammatory properties.

This article discusses what quercetin is used for, what research says about it, and what to look for when buying a supplement.

What Is Quercetin Used For?

In alternative medicine, quercetin is said to help with the following conditions:

• Allergies
• Cardiovascular disease
• Inflammation
• Colds

So far, results to support the benefits of quercetin are mixed. Furthermore, its use for some conditions has only been examined in test tube (in vitro) or animal studies.

If you're considering using it, consult your primary care provider first. Self-treating a condition and avoiding or delaying standard care may have serious consequences.

Here's a look at some research highlights.

Allergy Relief

Quercetin is thought to prevent the body's release of histamine, an inflammatory chemical involved in allergic symptoms such as sneezing and itching.

Although lab experiments suggest quercetin may fight conditions like allergic rhinitis, there is little research on this use in humans. Researchers recommend further studies on people to prove a correlation.

High Blood Pressure

A 2016 review of high-quality trials found quercetin significantly reduced both systolic and diastolic blood pressure. This was noted particularly in those with diabetes who were given at least 500 milligrams (mg) per day.

However, the precise dosage and duration needed to see the most benefits is not yet clear.

Athletic Endurance

Quercetin may be no better than a placebo, or sham treatment, when it comes to enhancing athletic performance, according to a 2011 review of research. All 11 studies included showed a boost in exercise endurance (measured by oxygen consumption) when people took quercetin. However, the effect was minimal.

Another study found a more impressive link. A 2013 study analyzed 60 male students who had participated in athletics for at least three years. They saw improved lean body mass, hydration levels, calories burned at rest (basal metabolic rate), and total energy expenditure after taking quercetin.

Cancer

Studies on cell cultures have shown that quercetin may help slow the growth of some types of cancer cells. Some in vitro and animal-based research indicates that quercetin may protect against certain types of cancer, such as leukemia and lung cancer.

For example, a 2010 study looked at the relationship between quercetin intake and lung cancer risk. Researchers examined 38 samples of lung tissue from patients with adenocarcinoma, a type of cancer. They collected survey results from 1,822 patients with lung cancer and 1,991 without lung cancer.

The study found that a quercetin-rich diet was associated with lower lung cancer risk. From the samples, they also identified genetic changes affected by quercetin intake that may protect against lung cancer. The researchers noted that more research into this relationship is needed.

There's currently a lack of clinical human studies on quercetin's cancer-fighting effects. Therefore, it's too soon to tell whether quercetin might play a significant role in cancer prevention.

Possible Side Effects

Quercetin is generally well-tolerated when used in appropriate amounts. Some have reported tingling in the arms and legs, as well as upset stomach and headaches when taking quercetin supplements.

Very high doses—greater than 1 gram per day—might cause kidney damage.

Avoid taking quercetin if you're pregnant, nursing, or have a kidney condition. It may also interfere with some antibiotics or blood thinners.

Check with your doctor before using a quercetin supplement. Be sure to mention any medications you are taking and any conditions you have.

Dosage and Preparation

With medical supervision, quercetin has been safely used in amounts up to 1,000 mg twice daily for 12 weeks. There is not enough evidence to know if it is safe for long-term use.

The appropriate dose for you may depend on factors including your age, gender, and medical history. Speak to your healthcare provider to get personalized advice if you choose to take this supplement.

What to Look For

Food sources of quercetin include teas, onions, apples, buckwheat, and pau d'arco.

When taking quercetin in supplement form, it may be beneficial to choose a product that also contains papain and/or bromelain. These are plant-derived enzymes (fruit extracts) shown to increase the intestine's absorption of quercetin.

Remember that dietary supplements are largely unregulated. The content of some products may differ from what's on the product label, and their safety is not guaranteed. Choosing a product that is certified by an independent third party, like NSF, can ensure that a product's contents are as advertised and pure.

Summary

Quercetin is a plant chemical naturally found in certain foods and drinks, like apples and tea. It's thought to help benefit certain health conditions, including allergies, cardiovascular disease, and inflammation.

There's conflicting research on quercetin's actual health benefits. Researchers have found evidence that quercetin can help reduce blood pressure, particularly in those with diabetes. However, it's not clear what dosage is needed for the most benefit.

Check with your doctor before using any supplement, including quercetin.