Quercetin is a pigment that adds color to many fruits and vegetables. It’s found mainly in the skins and leaves of plants. Light stimulates the production of quercetin, so an apple at the top of a tree may have more quercetin than one that doesn't get direct sunlight.
Quercetin may be referred to as a phytochemical, polyphenol, or flavonoid. Phytochemicals are substances produced by plants that may have health benefits for humans. Polyphenols and flavonoids are types of phytochemicals.
Quercetin is a powerful antioxidant. Antioxidants help fight free radicals, which are molecules that contain unpaired electrons. Because electrons naturally want to pair up, free radicals roam around the body, pulling electrons away from other molecules. This process can damage cells and DNA. Quercetin "cleans up" free radicals by pairing with their single electrons so they can no longer cause damage.
Dietary intakes of quercetin in the U.S. have been reported to be around 6-18 milligrams (mg) per day. However, if you’re eating several servings of fruits and vegetables on a daily basis, you’re likely consuming much more quercetin.
Why You Need Quercetin
Research shows that quercetin has many health benefits, including:
Quercetin has been shown to support the cardiovascular system by lowering blood pressure and cholesterol and relaxing blood vessels. Because reduced blood flow can cause erectile dysfunction, flavonoids like quercetin can also improve men's sexual health.
Improved circulation improves brain health as well. But quercetin can protect the brain in other ways, too. It may reduce inflammation and protect brain cells from toxins. Its antioxidant powers could lower the risk of Alzheimer's disease and other degenerative diseases of the brain.
When free radicals damage cells in the body, those cells sometimes develop into cancer. Quercetin and other antioxidants reduce the risk of cancer by combating free radicals. A few studies have targeted quercetin particularly. In one, it slowed tumor growth. In another, it lowered the risk of lung cancer. The third was a lab study, which found that quercetin had the ability to attack leukemia cells.
Foods with Quercetin
Quercetin is one of the most common flavonoids, and is present in many foods. It also appears in red wine, black tea, and green tea. When you get your quercetin from fruits and vegetables, you reap the other benefits of those foods. You also increase your intake of fiber, which is lacking in the standard American diet.
Quercetin is available as a supplement, often with a recommended dose of 500-1,000 mg per day. It’s considered safe to use, but may interact with several medications, including antibiotics and blood thinners. Doses over one gram may damage the kidneys.
There’s no recommended daily allowance for quercetin, but these commonly eaten foods are good sources of it:
All onions contain quercetin, but since it’s a pigment, red and yellow onions contain the most. To keep the quercetin, peel off as little as possible of the outer layers. Onions contain many other vitamins and minerals, and they are especially rich in the vitamin biotin.
Kale has a well-deserved reputation as a nutritional powerhouse. Besides quercetin, it’s also a rich source of vitamin A, vitamin C, and fiber. Half a cup contains only 18 calories.
All tomatoes are good sources of quercetin. Cherry tomatoes are the best because they have the highest ratio of skin to flesh. They make a great low-calorie snack, with only 37 calories per dozen.
Along with quercetin, broccoli is rich in vitamin K and vitamin C. It also contains another flavonoid, kaempferol, that offers potent antioxidant capabilities. A one-cup serving provides 5 grams of fiber.
Blueberries are antioxidant superstars. On top of quercetin, they contain 17 different phytochemicals including resveratrol, the antioxidant best known for appearing in red wine.
Apples contain four different phytochemicals along with the beneficial fiber known as pectin. To get the benefits of quercetin, eat your apples unpeeled. Apple juice doesn’t provide the same benefits as the whole fruit.