From Dr. Mercola:

Eighty-seven percent of U.S. adults say they like onions,1 which is great news since they’re one of the healthiest foods you can eat. Rich in vitamin C, sulphuric compounds, flavonoids and other phytochemicals,2 an onion a day may help keep the doctor away.

Onions are surprisingly high in beneficial polyphenols, which play an important role in preventing and reducing the progression of diabetes, cancer, and neurodegenerative and cardiovascular diseases.

Polyphenols also play an important role as a prebiotic, increasing the ratio of beneficial bacteria in your gut, which is important for health, weight management, and disease prevention.

Onions contain more polyphenols than even garlic or leeks, and are one of the best sources of a type of polyphenol called flavonoids, especially the flavonoid quercetin.

Onions Provide Disease-Fighting Quercetin

Quercetin is a powerful antioxidant with anti-inflammatory properties that may help fight chronic diseases like heart disease and cancer. In lab studies, quercetin was shown to prevent histamine release (histamines are the chemicals that cause allergic reactions.3

This makes quercetin-rich foods like onions “natural antihistamines.” In addition, quercetin may:4

Reduce the risk of atherosclerosis Help prevent death from heart disease Reduce blood pressure Relieve symptoms of interstitial cystitis Reduce symptoms of prostatitis Inhibit the growth of cancer cells from breast, colon, prostate, ovarian, endometrial and lung tumors Lower lung cancer risk, especially among smokers    

While apples and tea also contain quercetin, onions appear to be a particularly good source. Research from Wageningen Agricultural University in the Netherlands showed quercetin absorption from onions is double that from tea and three times that from apples.5

Research from the University also showed consuming onions leads to increased quercetin concentrations in the blood. As reported by The World’s Healthiest Foods:6

“ on an ounce-for-ounce basis, onions rank in the top 10 of commonly eaten vegetables in their quercetin content. The flavonoid content of onions can vary widely, depending on the exact variety and growing conditions.

Although the average onion is likely to contain less than 100 milligrams of quercetin per 3-1/2 ounces, some onions do provide this amount.

And while 100 milligrams may not sound like a lot, in the United States, moderate vegetable eaters average only twice this amount for all flavonoids (not just quercetin) from all vegetables per day.”

Quercetin is available in supplement form, but there are a couple of reasons why getting this flavonoid from onions makes more sense:7

One animal study found that animals received greater protection against oxidative stress when they consumed yellow onion in their diet as opposed to consuming quercetin extracts. Quercetin is not degraded by low-heat cooking, such as simmering. When preparing a soup with onions, the quercetin will be transferred into the broth of the soup, making onion soup an easy-to-make superfood. Onions Provide the Valuable Prebiotic Inulin and May Help Prevent Ulcers

Prebiotics are indigestible to you, but they help nourish beneficial bacteria in your body. These beneficial bacteria in turn assist with digestion and

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