Onion Power!

health benefits onions

Story at-a-glance -

  • Onions are an excellent source of heart-healthy, cancer-fighting quercetin
  • Onions contain more disease-fighting polyphenols than even garlic or leeks
  • The sulfur compounds in onions are thought to have anti-clotting properties as well as help to lower triglycerides
  • Flavonoid-rich foods like onions may also inhibit the growth of H. pylori, a type of bacteria responsible for most ulcers

By 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


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

"… [O]n 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.
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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 absorption of your food, and play a significant role in your immune function. One such prebiotic is inulin, a water-soluble form of dietary fiber that's found in onions.8

Inulin has multiple benefits to your health. Among obese women, consuming inulin beneficially changed the gut microbiota composition in a way that might help promote weight loss or lower the risk of diabetes.9

Further, among women with type 2 diabetes, those who took inulin had improved glycemic control and increased antioxidant activity.10 Inulin may even help relieve constipation.11 Flavonoid-rich foods like onions may also inhibit the growth of H. pylori, a type of bacteria responsible for most ulcers.12

Up to 20 percent of Americans over the age of 40 have H. pylori living in their digestive tract, although most will not develop ulcers. Eating onions and other flavonoid-rich foods may lower your risk further.

What Else Are Onions Good For?

Onions, which are very popular in French cuisine, are thought to play a role in the so-called "French Paradox" — the low incidence of heart disease among the French, despite their relatively high-calorie diet.

The sulfur compounds in onions, for instance, are thought to have anti-clotting properties as well as help to lower cholesterol and triglycerides. The allium and allyl disulphide in onions have also been found to decrease blood vessel stiffness by releasing nitric oxide.

This may reduce blood pressure, inhibit platelet clot formation and help decrease the risk of coronary artery disease, peripheral vascular diseases and stroke. The quercetin in onions is also beneficial, offering both antioxidant and anti-inflammatory properties that may boost heart health.

Further, as reported by the National Onion Association, onions are considered a dietary anti-carcinogen:13

"The inhibitory effects of onion consumption on human carcinoma have been widely researched …

In a review on the effects of quercetin … persons in the highest consumption category versus the lowest had a 50 percent reduced risk of cancers of the stomach and alimentary and respiratory tracts.

Organosulfur compounds [in onions] such as diallyl disulfide (DDS), S-allylcysteine (SAC), and S-methylcysteine (SMC) have been shown to inhibit colon and renal carcinogenesis.

… Mechanisms of protection ranged from induced cancer cell apoptosis and gene transcription inhibition to protection against UV-induced immunosuppression."

The many phytochemicals and other nutrients in onions work together to provide synergistic health benefits that reach body-wide. For instance, research has show including onions in your diet may offer the following benefits:14

Prevent inflammatory processes associated with asthma

Reduce symptoms associated with diabetes

Lower levels of cholesterol and triglycerides

Reduce symptoms associated with osteoporosis and improve bone health

Maintain gastrointestinal health by sustaining beneficial bacteria

Diminish replication of HIV

Reduced risk of neurodegenerative disorders

Lower your risk of cataract formatio


Antimicrobial properties that may help reduce the rate of food-borne illness

Improvement of intestinal flora, improved absorption of calcium and magnesium due to the fructans they contain

Antibacterial and antifungal properties

Lower risk of certain cancers

Be Careful Not to 'Overpeel' Onions

When removing the outer skin of an onion, take care to remove as little as possible. The outer layers of the flesh are thought to be the most nutritious, including concentrating the highest amounts of flavonoids.15 As reported by the George Mateljan Foundation:16

"The flavonoids in onion tend to be more concentrated in the outer layers of the flesh. To maximize your health benefits, peel off as little of the fleshy, edible portion as possible when removing the onion's outermost paper layer. Even a small amount of 'overpeeling' can result in unwanted loss of flavonoids. For example, a red onion can lose about 20 percent of its quercetin and almost 75 percent of its anthocyanins if it is 'overpeeled.'"

If you find it difficult to accomplish this because you're trying to chop your onion as quickly as possible to avoid "crying," there are tricks that may help. Onions release a gas called lachrymatory factor (LF), which causes tearing. To lessen the rate of LF production, try chilling the onion for an hour before cutting.17

How to Prepare Different Types of Onions

If learning about their health benefits has inspired you to eat more onions, you're in luck as they are incredibly versatile and come in a variety of colors and flavors. The chart below, from the National Onion Association,18 provides an excellent breakdown of which type of onion to use in your cooking.

Yellow Onion:

All-purpose and most popular, approximately 87 percent of the U.S. onion crop is comprised of yellow varieties. The most well-known sweet onions are yellow. The best type of onion for caramelizing is a yellow storage variety. Cooking brings out this variety's nutty, mellow, often sweet, quality when caramelized.

Variety or Type Availability Raw Flavor/Texture Best Usage
Sweet March to September Crisp, juicy, mild flavor with a slightly sweet ending with little to no after-taste Raw, lightly cooked, sautéed, or grilled
Fresh, Mild March to August Crisp, juicy, mild to slightly pungent with a faint after-taste Raw, lightly cooked, sautéed, or grilled
Storage August to May Strong onion flavor, mild after-taste Grilled, sautéed, caramelized, baked, or roasted

Source: National Onion Association, All About Onions

Red Onion:

About 8 percent of the U.S. onion crop is red. They have gained popularity in the past decade, especially in food service on salads and sandwiches because of their color.

Variety or Type Availability Raw Flavor/Texture Best Usage
Sweet March to September Crisp, very mild onion flavor Raw, grilled, or roasted
Fresh, Mild March to September Bright tones, slightly less water content than yellow with a slightly pungent ending Raw, grilled, or roasted
Storage August to May Sharp, spicy, and moderate to very pungent Raw, grilled, or roasted

Source: National Onion Association, All About Onions

White Onion:

Approximately 5 percent of U.S. onion production is dedicated to white onions. They are commonly used in white sauces, potato and pasta salads, and in Mexican or Southwest cuisine. Due to the compact nature of their cell structure, white onions do not store quite as long as other varieties.

Variety or Type Availability Raw Flavor/Texture Best Usage
Fresh, Mild March to August Moderately pungent and clean finish, very little after-taste Raw, grilled, sautéed, or lightly cooked
Storage August to May Moderately pungent to very pungent and full flavored, but finishes with a cleaner and crisper flavor in comparison to yellow and red storage varieties Raw, grilled, sautéed, or lightly cooked

Source: National Onion Association, All About Onions

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Remember, knowing what's in your food is the first step to choosing and preparing nutritious meals each and every day. So visit Mercola Food Facts today to get started.