Onions are rich in sulfur-containing compounds and other phytochemicals that may boost heart health, offer protection against cancer and diabetes, boost bone health, and more.
If you’ve been eating an apple a day to keep the doctor away, you would be wise to add an onion a day to that regimen. This humble vegetable is a member of the Allium genus, making it closely related to other superfoods like garlic, leeks, scallions, and chives.
This means onions are rich in sulfur-containing compounds that give them both their characteristic odor and much of their health-boosting potential.
As one of the oldest cultivated plants, onions do not disappoint in terms of nutrition. They’re a very good source of vitamins C and B6, iron, folate, and potassium. But it’s their phytochemicals – including the flavonoid quercetin and allyl disulphide – that are most exciting to researchers.
To date, onions have shown a wealth of beneficial properties; they’re anti-allergic, anti-histaminic, anti-inflammatory, and antioxidant,1 all rolled into one. And if you take even a quick glance at the available research, you’ll quickly understand why onions deserve to make a very frequent appearance at your dinner table.
Onions Are Polyphenol Superstars
Polyphenols are plant compounds recognized for their disease prevention, antioxidant, and anti-aging properties. Onions have a particularly high concentration, with more polyphenols than garlic, leeks, tomatoes, carrots, and red bell peppers.2
In particular, onions are especially rich in polyphenol flavonoids called quercetin. Quercetin is an antioxidant that many believe prevent histamine release—making quercetin-rich foods “natural antihistamines.” As reported by The World’s Healthiest Foods:3
“…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 naturally from onions makes more sense:4
- 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.
Eating Onions May Lower Your Risk of Cancer
If you’re interested in using food to lower your risk of cancer, eat onions. People who eat more onions, as well as other allium vegetables, have a lower risk of many types of cancer, including:5
- Prostate and breast
- Ovarian and endometrial
- Colorectal and gastric
- Esophageal and laryngeal
- Renal cell
Onions contain numerous anti-cancer compounds, including quercetin, which has been shown to decrease cancer tumor initiation as well as inhibit the proliferation of cultured ovarian, breast, and colon cancer cells.6 As reported by the National Onion Association, onions are considered a dietary anti-carcinogen:7
“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% 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.”
It’s unclear exactly how much onion consumption is necessary for cancer protection, but research shows benefit from even moderate consumption. Even one to seven servings of onions a week may be protective, although some research suggests a daily serving of onion (one-half cup) is best.
Heart Health: Are Onions Responsible for the French Paradox?
The so-called “French Paradox” — the low incidence of heart disease among the French, despite their relatively high-calorie diet – has often been credited to the antioxidants in the red wine they often consume.
But onions, which are very popular in French cuisine, may be another contributing factor to their good health, particularly heart health. The sulfur compounds in onions, for instance, are thought to have anti-clotting properties, as well as, improve blood lipid profiles. The allium and allyl disulphide in onions have also been found to decrease blood vessel stiffness by enhancing nitric oxide release.
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.
Onions Are Beneficial for Your Bones, Digestive Tract, Diabetes Prevention, and More
The more we learn about onions, the more it becomes clear that they offer whole body benefits. That is the beauty of eating whole foods, after all, because they typically contain many beneficial phytochemicals that enhance your health in numerous synergistic ways. As for onions, research has shown that including onions in your diet may offer the following benefits:8
|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|
|Reduce risk of neurodegenerative disorders||Lower your risk of cataract formation||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|
Onions Were a Prehistoric Staple Food
If there were any doubt as to how valuable onions have been through the ages, it’s thought that wild onions have been enjoyed since the very early ages, and were likely a staple in the prehistoric diet. The National Onion Association stated:9
“Most researchers agree the onion has been cultivated for 5,000 years or more. Since onions grew wild in various regions, they were probably consumed for thousands of years and domesticated simultaneously all over the world. Onions may be one of the earliest cultivated crops because they were less perishable than other foods of the time, were transportable, were easy to grow, and could be grown in a variety of soils and climates.
In addition, the onion was useful for sustaining human life. Onions prevented thirst and could be dried and preserved for later consumption when food might be scarce. While the place and time of the onion’s origin is still a mystery, many documents from very early times describe its importance as a food and its use in art, medicine, and mummification.”
Since ancient times, onions have been enjoyed around the world for many uses:10
- The onion symbolized “eternity” to Egyptians, who would bury them along with their Pharaohs
- In India, onions were valued as a diuretic and good for digestion, the heart, the eyes, and the joints
- During the Middle Ages in Europe, onions were widely consumed and prescribed medicinally for headaches, snakebites, and hair loss
- Native American Indians used wild onions in cooking as well as in poultices, dyes, and even as toys
Tips for Choosing and Preparing Onions
The average American eats about 20 pounds of onions a year, which may sound like a lot until you learn that in Libya, which has the highest onion consumption rate in the world, the average person eats nearly 67 pounds a year.11 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,12 provides an excellent breakdown of which type of onion to use in your cooking.
|Color||Variety or Type||Availability||Raw Flavor/Texture||Best Usage|
|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.||Sweet||March-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 – August||Crisp, juicy, mild to slightly pungent with a faint after-taste||Raw, lightly cooked, sautéed, or grilled|
|Storage||August-May||Strong onion flavor, mild after-taste||Grilled, sautéed, caramelized, baked, or roasted|
|Red Onion:About eight percent of the U.S. onion crop is red. They have gained popularity in the past decade, especially in foodservice on salads and sandwiches because of their color.||Sweet||March-September||Crisp, very mild onion flavor||Raw, grilled, or roasted|
|Fresh, Mild||March-September||Bright tones, slightly less water content than yellow with a slightly pungent ending||Raw, grilled, or roasted|
|Storage||August-May||Sharp, spicy, and moderate to very pungent||Raw, grilled, or roasted|
|White Onion:Approximately five 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.||Fresh, Mild||March-August||Moderately pungent and clean finish, very little after-taste||Raw, grilled, sautéed, or lightly cooked|
|Storage||August-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|
A Trick to Avoid ‘Crying’ While You Cut Onions
Onions release a gas called lachrymatory factor (LF), which causes tearing. Japanese researchers developed an onion that lacked the enzyme necessary to produce LF, and therefore wouldn’t cause tearing, but it also altered the beneficial sulfur-containing compounds in the onion. So while the fact that onions make you tear up is a bit inconvenient, it’s also a reminder of the many potent health compounds they contain. That being said, the World’s Healthiest Foods shared a few tips to cutting onions that should help lessen eye irritation and tearing. If this is an issue for you, don’t give up on onions. Try these tips instead:13
“If cutting onions irritates your eyes, there are a few tricks that you can employ. Use a very sharp knife and always cut the onions while standing; that way your eyes will be as far away as possible. Consider cutting onions by an open window. If cutting onions really makes you cry, consider wearing glasses or goggles.
Chill the onions for an hour or so before cutting; this practice can slow down the onion’s metabolism and thereby lessen the rate of LF gas production. Cutting onions under cold, running water is a method that is often used to cut back on eye irritation, but it’s a method we view as a second-best choice since some of the nutrients found in onion can be lost into the flow of water.”