Onions belong to the Allium family of plants, which also includes chives, garlic, and leeks. These vegetables have characteristic pungent flavors. The medicinal properties of onions have been recognized since ancient times, when they were used to treat ailments like headaches, heart disease and mouth sores.
Onions vary in size, shape, color, and flavor. The most common types are red, yellow, and white onions. The taste can range from sweet and juicy to sharp, spicy, and pungent.
According to the Food and Agriculture Organization of the United Nations, China is the biggest producer of onions worldwide.
Onions are nutrient-dense, meaning they’re low in calories but high in vitamins and minerals. One medium onion has just 44 calories with plenty of vitamins, minerals and fiber.
Onions are particularly high in vitamin C, a nutrient involved in regulating immune health, collagen production, tissue repair and iron absorption. Vitamin C also acts as a powerful antioxidant in your body, protecting your cells against damage caused by free radicals.
Onions are also rich in B vitamins, including folate (B9) and pyridoxine (B6) which play key roles in metabolism, red blood cell production and nerve function. Onions are also a good source of potassium.
Onions contain antioxidants and compounds that fight inflammation, decrease triglycerides and reduce cholesterol levels. In fact, they contain over 25 different varieties of flavonoid antioxidants. Recently, health researchers have noted a relationship between messaging molecules called oxylipins and high cholesterol management. A 2016 study in the journal Redox Biology found that consuming onions increases oxylipins that help regulate blood fat levels and levels of cholesterol.
Onions’ anti-inflammatory properties may also help reduce high blood pressure and protect against blood clots. Quercetin is a flavonoid antioxidant that’s highly concentrated in onions. Since it’s a potent anti-inflammatory, it may help decrease heart disease risk factors, such as high blood pressure. A study in 70 overweight people with high blood pressure found that a dose of 162 mg per day of quercetin-rich onion extract significantly reduced systolic blood pressure by 3–6 mmHg compared to a placebo.
A study in 54 women with polycystic ovarian syndrome (PCOS) found that consuming large amounts of raw red onions (40–50 grams/day if overweight and 50–60 grams/day if obese) for eight weeks reduced total and LDL cholesterol compared to a control group.
Red onions, in particular, contain anthocyanins, special plant pigments in the flavonoid family that give red onions their deep color. Multiple population studies have found that people who consume more foods rich in anthocyanins have a reduced risk of heart disease. Additionally, anthocyanins have been found to protect against certain types of cancer and diabetes.
Eating vegetables of the Allium genus like garlic and onions has been linked to a lower risk of certain cancers, including stomach and colorectal. A review of 26 studies showed that people who consumed the highest amount of allium vegetables were 22% less likely to be diagnosed with stomach cancer than those who consumed the least amount. A review of 16 studies with 13,333 people revealed that participants with the highest onion intake had a 15% reduced risk of colorectal cancer compared to those with the lowest intake.
A study in 42 people with type 2 diabetes demonstrated that eating 3.5 ounces of fresh red onion reduced fasting blood sugar levels by about 40 mg/dl after four hours. Specific compounds found in onions, such as quercetin and sulfur compounds, have antidiabetic effects.
Quercetin has been shown to interact with cells in the small intestine, pancreas, skeletal muscle, fat tissue and liver to control whole-body blood sugar regulation. Quercetin extracted from onions seems to be a particularly powerful way to fight bacteria. Test-tube study demonstrated that quercetin extracted from yellow onion skin successfully inhibited the growth of Helicobacter pylori (H. pylori) and Methicillin-resistant Staphylococcus aureus (MRSA). H. pylori is a bacteria associated with stomach ulcers and certain digestive cancers, while MRSA is an antibiotic-resistant bacteria that causes infections in different parts of the body. Another test-tube study found that quercetin damaged the cell walls and membranes of E. coliand S. aureus.
A study in 24 middle-aged and postmenopausal women showed that those who consumed 3.4 ounces of onion juice daily for eight weeks had improved bone mineral density and antioxidant activity compared to a control group. Another study in 507 perimenopausal and postmenopausal women found that those who ate onions at least once a day had a 5% greater overall bone density than individuals who ate them once a month or less. This study also demonstrated that older women who most frequently ate onions decreased their risk of hip fracture by more than 20% compared to those who never ate them.
Onions can fight potentially dangerous bacteria, such as Escherichia coli (E. coli), Pseudomonas aeruginosa, Staphylococcus aureus (S. aureus) and Bacillus cereus. Onion extract has been shown to inhibit the growth of Vibrio cholerae, a bacteria that is a major public health concern in the developing world.
Onions are a rich source of fiber and prebiotics, which are necessary for optimal gut health. Prebiotics are non-digestible types of fiber that are broken down by beneficial gut bacteria. Gut bacteria feed on prebiotics and create short-chain fatty acids, including acetate, propionate and butyrate.
Research has shown that these short-chain fatty acids strengthen gut health, boost immunity, reduce inflammation and enhance digestion. Consuming foods rich in prebiotics helps increase probiotics, such as Lactobacillus and bifidobacteria strains, which benefit digestive health.
While not especially serious, eating onions can cause problems for some people. The carbohydrates in onions may cause gas and bloating, according to National Digestive Diseases Information Clearinghouse. Onions, especially if consumed raw, can worsen heartburn in people who suffer from chronic heartburn or gastric reflux disease, according to one 1990 study in the American Journal of Gastroenterology.
Eating a large amount of green onions or rapidly increasing your consumption of green onions may interfere with blood thinning drugs, according to the University of Georgia. Green onions contain a high amount of vitamin K, which can decrease blood thinner functioning.
It is also possible to have a food intolerance or an allergy to onions, but cases are rare, according to an article in the Journal of Allergy and Clinical Immunology. People with onion allergies may experience red, itchy eyes and rashes if an onion comes into contact with the skin. People with an intolerance to onions may experience nausea, vomiting and other gastric discomfort.
To avoid “onion breath,” eat a sprig of parsley, or rinse your mouth with equal parts lemon juice and water, or chew a citrus peel.
How to Buy
When you buy onions, be sure to look for firm onions that are free of cuts and blemishes.
There are two main classifications of onions: the green onion (or scallion) and the dry onion.
Green onions are often eaten raw on salads or used as a garnish. Scallions, or green onions, are actually immature yellow, red or white onions, harvested before the bulb begins to form. “Spring onions” and “salad onions” are other names for immature onions.
Shallots have a distinctive taste, but the flavor is closer to that of mature onions than to that of scallions.
The dry onions have a juicy flesh and are covered with a dry papery skin. They come in a wide range of sizes, shapes and flavors.
If a recipe just says “chopped onions” buy the “yellow” onions. 88% of all onions eaten are the yellow onion. Another type of yellow onion is the Vidalia onion which is the sweetest and juiciest of them all. Red or purple onions, another example of the dry onion, are often eaten raw or on salads and hamburgers.
How to Store
It is an urban legend that cut onions are a magnet for bacteria and viruses. In fact, the cut surface of an onion is acidic and has sulfur compounds which inhibit bacteria and mold. This means it’s perfectly fine to hang on to a leftover onion, whether you’re dealing with half an onion, slices or chopped bits. Store your extras in a sealed container in the refrigerator, and use them within 7 to 10 days. This advice comes directly from the USDA and the National Onion Council.
Sweet onions, produced in early summer, have a high moisture content, so they don’t store well. To extend the storage life of sweet onions, the National Onion Council recommends wrapping each onion in a paper towel and keeping them in your refrigerator. Even if you do this, you’ll still need to use your sweet onions within a few weeks; all that moisture makes them prone to mold. Toss (or compost) your onions sooner, if you see signs of mold.
The pungent onions harvested in late summer and early fall can be stored for months. They contain sulfurous compounds (the stuff that makes you cry when you cut them) that help to preserve them. To maximize their storage life, store your dry bulb onions in a cool, dry, well-ventilated spot. Do not store them in plastic bags; they need to be able to breathe.
How to Cook
If cutting onions makes your eyes water, a really sharp knife actually makes a difference. Some say “the sharper the knife, the less you cry.”
Peeling the onion under running water can help a little if the onion is really bothering your eyes.
Slicing onions makes you cry because when you cut into it, the onion produces a sulfur-based gas. The gas reacts with the water in your eyes and forms sulfuric acid. To rid your eyes of this irritant, your tear ducts work overtime. For fewer tears, try moving your face farther away from the onion so the gas disperses before reaching your eyes. Another suggestion for reducing tears is to first chill the onions for 30 minutes. Then, cut off the top and peel the outer layers leaving the root end intact.
Here are some ideas for incorporating onions into your diet:
- Use raw onions to add flavor and crunch to your guacamole recipe.
- Add caramelized onions to savory baked goods.
- Combine cooked onions with other vegetables for a side dish.
- Add thinly sliced red onions to your favorite salad.
- Make a fiber-rich salad with chickpeas, chopped onions and red peppers.
- Use onion and garlic as a base for stocks and soups.
- Throw onions into stir-fry dishes.
- Top tacos, fajitas and other Mexican dishes with chopped raw onions.
- Make a homemade salsa with onions, tomatoes and fresh cilantro.
- Prepare a hearty onion and vegetable soup.
- Add onions to chili recipes for a flavor boost.
- Blend raw onions with fresh herbs, vinegar and olive oil for homemade salad dressing.