Onions are highly nutritious vegetables that may have several benefits, including improved heart health, better blood sugar regulation, and increased bone density.

Onions are members of the Allium genus of flowering plants, which also includes garlic, shallots, and leeks.

They’re delicious, versatile, and relatively cheap, and they boast a wide range of healthy vitamins, minerals, and plant compounds.

The medicinal properties of onions have been recognized for thousands of years. Athletes in ancient Greece supposedly used onions to purify their blood, while medieval and traditional doctors prescribed them to help treat headaches, heart disease, and mouth sores.

Read on to discover 9 health benefits of onions.

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Onions are nutrient-dense, meaning they’re low in calories but high in vitamins, fiber, and minerals.

One medium onion (110 grams [g]) contains:

  • Calories: 44
  • Protein: 1.2 g
  • Carbs: 10.3 g
  • Sugar: 4.7 g
  • Fiber: 1.9 g
  • Fat: 0.1 g
  • Potassium: 3.4% of Daily Value (DV)
  • Vitamin C: 9% of the DV

Onions are high in vitamin C, which may help regulate your immune health, collagen production, and iron absorption.

It’s also a powerful antioxidant that could help protect your cells from unstable, damaging molecules called free radicals.

Onions are rich in B vitamins, including folate and vitamin B6. These play key roles in metabolism, red blood cell production, and nerve function.

Lastly, onions are a good source of potassium, a mineral that may help with:

  • cellular function
  • fluid balance
  • nerve transmission
  • kidney function
  • muscle contraction

The average potassium intake of Americans is less than half the recommended DV of 4,700 milligrams (mg). So, adding onions to your diet is a great way to increase your potassium intake.

Onions contain antioxidants and compounds that may reduce your risk of heart disease by fighting inflammation and lowering triglyceride and cholesterol levels.

They contain a large amount of quercetin, a flavonoid antioxidant and anti-inflammatory that may help lower high blood pressure.

A small 2015 study in 70 people with overweight and hypertension suggests that a daily dose of 162 mg of quercetin-rich onion extract may significantly reduce systolic blood pressure by 3.6 millimeters of mercury.

Also, a small 2014 study in 54 females with polycystic ovary syndrome found that consuming 80–120 g of raw red onions per day for 8 weeks lowered total and LDL (bad) cholesterol levels.

However, more research is needed.

Antioxidants are compounds that inhibit oxidation, a process that may lead to cellular damage and contribute to diseases such as cancer, diabetes, and heart disease.

Onions are an excellent source of antioxidants and contain at least 17 types of flavonoids.

Red onions, in particular, contain anthocyanins, plant pigments in the flavonoid family that give red onions their deep color. These may protect against diabetes and certain types of cancer.

In a 2016 study involving 43,880 males, researchers found that habitual anthocyanin intakes up to 613 mg were correlated with a 14% lower risk of nonfatal heart attacks.

Similarly, the authors of a 2019 review concluded that consuming more anthocyanin-rich foods was associated with a lower risk of heart disease and of death from heart disease.

Allium vegetables such as onions and garlic may lower your risk of developing certain types of cancer, including stomach and colorectal cancers.

In a 2015 review of 26 studies, researchers concluded that people who consumed the most allium vegetables were 22% less likely to receive a diagnosis of stomach cancer than those who consumed the least.

And in a 2014 review of 16 studies involving a total of 13,333 people, researchers suggested that people with the greatest onion intake had a 15% lower risk of colorectal cancer than those with the lowest intake.

Test-tube studies suggest that onionin A, a sulfur-containing compound in onions, may help decrease tumor development and slow the spread of ovarian cancer.

Onions also contain fisetin and quercetin, which are flavonoid antioxidants that may inhibit tumor growth.

Eating onions may help regulate blood sugar levels, which is significant for people with diabetes or prediabetes.

A small 2010 study in 84 people with type 1 or type 2 diabetes found that eating 100 g of raw red onion significantly reduced fasting blood sugar levels after 4 hours.

A 2020 study showed that rats with diabetes who ate food containing 5% dried onion powder for 8 weeks had decreased fasting blood sugar levels and lower triglyceride and cholesterol levels than a control group.

Quercetin has also been shown to help regulate whole-body blood sugar balance by interacting with cells in the:

  • small intestine
  • pancreas
  • skeletal muscle
  • fat tissue
  • liver

Dairy gets much of the credit for boosting bone health, but other foods, including onions, may also help support strong bones.

A small 2016 study in 24 middle-aged and postmenopausal females found that those who consumed 100 milliliters of onion juice daily for 8 weeks had improved bone mineral density and antioxidant activity compared to a control group.

Also, a 2009 study in 507 perimenopausal and postmenopausal females found that those who ate onions at least once per day had a 5% greater overall bone density than those who ate onions once per month or less often.

Onions may help reduce oxidative stress, boost antioxidant levels, and decrease bone loss. This may help prevent osteoporosis and improve bone density.

Onions may help fight potentially dangerous bacteria such as:

A 2010 test-tube study suggests that onion extract might inhibit the growth of Vibrio cholerae, a type of bacteria that is a major public health concern in some parts of the world.

Quercetin extracted from onions may also reduce bacteria growth.

One review suggests that it could inhibit the growth of several strains of bacteria, including Helicobacter pylori, a type of bacteria associated with stomach ulcers and certain digestive cancers.

Another test-tube study found that quercetin damaged the cell walls and membranes of E. coli and S. aureus.

Onions are a rich source of fiber and prebiotics, which are necessary for optimal gut health.

Prebiotics are nondigestible types of fiber that are broken down by beneficial gut bacteria.

Gut bacteria feed on prebiotics and create short-chain fatty acids, which may help:

  • strengthen gut health
  • boost immunity
  • reduce inflammation
  • enhance digestion

Consuming prebiotic foods may also help increase probiotics, such as Lactobacillus and Bifidobacterium strains, which benefit digestive health.

Onions are rich in the prebiotics inulin and fructooligosaccharides, which may help increase the number of friendly bacteria in your gut and improve immune function.

Onions are a fresh and versatile staple in kitchens around the world. They can be cooked, fried, eaten raw, and more.

To incorporate onions into your diet, you can try:

  • using them in soups such as French onion soup
  • using them in dips and spreads such as guacamole, salsa, and ranch
  • adding them to egg dishes such as omelets, frittatas, and quiches
  • making cooked toppings, such as caramelized onions, to top meat or tofu or add to savory baked goods
  • using them raw as a topping for tacos, fajitas, and other Mexican dishes and savory baked goods
  • adding them to salads, such as a chickpea, chopped onion, and red pepper salad
  • using them in stir-fries, pasta sauces, or curries

Onions are nutrient-packed vegetables that contain powerful compounds that may help decrease your risk of heart disease and certain cancers.

They have antibacterial properties and promote digestive health, which may improve immune function.

What’s more, they’re versatile and can be used to heighten the flavor of any dish.