10 Research-Based Health Benefits of Onion

Onions are a staple ingredient of almost all cuisines around the world. But do they add any health benefit to the dishes, or are they used only for flavor?

We try to incorporate various vegetables and fruits into our diet to get many minerals and vitamins. Onions are often ignored in this process.


Many of us are not aware that this pale vegetable is a nutritional powerhouse. Onions are a good source of vitamin C, vitamin B6, potassium, and folate.

They are also high in antioxidants and sulfur-containing compounds.

Facts about onions

Onions (Allion cepa) are a part of the allium family of vegetables, which includes chides, garlic, scallions, and leeks.

Several types of onion are grown around the world. The most common varieties of onions are red, yellow, and white onions.

Around 105 billion pounds of onions are harvested every year worldwide.

History of onions

Many archaeologists and botanists believe that onions originated in central Asia. Some research also suggests that they were first grown in Iran and West Pakistan.

Onions have been cultivated for over 5000 years. They are considered to be one of the first cultivable crops because they were less perishable and could easily be transported.

In China, onions were grown as early as 5000 years ago. Some ancient Vedic writings also mention the use of onions.

In Egypt, onions were grown as far back as 3500 BC. Here they were considered as an object of worship.

To Egyptians, onions symbolized eternity and were buried along with the pharaohs.

The first Pilgrims brought onions with them to America. They later found that some strains of wild onions grew throughout North America and that the native American Indians used them in a variety of ways for cooking.

Nutritional profile of onions

One cup (160 grams) of chopped onions contains (1) –

  • Calories: 60
  • Carbohydrates: 15 grams
  • Dietary fiber: 3 grams
  • Sugars: 7 grams
  • Protein: 2 grams
  • Vitamin C: 11.8 mg (20% DV)
  • Vitamin B6: 0.2 mg (10% DV)
  • Folate :  30.4 mcg (8% DV)
  • Thiamin : 0.1 mg (5% DV)
  • Manganese: 0.2 mg (10% DV)
  • Potassium: 234 mg (7% DV)
  • Phosphorus : 46.4 mg (5% DV)
  • Calcium : 36.8 mg (4% DV)
  • Magnesium: 16 mg (4% DV)

Onions also contain antioxidants and sulfur-containing compounds. These include –

Anthocyanins: Anthocyanins are potent antioxidants that give onions their typical reddish color.

Quercetin: Quercetin is an antioxidant flavonoid that helps lower blood pressure.

Sulfur compounds: These sulfides and polysulfides help protect against cancer.

Thiosulfinates: These sulfur-containing compounds help prevent blood clots and inhibit the growth of harmful microorganisms.

Health benefits of onions

1. Reduces the risk of cancer

Allium vegetables like onions and garlic contain an organosulfur compound that is known to lower the risk of developing cancer.

According to a meta-analysis of 26 studies, people who consume high amounts of allium vegetables are 22 % less likely to be diagnosed with stomach cancer, as compared to the people who eat little amounts of allium vegetables (2).

Onions contain Onionin A, a sulfur-containing compound which impairs tumor development and lung metastasis.

This compound may be a potential tool for antitumor therapy and tumor prevention (3).

Various studies have found that consuming onions can help lower the risk of colon, prostate, stomach, and esophageal cancer as well (4, 5, 6).

2. Controls blood sugar

Eating onions can help control blood sugar levels. Onions contain quercetin and other sulfur compounds that have anti-diabetic effects.

Quercetin interacts with many molecular targets in the small intestine, pancreas, skeletal muscle, adipose tissue, and liver to control whole-body glucose homeostasis (7).

According to a 2009 study from the Republic of Korea, onion extract intake can be useful in lowering plasma glucose concentrations (8).

In 2010, a study was conducted to study the effects of red onion in type 1 and type 2 diabetic patients.

The researchers concluded that onion produced hypoglycaemic effects, so it could be used as a dietary supplement in the management of type 1 and type 2 diabetes mellitus (9).

3. Prevents osteoporosis

Onions can help prevent osteoporosis, the thinning of bones that often comes with age.

Onions contain flavonoids like quercetin that protect the bones from free-radical damage.

They also inhibit the development and differentiation of osteoclasts, which prevents a bone break down.

Onions also contain sulfur compounds that the body needs to produce glutathione, a major intracellular antioxidant, and prevents excessive homocysteine accumulation, which damages bones and arteries.

During a study, 24 postmenopausal women took 100 ml onion juice or a placebo every day for eight weeks.

The results showed that the group that took onion juice had improved bone mineral density and antioxidant activity as compared to the group that took a placebo (10).

Another study from the Medical University of South Carolina showed that onion consumption has a beneficial effect on the bone density of perimenopausal and postmenopausal non-Hispanic white women over 50.

The study also concluded that older women who consume onions more frequently may decrease the risk of hip fractures by more than 20 % as compared to those who never consumed onions (11).

4. Improves heart health

Onions improve heart health by lowering blood pressure and reducing the risk of heart attacks.

The sulfur in onions acts as a natural blood thinner and prevents blood platelets from aggregating. When platelets cluster, they increase the risk for heart attacks and stroke.

Onion contains the polyphenol quercetin, which can prevent cardiovascular disease due to its antihypertensive and vasorelaxant properties.

According to a 2015 study from Germany, when 70 overweight people with high blood pressure were given 162 mg of onion extract, it helped reduce systolic blood pressure by 3 to 6 mm Hg compared to the placebo (12).

According to a 2016 study, consuming onions increases oxylipins that help regulate blood fat levels and levels of cholesterol (13).

During an investigation, 54 overweight women with polycystic ovary syndrome were asked to consume raw red onion for eight weeks.

The results showed that onion significantly decreased the levels of total cholesterol in these women (14).

5. Prevents bacterial infections

Onions have strong antibacterial properties. It is effective against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus cereus (15).

Onion is also effective against Vibrio cholerae. Cholera is a significant problem in the developing world.

Due to the spread of microbial drug resistance, newer antimicrobials are required to overcome this problem.

Thanks to the inhibitory effect of onion on V. cholerae, onion can be used as a therapeutic agent (16).

Research has also shown that certain quercetin oxidation products from yellow onion were effective against MRSA and H. pylori (17).

6. Improves digestion

The dietary fibers in onions help to keep you regular and promote healthy digestion.

Onions contain a particular type of soluble fiber called oligofructose, which is known to promote the growth of good bacteria in the intestines.

According to a 2015 study from the UK, prebiotic oligofructose can alter fecal bacterial flora and in addition to antibiotic treatment, and reduce the rate of relapse from C difficile infection (18).

Onions also contain phytochemicals that help reduce the risk of developing gastric ulcers.

7. Boosts immunity

Onions contain selenium, which is known to reduce inflammation and stimulate the immune system.

According to a study published in the journal Antioxidants and Redox Signaling, Adequate levels of selenium are essential for initiating immunity.

It is also involved in regulating excessive immune response and chronic inflammation. Selenium deficiency can harm the immune cells during activation, differentiation, and proliferation (19).

Onion also contains a significant amount of vitamin C, which can help boost immunity.

8. Improve eye health

Onions contain B vitamins, the deficiency of which may lead to pink eye. Onions are also rich in vitamin C, which is a powerful antioxidant and helps reduce inflammation in the eye.

Onions are also rich in the sulfur compound called allyl propyl disulfide, which irritates the tear ducts and makes you cry. Crying cleanses the cornea of the eyes and also helps treat dry eyes.

Onions also contain glutathione, a sulfur-rich protein which protects eyes from diseases like glaucoma, cataracts and macular degeneration.

Quercetin content in onions helps reduce eye inflammation and relieve eye irritation.

9. Improve skin and hair

Onions are rich in sulfur, several B vitamins and vitamins A, C, E. They help in improving blood circulation, rejuvenating the dull complexion, and protect the body from UV rays.

Onions contain quercetin, a powerful antioxidant, that helps fight free radicals the keeps the skin looking healthy and young.

Onions also help to fight acne, thanks to its antibacterial, antimicrobial, antiseptic and anti-inflammatory properties.

Raw onion juice mixed with olive oil is also known to strengthen hair follicles and stimulate healthy hair growth.

10. Improves sleep and reduces depression

The folate content in onion help to alleviate depression. Homocysteine, a common amino acid, prevents blood and other nutrients from reaching the brain and leads to depression (20).

Excess homocysteine can also interfere in the production of serotonin, dopamine, and norepinephrine, which regulate mood and sleep.

The folate content in onions helps prevent homocysteine from building up. According to an animal-based study from Japan, quercetin-rich onions exert antidepressant-like activity in rats (21).

Side effects of onions

While onions provide many health benefits, eating excessive amounts of onions can have some side effects.

1. Allergies

Some people develop an allergic reaction while eating or coming in contact with onions.

The symptoms include reddening of the skin, itchy eyes, swelling and tingling, difficulty breathing, and a drop in blood pressure.

2. Gas and bloating

Onions contain natural sugars like fructose, which can cause intestinal gas. Onions may cause abdominal bloating, discomfort, and increased flatulence.

3. Heartburn

According to the American Journal of Gastroenterology, eating excessive amounts of onions can cause problems like heartburn, especially in pregnant women.

4. Drug interactions

Green onions are rich in vitamin K. Having too much vitamin K can interfere with certain blood-thinning medications like Coumadin.

Onions are also a good source of potassium, which is a vasodilator. Potassium helps regulate blood pressure and reduces the risk of hypertension.

However, too many onions in the diet can lead to hypotension (low blood pressure), which can cause fatigue, dizziness, nausea, blurry vision, and light-headedness.


5. Dangerous for pets

Onions can be hazardous for animals like dogs, cats, horses, and monkeys. They contain sulfides and sulfoxides, which can cause Heinz body anemia.

This illness can damage the red blood cells in animals and lead to anemia.

Final thoughts

Onions, a part of the allium family of vegetables, provide an impressive variety of health benefits.

They are a rich source of vitamin C, folate, vitamin B6, potassium, calcium, and manganese.

Onions also contain antioxidants and sulfur-containing compounds like quercetin, anthocyanins, and thiosulfinates.

Consuming onions can lower the risk of cancer, control blood sugar, prevent osteoporosis, improve heart health, prevent bacterial infections, and improve digestion.

However, avoid consuming an excessive amount of onions as they can lead to gas, heartburn.

Onions can have an allergic reaction in some people. If you are taking blood thinners like Coumadin, avoid excessive amounts of onions as they may interact with these medicines.


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