Onions (Allium cepa) are bulb-shaped vegetables that grow underground.

Also known as bulb onions or common onions, they are grown worldwide and closely related to chives, garlic, scallions, shallots, and leeks.

Onions may have several health benefits, mostly due to their high content of antioxidants and sulfur-containing compounds.

They have antioxidant and anti-inflammatory effects and have been linked to a reduced risk of cancer, lower blood sugar levels, and improved bone health.

Commonly used as a flavoring or side dish, onions are a staple food in many cuisines. They can be baked, boiled, grilled, fried, roasted, sautéed, powdered, or eaten raw.

Onions vary in size, shape, and color, but the most common types are white, yellow, and red. The taste ranges from mild and sweet to sharp and spicy, depending on the variety and season.

Onions can also be consumed when immature, before the bulb reaches full size. They are then called scallions, spring onions, or summer onions.

This article tells you everything you need to know about onions.

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Raw onions are very low in calories, with only 40 calories per 3.5 ounces (100 grams).

By fresh weight, they are 89% water, 9% carbs, and 1.7% fiber, with tiny amounts of protein and fat.

The main nutrients in 3.5 ounces (100 grams) of raw onions are (1):

  • Calories: 40
  • Water: 89%
  • Protein: 1.1 grams
  • Carbs: 9.3 grams
  • Sugar: 4.2 grams
  • Fiber: 1.7 grams
  • Fat: 0.1 grams

Carbs

Carbohydrates make up about 9–10% of both raw and cooked onions.

They consist mostly of simple sugars, such as glucose, fructose, and sucrose, as well as fiber.

A 3.5-ounce (100-gram) portion contains 9.3 grams of carbs and 1.7 grams of fiber, so the total digestible carb content is 7.6 grams.

Fibers

Onions are a decent source of fiber, which accounts for 0.9–2.6% of the fresh weight, depending on the type of onion.

They are very rich in healthy soluble fibers called fructans. In fact, onions are among the main dietary sources of fructans (2, 3).

Fructans are so-called prebiotic fibers, which feed the beneficial bacteria in your gut.

This leads to the formation of short-chain fatty acids (SCFAs), such as butyrate, which may improve colon health, reduce inflammation, and cut your risk of colon cancer (4, 5, 6).

However, fructans are considered FODMAPs, which may cause unpleasant digestive symptoms in sensitive individuals, such as those with irritable bowel syndrome (IBS) (7, 8, 9).

SUMMARY Onions consist mostly of water, carbs, and fiber. Their main fibers, fructans, can feed the friendly bacteria in your gut, though they may cause digestive problems in some people.

Onions contain decent amounts of several vitamins and minerals, including:

  • Vitamin C. An antioxidant, this vitamin is needed for immune function and maintenance of skin and hair (10, 11, 12).
  • Folate (B9). A water-soluble B vitamin, folate is essential for cell growth and metabolism and especially important for pregnant women (13).
  • Vitamin B6. Found in most foods, this vitamin is involved in the formation of red blood cells.
  • Potassium. This essential mineral can have blood pressure-lowering effects and is important for heart health (14, 15).
SUMMARY Onions contain decent amounts of vitamin C, folate, vitamin B6, and potassium, which provide a number of benefits.

The health benefits of onions are attributed to their antioxidants and sulfur-containing compounds (3).

In many countries, onions are also among the main dietary sources of flavonoids, specifically a compound called quercetin (16, 17, 18).

The most abundant plant compounds in onions are:

  • Anthocyanins. Only found in red or purple onions, anthocyanins are powerful antioxidants and pigments that give these onions their reddish color.
  • Quercetin. An antioxidant flavonoid, quercetin may lower blood pressure and improve heart health (19, 20).
  • Sulfur compounds. These are mainly sulfides and polysulfides, which may protect against cancer (21, 22, 23).
  • Thiosulfinates. These sulfur-containing compounds may inhibit the growth of harmful microorganisms and prevent the formation of blood clots (24).

Red and yellow onions are richer in antioxidants than other types. In fact, yellow onions may contain almost 11 times more antioxidants than white onions (25).

Cooking can significantly reduce levels of some antioxidants (26).

SUMMARY Onions are rich in plant compounds and antioxidants, especially quercetin and sulfur-containing compounds. Colorful varieties, such as yellow or red ones, pack more antioxidants than white ones.

Onions have been shown to have strong antioxidant and anti-inflammatory properties (3, 28, 29, 30).

Blood sugar regulation

Type 2 diabetes is a common disease, characterized primarily by high blood sugar levels.

Animal studies suggest that onions can lower blood sugar levels (31, 32, 33).

The same results have been shown in humans. One study in people with type 2 diabetes found that eating 3.5 ounces (100 grams) of raw onions per day led to a significant reduction in blood sugar levels (34).

Raw onions may help control both type 1 and 2 diabetes, but more research is needed (35, 36).

Bone health

Osteoporosis is a common health problem, especially in postmenopausal women. A healthy diet is one of the main preventive measures (37, 38).

Animal studies reveal that onions protect against bone deterioration and may even increase bone mass (39, 40, 41).

A large observational study in women over 50 years of age found that regular onion consumption is linked to increased bone density (42).

Further research indicates that intake of selected fruit, herbs, and vegetables, including onions, may reduce bone loss in postmenopausal women (43).

Reduction of cancer risk

Cancer is a common disease, characterized by uncontrolled cell growth. It is one of the world's leading causes of death.

Observational studies have linked increased consumption of onions to a reduced risk of several types of cancers, such as those of the stomach, breast, colon, and prostate (44, 45, 46, 47, 48, 49).

SUMMARY Onions have antioxidant and anti-inflammatory effects. They may lower blood sugar levels, improve bone health, and reduce the risk of several types of cancers.

Eating onions can lead to bad breath and an unpleasant body odor.

Several other downsides may make this vegetable unsuitable for some people.

Onion intolerance and allergy

Onion allergy is relatively rare, but intolerance to raw varieties is fairly common.

Symptoms of onion intolerance include digestive disruption, such as upset stomach, heartburn, and gas (50).

Some people may experience allergic reactions from touching onions, whether or not they’re allergic to eating them (51).

FODMAPs

Onions contain FODMAPs, which are a category of carbs and fibers that many people cannot tolerate (9, 52, 53).

They may cause unpleasant digestive symptoms, such as bloating, gas, cramping, and diarrhea (7, 8).

Individuals with IBS are often intolerant to FODMAPs and may want to avoid onions.

Eye and mouth irritation

The most common issue with preparing and cutting onions is eye irritation and tear production. When cut, an onion's cells to release a gas called lachrymatory factor (LF) (54).

The gas activates neurons in your eyes that cause a stinging sensation, followed by tears that are produced to flush out the irritant.

Leaving the root end intact while cutting may reduce irritation, as the onion base has a higher concentration of these substances than the bulb.

Cutting onions under running water may also prevent this gas from dissolving into the air.

LF is also responsible for the burning sensation in your mouth when onions are eaten raw. This burning sensation is reduced or eliminated by cooking (55).

Dangerous for pets

While onions are a healthy component of human diets, they can be deadly for some animals, including dogs, cats, horses, and monkeys (56).

The main culprits are sulfoxides and sulfides, which can induce a disease called Heinz body anemia. This illness is characterized by damage within animals’ red blood cells, which leads to anemia (57).

Make sure not to feed onions to your pet, and keep anything flavored with onions out of reach if you have an animal in your home.

SUMMARY Onions may cause adverse digestive effects in some people, and raw onion can cause eye and mouth irritation. Onions may be toxic to some animals.

Onions are a root vegetable with a variety of benefits.

They’re high in antioxidants and sulfur-containing compounds, some of which may have a number of beneficial effects.

Although more research is needed, onions have been linked to improved bone health, lower blood sugar levels, and a reduced risk of cancer.

On the other hand, they may cause digestive problems in some people.

If you enjoy them, onions can be a valuable component of a healthy diet.