Onions are vegetables that grow as underground bulbs at the base of plants known scientifically as Allium cepa.
Also known as bulb onions or common onions, they are grown worldwide, and are 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 reduced risk of cancer, lower blood sugar levels and improved bone health.
Onions are commonly used as a flavoring or side dish, and are a staple food in Indian cooking.
There are countless ways to enjoy onions. They can be baked, boiled, grilled, fried, roasted, sauteed, powdered or eaten raw in salads.
Onions can vary in size, shape and color, but the most common types are white, yellow and red.
The taste can vary from mild and sweet to sharp and spicy, depending on 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.
Raw onions are very low in calories, with only 40 calories per 100 grams (3.5 oz).
By fresh weight, they are 89% water, 9% carbs and 1.7% fiber, with tiny amounts of protein and fat.
The table below contains information on all the main nutrients in onions (1).
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 fibers.
A 100 gram (3.5 oz) portion of onions contains 9.3 grams of carbs, and 1.7 grams of fiber, so the total digestible carbohydrate content is 7.6 grams.
Onions are a decent source of fiber, which accounts for 0.9-2.6% of the fresh weight, depending on the type of onion.
Fructans are so-called prebiotic fibers. In other words, the beneficial bacteria in the gut can use them for fuel.
However, fructans are also known FODMAPs (fermentable oligo-, di-, monosaccharides and polyols), which some people cannot digest.
Bottom line: Onions consist mostly of water, carbs and fiber. The main fibers in them, fructans, can feed the friendly bacteria in the gut, but they can also cause digestive problems in some people.
Onions contain decent amounts of several vitamins and minerals.
The main ones are listed below:
- Vitamin C: An antioxidant vitamin that is needed for immune function and maintenance of skin and hair (10, 11, 12).
- Folate (B9): A water-soluble B-vitamin that is essential for cell growth and metabolism, and is 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).
Bottom line: Onions contain decent amounts of vitamin C, folate, vitamin B6 and potassium.
The health benefits of onions are attributed to their antioxidants and sulfur-containing compounds (3).
The most abundant plant compounds in onions are listed below:
- Anthocyanins: Only found in red or purple onions, anthocyanins are powerful antioxidants and pigments that give onions their reddish color.
- Quercetin: An antioxidant flavonoid that may lower blood pressure and improve heart health (19, 20).
- Sulfur-compounds: Mainly sulfides and polysulfides, which may have protective effects against cancer (21, 22, 23).
- Thiosulfinates: Sulfur-containing compounds that may inhibit the growth of harmful microorganisms and prevent formation of blood clots (24, 25).
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 (26).
Cooking onions can significantly decrease some of the antioxidants (27).
Bottom line: Onions are rich in plant compounds and antioxidants, especially quercetin and sulfur-containing compounds. Colored varieties (yellow and red) contain more antioxidants than white ones.
There are many microorganisms in the environment, as well as inside our bodies. Some of them can cause harm.
Blood Sugar Regulation
Diabetes is a common disease, characterized primarily by high blood sugar levels.
This has also been shown in humans. One study in diabetics found that 100 grams of raw onions per day led to a significant reduction in blood sugar levels (37).
A very large observational study in women over 50 years of age found that regular onion consumption is linked with increased bone density (45).
A new controlled trial also showed that intake of selected fruit, herbs and vegetables, including onions, may reduce bone loss in postmenopausal women (46).
Cancer is a common disease, characterized by uncontrolled growth of cells in the body. It is one of the world's leading causes of death.
Bottom line: Onions have antioxidant and anti-inflammatory effects. They may help fight infections, lower blood sugar, improve bone health and reduce the risk of several types of cancers.
Eating onions can lead to bad breath and an unpleasant body odor.
There are also several other concerns with onions, which may make them unsuitable for some people.
Onion Intolerance and Allergy
Allergy to onions is relatively rare, but intolerance to raw onion is fairly common.
Symptoms of onion intolerance include digestive symptoms, such as upset stomach, heartburn and gas (53).
Some people may experience allergic reactions just from touching onions, regardless of whether they are allergic to onions that they eat (54).
Individuals who have irritable bowel syndrome (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.
This happens when the onion is cut. This causes the onion's cells to release a gas called lachrymatory factor (LF) (57).
The gas activates neurons in the eyes that cause a stinging sensation, followed by tears that are produced to flush out the irritant gas.
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 underwater or under running water may also prevent the gas from dissolving into the air.
The lachrymatory factor (LF) is also responsible for the burning sensation in the mouth when onions are eaten raw (58).
This burning sensation is reduced or eliminated by cooking.
Danger for Pets
While onions are a healthy component of the diet of humans, they can be deadly for some animals, including dogs, cats, horses and monkeys (59).
The main culprits are compounds called sulfoxides and sulfides, which can induce a disease called Heinz body anemia.
Heinz body anemia is characterized by damage within the red blood cells, which leads to anemia (60).
Make sure not to feed onion to your pet, and to keep anything flavored with onions out of reach if you have an animal in your home.
Bottom line: Onions may cause adverse digestive effects in some people, and raw onion can cause eye and mouth irritation. Onions may be toxic for some animals.
Same as with many other foods, onions have both "good" and "bad" aspects.
They are high in antioxidants and sulfur-containing compounds, some of which may have a number of beneficial health effects.
Although more research is needed, onions have been linked to improved bone health, lower blood sugar levels and reduced risk of cancer.
On the other hand, they may cause problems in some people, especially digestive problems in those who are intolerant to FODMAPs.
All of that being said, onions are usually consumed in small quantities anyway. Their main purpose is providing flavor, not nutrition.
For individuals who tolerate (and enjoy) onions, they can be a valuable component of a healthy diet.