The onion (Allium cepa L., from Latin cepa “onion”), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus Allium. Its close relatives include the garlic, scallions, shallots, leek, chive, and Chinese onion.
This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (Allium fistulosum), the tree onion (A. xproliferum), and the Canada onion (Allium canadense). The name “wild onion” is applied to a number of Allium species, but A. cepa is exclusively known from cultivation. Its ancestral wild original form is not known, although escapes from cultivation have become established in some regions. The onion is most frequently a perennial plant, but is usually treated as an annual and harvested in its first growing season.
The onion plant has a fan of hollow, bluish-green leaves and its bulbs at the base of the plant begins to swell when a certain day-length is reached. The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem.
In the autumn (or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become dry and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly the onion fly, the onion eelworm, and various fungi cause rotting. Some varieties of A. cepa, such as shallots and potato onions, produce multiple bulbs.
Onions are cultivated and used around the world. As a food item, they are usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make pickles or chutneys. They are pungent when chopped and contain certain chemical substances which irritate the eyes.
HEALTH BENEFITS OF ONIONS
■ PACKED WITH NUTRIENTS
Onions are nutrient-dense, meaning they’re low in calories but high in vitamins and minerals.
One medium onion has just 44 calories but delivers a considerable dose of vitamins, minerals and fiber.
This vegetable is particularly high in vitamin C, a nutrient involved in regulating immune health, collagen production, tissue repair and iron absorption.
Vitamin C also acts as powerful antioxidant in your body, protecting your cells against damage caused by unstable molecules called 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.
Lastly, they’re a good source of potassium, a mineral in which many people are lacking.
In fact, the average potassium intake of Americans is just over half the recommended daily value (DV) of 4,700 mg.
Normal cellular function, fluid balance, nerve transmission, kidney function and muscle contraction all require potassium.
Onions are low in calories yet high in nutrients, including vitamin C, B vitamins and potassium.
■ MAY BENEFIT HEART HEALTH
Onions contain antioxidants and compounds that fight inflammation, decrease triglycerides and reduce cholesterol levels – all of which may lower hearts disease risk.
Their potent 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.
Onions have also been shown to decrease cholesterol levels.
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 “bad” LDL cholesterol compared to a control group.
Additionally, evidence from animal studies supports that onion consumption may reduce risk factors for hearts disease, including inflammation, high triglyceride levels and blood clot formation.
Research shows that eating onions may help reduce heart disease risk factors, such as high blood pressure, elevated triglyceride levels and inflammation.
■ LOADED WITH ANTIOXIDANTS
Antioxidants are compounds that inhibit oxidation, a process that leads to cellular damage and contributes to diseases like cancer, diabetes and heart disease.
Onions are an excellent source of antioxidants. In fact, they contain over 25 different varieties of flavononoid antioxidants.
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.
A study in 43,880 men showed that habitual intakes as high as 613 mg per day of anthocyanin were correlated to a 14% lower risk of nonfatal heart attacks.
Similarly, a study in 93,600 women observed that those with the highest intake of anthocyanin-rich foods were 32% less likely to experience a heart attack than women with the lowest intake.
Additionally, anthocyanins have been found to protect against certain types of cancer and diabetes.
Red onions are rich in anthocyanins, which are powerful plant pigments that may protect against heart disease, certain cancers and disease.
■ CONTAIN CANCER-FIGHTING COMPOUNDS
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.
Moreover, a review of 16 studies in 13,333 people demonstrated that participants with the highest onion intake had a 15% reduced risk of colorectal cancer compared to those with the lowest intake.
These cancer-fighting properties have been linked to the sulfur compounds and flavonoid antioxidants found in allium vegetables.
Onions provide onion in A, a sulfur-containing compound that has been shown to decrease tumor development and slow the spread of ovarian and lung cancer in test-tube studies.
Onions also contain fisetin and quercetin, flavonoid antioxidants that may inhibit tumor growth.
A diet rich in allium vegetables like onions may have a protective effect against certain cancers.
■ HELP CONTROL BLOOD SUGAR
Eating onions may help control blood sugar, which is especially significant for people with diabetes or pre-diabetes.
A study in 42 people with type 2 diabetes demonstrated that that eating that eating 3.5 ounces (100 grams) of fresh red onion reduced fasting blood sugar levels by about 40 mg/dl after four hour.
Additionally, multiple animal studies have shown that onion consumption may benefit blood sugar control.
A study showed that diabetic rats fed food containing 5% onion extract for 28 days experienced decreased fasting blood sugar and had substantially lower body fat than the control group.
Specific compounds found in onions, such as quercetin and sulfur compounds, possess 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.
Due to the many beneficial compounds found in onions, consuming them may help reduce high blood sugar.
■ MAY BOOST BONE DENSITY
Though dairy gets much of the credit for boosting bone health, many other foods, including onions, may help support strong bones.
A study in 24 middle-aged and postmenopausal women showed that those who consumed 3.4 ounces (100 ml) 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.
Plus, the study demonstrated than older women who most frequently ate onions decreased their risk of hip fracture by more 20% compared to those who never ate them.
It’s believed that onions help reduce oxidative stress, boost antioxidant levels and decrease bone loss, which may prevent osteoporosis and boost bone density.
Studies show that onion consumption is associated with improved bone mineral density.
■ HAVE ANTIBACTERIAL PROPERTIES
Onions can fight potentially dangerous bacteria, such as Escherichia coli (E. coli), Pseudomonas aeruginosa, Staphylococcus aureus (S. aureus) and Bacillus cereus.
Furthermore, 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.
Quercetin extracted from onions seems to be a particularly powerful way to fight bacteria.
A 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 cells walls and membranes of E. coli and S. aureus.
Onions have been shown to inhibit the growth of potentially harmful bacteria like E. coli and S. aureus.
■ MAY BOOST DIGESTIVE HEALTH
Onions are a rich source of fiber and prebiotic, which are necessary for optimal gut health.
Prebiotic are nondigestible 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.
Additionally, consuming food rich in prebiotic helps increase probiotics, suc Lactobacillus and bifidobacteria strains, which benefit digestive health.
A diet rich in prebiotic may help improve the absorption of important minerals like calcium, which may improve bone health.
Onions are particularly rich in the prebiotic inulin and fructooligosaccharides. These help increase the number of friendly bacteria in your gut and improve immune function.
Onions are a rich source of prebiotic, which help boost digestive health, improve bacterial balance in your gut and benefit your immune system.