Onions are members of the Allium genus of flowering plants and are related to garlic, shallots, leeks and chives.

Onions contain various vitamins, minerals and potent plant compounds that have been shown to promote health in many ways.

Onion is among the oldest cultivated plants, and is used for multiple purposes. In addition to its nutritional properties,  the antioxidant and  antimicrobial activities continue  to be extensively investigated.

Onions Have Antibacterial Properties

Onions can fight potentially dangerous bacteria, such as Escherichia coli (E. coli), Pseudomonas aeruginosa, Staphylococcus aureus (S. aureus) and Bacillus cereus (29Trusted Source).

Onion extract has also 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.

Interestingly, in lab conditions, which bacteria can be fought depends on the type of onion: green, yellow, or red. More can be read about those distinctions in this Research Gate Study.

Onions Help Control Blood Sugar

Eating onions may help control blood sugar, which is especially significant for people with diabetes or prediabetes. A study of 42 people with type 2 diabetes demonstrated that eating 3.5 ounces of fresh red onion reduced fasting blood sugar levels by about 40 mg/dl after four hours.

A 2014 review article published in Nutrition reported that preliminary research indicates that onions may have a hypoglycemic effect for people with diabetes. The authors of the article note that the sulfur compounds in onions, namely S-methylcysteine and the flavonoid quercetin, may be responsible for the effects on blood sugar. 

Onions May Benefit Heart Health

Onions contain antioxidants and compounds that fight inflammation, decrease triglycerides and reduce cholesterol levels — all of which may lower heart 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.

Onions are Packed with Antioxidants

Onions are low in calories and a source of many nutrients that promote good health, including fiber, B vitamins and a variety of antioxidants. A 1-cup serving of raw sliced onions has 46 calories, 10 grams of carbs, 2 grams of fiber and 1 gram of protein. They also meet 14 percent of the daily value for vitamin C, 7 percent of the daily value for manganese and vitamin B-6, and 5 percent of the daily value for folate.

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 flavonoid antioxidants (12Trusted Source).

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.