Onions are low in calories yet add abundant flavor to a wide variety of foods. With only 30 calories per serving, onions are sodium, fat, and cholesterol free, and provide dietary fiber, vitamin C, vitamin B6, potassium, and other key nutrients.

Higher intakes of fruits and vegetables have been associated with a variety of health benefits. Research shows that onions may help guard against many chronic diseases. That's probably because onions contain generous amounts of a flavonoid called quercetin. Other sources are tea and apples, but research shows that absorption of quercetin from onions is twice that from tea and more than three times that from apples. Studies have shown that quercetin protects against cataracts, cardiovascular disease, and cancer.

In addition, onions contain a variety of other naturally occurring chemicals known as organosulfur compounds that have been linked to lowering blood pressure and cholesterol levels.