Food Facts: Seven Things You Might Not Know About Onions

Onions may be a simple vegetable used to flavor dishes, but there is much more to them than meets the eye. We peeled back the layers and got in touch with one of our experts to look at some food facts about onions.

Onions contain flavonoids, unique compounds found in plants that also have health benefits. “They contain mainly the bioactive compound quercetin, which has been implemented as an antioxidant, anticancer and anti-inflammatory compound,” said Dr. Ruth McDonald, Chair and Professor of the Department of Food Science & Human Nutrition, Iowa State University.
Besides flavor, onions do not contribute much to the diet. “Onions are not very nutrient dense,” Dr. McDonald explained. One cup of chopped onion contains 234 mg of potassium, compared to about 537 mg in a banana. (One cup is a lot of onions!)
Onions contain sulfur compounds. That’s why your eyes water when you chop them and why the odor lingers on your breath.
These sulfur compounds led to onions being used for medical purposes in many cultures, such as treating wounds. However, the effect was minimal. “There is folklore that onions are antibacterial because of the presence of sulfur-containing compounds. But they are unlikely to have sufficient potency to treat infections,” Dr. McDonald said.
This belief in the antibacterial effect led to the practice of placing a cut onion by the bedside to prevent illness, with the idea that the onion would soak up germs. “Absorbing bacteria from the air is total fiction,” Dr. McDonald said. While science has not found evidence that onions prevent illness, some still follow this folklore remedy and you can follow the discussion.
It’s okay to eat leftover onions. A social media report has circulated stating that onions become toxic after you cut them. “I suppose that comes from the concept that when you cut onions there is release of the sulfur-compounds, but I don’t know of any health risks from those compounds,” Dr. McDonald said.
Onions can prevent hair loss. You just have to rub onion juice directly on your scalp.

Onions can pack a powerful punch when added to foods. Although they cannot prevent illness, onions do have some health benefits and there are many reasons they are so appealing.