Health risks While not especially serious, eating onions can cause problems for some people. The carbohydrates in onions may cause gas and bloating, according to National Digestive Diseases Information Clearinghouse. Onions, especially if consumed raw, can worsen heartburn in people who suffer from chronic heartburn or gastric reflux disease, according to one 1990 study in the American Journal of Gastroenterology.
Eating a large amount of green onions or rapidly increasing your consumption of green onions may interfere with blood thinning drugs, according to the University of Georgia. Green onions contain a high amount of vitamin K, which can decrease blood thinner functioning.
It is also possible to have a food intolerance or an allergy to onions, but cases are rare, according to an article in the Journal of Allergy and Clinical Immunology. People with onion allergies may experience red, itchy eyes and rashes if an onion comes into contact with the skin. People with an intolerance to onions may experience nausea, vomiting and other gastric discomfort.
Lastly, Jarzabkowski encouraged people to make sure their onions are fresh. "Onions keep for a long time," she said, "but they still spoil." Onions spoil much faster if they are chopped or sliced. If you cut up your onions for later use, be sure to refrigerate them in a closed container. A 2015 study found that unrefrigerated yellow onions showed potential growth of E.coli and salmonella, though refrigerated ones did not.
I rarely bother to put the cut ones into a closed container. And sometimes one will be out overnight.