Some of the different nutrients you’ll find in 1 cup of cooked onions free of salt include 349 milligrams (mg) of potassium, 73.5 mg of phosphorus, 46.2 mg of calcium, and a little over 23 mg of magnesium (per U.S. Department of Agriculture).
Onions also contain a flavonoid known as quercetin. Researchers from a 2017 study published in Lipids in Health and Disease set out to better understand the relationship between quercetin and its potential effects on reverse cholesterol transport (RCT), or the process in which the body removes cholesterol from the cells. Using mice who had been fed a high-fat diet, it was found that quercetin boosted cholesterol transfer, lowered cholesterol levels, and enhanced “good” HDL cholesterol functioning.
Even alternate forms of onion have been shown to offer these same cholesterol-lowering benefits. In an 8-week study published in 2015 in Plant Foods for Human Nutrition, researchers found that drinking quercetin-rich onion juice significantly reduced LDL cholesterol levels in patients with mildly high cholesterol.