"… [O]n an ounce-for-ounce basis, onions rank in the top 10 of commonly eaten vegetables in their quercetin content. The flavonoid content of onions can vary widely, depending on the exact variety and growing conditions.Although the average onion is likely to contain less than 100 milligrams of quercetin per 3-1/2 ounces, some onions do provide this amount.And while 100 milligrams may not sound like a lot, in the United States, moderate vegetable eaters average only twice this amount for all flavonoids (not just quercetin) from all vegetables per day."
One animal study found that animals received greater protection against oxidative stress when they consumed yellow onion in their diet as opposed to consuming quercetin extracts.Quercetin is not degraded by low-heat cooking, such as simmering. When preparing a soup with onions, the quercetin will be transferred into the broth of the soup, making onion soup an easy-to-make superfood.
"The inhibitory effects of onion consumption on human carcinoma have been widely researched …In a review on the effects of quercetin … persons in the highest consumption category versus the lowest had a 50 percent reduced risk of cancers of the stomach and alimentary and respiratory tracts.Organosulfur compounds [in onions] such as diallyl disulfide (DDS), S-allylcysteine (SAC), and S-methylcysteine (SMC) have been shown to inhibit colon and renal carcinogenesis.… Mechanisms of protection ranged from induced cancer cell apoptosis and gene transcription inhibition to protection against UV-induced immunosuppression."