Quercetin is a dietary flavonoid abundant in a variety of foods including apples, berries, brassica vegetables, grapes, onions, shallots, tea, and tomatoes as well as many seeds, nuts, barks and leaves. Quercetin has a bitter flavour and is used as an ingredient in dietary supplements, beverages, and foods. Quercetin is one of the most abundant dietary flavonoids, with an average daily consumption of 25–50 milligrams. In red onions, higher concentrations of quercetin occur.
Quercetin has antioxidant and anti-inflammatory effects which might help reduce inflammation, kill cancer cells, control blood sugar, and help prevent heart disease. Some research suggests that eating foods rich in quercetin, such as tea, onions and apples, may reduce the risk of death due to heart disease in elderly men. quercetin reduces pain and stiffness in women with Rheumatoid arthritis.
Quercetin in cancer
A higher intake of quercetin as part of the diet has been linked with a lower risk of lung cancer in people who smoke.
Some research suggests that eating high amounts of quercetin in the diet might reduce the chance of developing pancreatic cancer, especially in men who smoke.
Precautions with Quercetin
Oral intake of quercetin in doses up to one gram per day over three months did not cause adverse effects. There have been reports of kidney damage at higher doses.
Side effects may include flushing, sweating, nausea, vomiting, difficulty breathing, or pain at the injection site.
There isn't enough reliable information to know if quercetin is safe to use when pregnant or breastfeeding. Stay on the safe side and avoid use.