Quercetin is contained in abundance in apples, raspberries, onions, red grapes, cherries, citrus fruits, and green leafy vegetables. Among vegetables and fruits, quercetin content is highest in onions. Preparation and storage of food can affect quercetin content in it. Fried or boiled foods contain lower quercetin content. Long-term storage of foods was found to change their quercetin content. While onions lose their quercetin content by up to 33% in the first 12 days of storage, quercetin level in strawberries can actually increase while stored.
A new study was just published evaluating the effect of supplementing quercetin as a nutraceutical on parameters of cardiovascular health. This involved a review and meta-analysis of 17 studies involving 896 participants given quercetin or placebo. Quercetin significantly lowered both systolic and diastolic blood pressure. Both measurements were lowered by about 3 mmHg. Blood glucose and lipid measurements were not significantly lowered. Quercetin may lower blood pressure by increasing nitric oxide. Nitric oxide is a substance that causes relaxation of blood vessels resulting in better blood pressure control.
Increasing your intake of quercetin by focusing on a higher intake of whole foods rich in it is advised for all. This is particularly true in follow up of another recent study demonstrating that eating 2 apples a day lowers blood lipid measurements. Supplemental quercetin is easily found in stores and online. Generally, doses of 50-500 mg a day have been found to be safe for most people. It is possible that it could interact with warfarin, chemotherapy medications, corticosteroids, and cyclosporine.
Quercetin can also be considered for allergies, asthma, and hives and is often combined with bromelain and vitamin C as QBC.