Natural Factors Quercetin capsules are enhanced with citrus bioflavonoids, bromelain and rutin. Quercetin, a popular bioflavonoid, is essential to optimize the body’s use of vitamin C. Bioflavonoids are brightly-colored substances that are found in fruits and vegetables, typically as companions to vitamin C. Bioflavonoids are essential for proper absorption of vitamin C and have been shown to have antioxidant properties, increase capillary strength and keep collagen healthy.