Vitamin C and Bio-Quercetin Phytosome from Life Extension provides natural immune health support and synergistic antioxidants. Vitamin C, as ascorbic acid, is a water-soluble substance found in citrus fruits and green vegetables. Because humans do not manufacture vitamin C internally, it must be obtained through dietary sources or supplements. Few people realize, however, that vitamin C is a water-soluble nutrient that is quickly oxidized and excreted by the body, which limits its efficacy. Fortunately, a flavonoid antioxidant known as Bio-Quercetin functions as a vitamin C "supercharger." This formula helps make the vitamin C more biologically active and available.