Dehydroascorbic acid DHA gives a pentacyclic dimer structure of indefinite stability. Recycling of ascorbate via active transport of DHA into cells, followed by reduction and reuse, mitigates the inability of humans to synthesize it from glucose. Vitamin C accumulates in mitochondria, where most of the free radicals are produced, by entering as DHA through the glucose transporters, GLUT10. Ascorbic acid protects the mitochondrial genome and membrane. Vitamin C does not pass from the bloodstream into the brain, although the brain is one of the organs that have the greatest concentration of vitamin C. Instead, DHA is transported through the blood–brain barrier via
Vitamin C efficiency decreases as amounts increase. Intravenous administration bypasses this. Doing so makes it possible to achieve plasma concentrations of 5 to 10 millimoles/liter (mmol/L), which far exceed the approximately 0.2 mmol/L limit from oral consumption. The theories of mechanism are contradictory. At high tissue concentrations ascorbic acid is described as acting as a pro-oxidant, generating hydrogen peroxide (HO) to kill tumor cells. The same literature claims that ascorbic acid acts as an antioxidant, thereby reducing the adverse effects of chemotherapy and radiation therapy. Research continues in this field, but a 2014 review concluded: "Currently, the use of high-dose intravenous vitamin