Vitamin C much of the basis of the controversy on current recommended dietary allowances. It is countered by arguments that humans are very good at conserving dietary vitamin C, and are able to maintain blood levels of vitamin C comparable with simians on a far smaller dietary intake, perhaps by recycling oxidized vitamin C. There are many different biosynthesis pathways for ascorbic acid in plants. Most of these pathways are derived from products found in glycolysis and other pathways. For example, one pathway goes through the plant cell wall polymers. The plant ascorbic acid biosynthesis pathway most principal seems to be -galactose.
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