Vitamin C plays vital roles in neutralizing free radicals, preventing cancer and scurvy, and other physiological functions. Rats synthesize vitamin C in the liver from glucose, using an enzyme called L-gulono oxidase. Humans do not synthesize vitamin C at all, due to the absence of this enzyme and perhaps also a second enzyme called D-glucurono reductase. Although most animal species can synthesize vitamin C, humans cannot. However, rats are poor predictors of human cancer risk.
Experimenters conducting such tests look for the development of cancer in rats.
Yet rats are susceptible to different types of cancers from those affecting humans. Therefore, a direct extrapolation of rat data cannot be made. In the male rat, the globulin is synthesized in the liver and secreted into the blood, from which it is then filtered by the kidney into the urine. The bound protein accumulates in kidney cells, leading to cell death. This leads to reactive cell proliferation, which eventually causes kidney tumors.