The evidence shows that this is just not true. I myself, twenty years ago or more, read this statement, probably made by Fred Stare, professor then at Harvard School of Public Health, and I decided to check. I collected my urine for 24 hours and analyzed it myself for the vitamin C content. Of course, some of the ingested ascorbate remains in the intestinal contents and doesn't get into the blood stream. It may be as much as a third.
Some evidence indicates that perhaps as much as a third remains in the intestinal contents.
Well, this does good, protecting the lower bowel against cancer by destroying carcinogens that are present in the fecal material and also does good because of the laxative effect of bringing water into the bowel so that the volume of the waste material is larger. There's also a smaller surface area which helps speed up the process of elimination of this material. The rest of it, two thirds perhaps 6.5 grams when I was taking 10 grams a day, gets into the blood stream but only 1.5 grams is eliminated in the urine. I'm sure, in fact we have direct experimental evidence for it, is that vitamin C is rapidly converted into other substances, oxidation products and these other substances, these oxidation products have been shown to have greater value against cancer than vitamin C itself. So if you take large doses of vitamin C you produce large amounts of these other substances, the value of which is still under investigation.