I don't no how this thread got onto vitamins D and C but i read some incorrect information, so i'll post what i learned from Linus Pauling with regard to C.
1. Vitamin C (ascorbic acid) is not an oxidizing agent. It is a reducing agent.
2. There is a substantial body of clinical studies done on Vitamin C. Much of it was done in England and usually involved massive doses.
3. Among other things, these studies showed that the humans could withstand very large doses of vitamin C with few adverse effects, mostly gastric disturbance. In at least one study a large dose of Vitamin C was administered intravenously to a comatose patient, if i recall correctly. The patient recovered.
4. It was shown rather convincingly that massive doses, similar to 20 grams over 24 hours, could cure the common cold caused by rhinovirus if administered when symptoms first appeared. It was less effective when administered later.
5. Clinical studies were repeated in the US but using much smaller doses. The results did not agree with those from the British studies using very large doses.
6. It requires an approximately 10-fold increase in ingested Vitamin C to double the level of Vitamin C in blood.
I heard Pauling lecture on these studies, and these are my recollections of what he said in that lecture. I know that Pauling (who was a Physical Chemist, not an M.D., took a large dose of Vitamin C daily for much of his life ( the number 2 grams with each meal sticks in my head, but i am not certain of that anymore as it was years ago that i heard him speak on this subject.) He died in his nineties of prostate cancer, but was healthy and vigorous until near the end. He was absolutely convinced that the MDR set by the FDA for both folic acid and for Vitamin C were well below optimal.
Incidentally, Pauling, years before scoliosis of the spine in newborns was connected to folic acid deficiencies, said that the FDA's MDR for folic acid was woefully in error and needed to be much higher. An interesting main point of his lecture was that the FDA MDR values were set at the minimums to prevent disease, but for many of the vitamins, were far from optimal. He also believed that American medicine was way too conservative, and often said so.
Pauling won two Nobel prizes, and in my opinion came very close to winning four.