In its article "Vitamin" the encyclopedia Britannica states:
Since they generally cannot be synthesized by an animal (or, if synthesized, the amounts are insufficient to meet body needs) and must be obtained from the diet or from some synthetic source, vitamins are called essential nutrients.
Vitamin C (also a water-soluble vitamin) can be synthesized by some organisms in sufficient amounts so that the dietary requirement is eliminated; vitamin C usually is considered a vitamin, however, because it must be included in the diet of man. Vitamins are distinct from many other compounds, which, although indispensable for proper animal functions, can be synthesized in adequate quantities.
It would appear from the above that vitamin C holds a special place in the definition of a vitamin. Inadequate synthesis is part of the definition of "vitamin", so while vitamin C is outside this definition for almost all plants and animals, it is still defined as a vitamin because man cannot synthesize vitamin C at all. Might vitamin C indeed be a special case and not a "vitamin" at all?
Irwin Stone was a leading researcher concerning vitamin C. Dr. Linus Pauling credits Stone for his (Pauling's) early interest in vitamin C research. In his book The Healing Factor "Vitamin C" Against Disease, Irwin Stone discusses at length our biological ancestry and vitamin C. If vitamin C is a necessary enzyme for almost all plants and animals and almost all plants and animals synthesize their required vitamin C, then it is truly not a vitamin because the definition of a vitamin requires that, if synthesis takes place at all, it must be "insufficient to meet body needs". So what we have is two classes of organisms, one that makes C in sufficient quantities (most all) and those that have no ability, or have lost the ability, to make this essential nutrient.
Vitamin C has been necessary for life before the evolutionary tree separated into the plant and animal kingdoms more than 600 million years ago. Since the inability to synthesize vitamin C is only present in a very few species and man possesses most of the mechanisms to synthesize vitamin C, it makes perfectly good sense that this could have developed from a genetic mutation. See another discussion of this genetic mutation on the Why Take C page.
If vitamin C requirements were viewed in the context of this genetic mutation rather than as a nutrient required in trace amounts (i.e. a "vitamin"), dosage takes on a whole new meaning. Type I diabetics learn to utilize insulin to produce the best state of health. Likewise, those of us suffering from the genetic mutation concerning vitamin C synthesis (i.e. everybody), should use vitamin C dosage in terms of its contribution to a higher state of health and homeostasis. This paradigm shift would open our eyes and our research to utilizing vitamin C in amounts more in line with what our bodies might synthesize if we still had that ability.
Indeed, much of the research that shows vitamin C to be only marginally effective was conducted based on this misconception of vitamin C's status and consequently dosages in those studies were most always very low. The conventional wisdom is that vitamin C will not prevent colds but that it will help shorten their duration somewhat. This, I believe, is entirely due to the colds and vitamin C research being done with much too low a dose. When taken in adequate amounts, vitamin C's effectiveness is dramatically better than this (see my Colds & Flu experiences page).
Convincing people of the requirement for high doses of C for optimum health is the hardest struggle for me. While I can often convince someone that vitamin C supplementation is advantageous, once I start talking about dosage I usually lose them. This is when, in their minds I think, their opinion of me changes from informed to kook. If vitamin C were viewed differently, as I have suggested above, perhaps this could change.