With very few exceptions, all animal species synthesize vitamin C in their bodies. Man does not. The generally accepted theory is that a mutation occurred some tens of millions of years ago to the ancestors of modern man that disabled this synthesis process (see the article C no Vitamin for additional discussion on this). Natural selection dictates that there was adequate C from dietary sources to allow a reasonable level of health, otherwise the mutation could not have taken hold. Indeed, natural selection favors those organisms with only the absolutely necessary machinery. A reader from Arizona State University, Tatiana Covington discussed this in a letter to me thus:
Scurvy is actually a species-wide enzyme deficiency disorder, exactly like Tay-Sachs, gauchers, or Niemann-Pick Type C. There are 4 enzymes which convert glucose into ascorbic acid--but we can only make the first 3. The gene for the 4th--gulonolactone oxidase--is massively damaged and totally useless. Thus we cannot make the last enzyme, and hence cannot turn gulonolactone, which is almost useless against reactive oxygen species, into ascorbic acid, which is the centerpiece of antioxidant, antiradical defense of essentially all air-exposed life on Earth. Without it, indeed, the biosphere would collapse and Earth would be a dead world within a century.
Our antioxidant protection is thus shot to hell. The correct vitamin C level for a human being--a mammal--is c. 2000-3000 mg/kg, and the correct production rate, typical for mammals, is 60 mg-kg/day. But we lack LO, and so our level is c. 25 mg/kg and our synthesis rate is 0. By the way, we are very poor absorbers of C, and thus find it almost impossible to get past 30 mg/kg. (see chart below, Rusty)
Thus the correct path is obvious: reinstall the *gene* for GLO and be done with it, thus curing the universal scurvy. Mammals with the enzyme--which is almost all of them--can live 8-10 times their maturity age. Mammals without it have a hard time reaching 3-4 Ma. Thus this shows that reinstallation of the GLO gene would extend the human life span to c. 300 years.
One mistake, one missing enzyme, one blocked pathway--and you are dead. Ask any hemophiliac.
Our diets have changed dramatically since then. We live in much less hospitable climates. We don't pick fresh fruits from trees or eat fresh, raw meat, both excellent sources of vitamin C, like our primate ancestors. Our diets are almost universally lacking in nutritional value. Please see the Food Sources Chart. For an excellent discussion of the value of supplementation in general, please visit Healthy.net.
Vitamin C is also not stored well in our bodies. We use it constantly and yet our intake is meager. Only with regular supplementation in quantities that fill our body's every need will we approach optimum health and the elimination of disease.
Dr. Cathcart talks about how C functions in our bodies depending on the level we are getting (i.e. enough to prevent scurvy, etc.) at the following link - The Third Face of C.