Vitamin C is an important aqueous phase reducing agent in vivo, which interacts with both glutathione and vitamin E to provide cellular antioxidant protection. The current recommended intake for vitamin C in the United Kingdom is 60mg, roughly the amount of vitamin C in one orange. However, higher intakes are possible if adhering to a diet high in fruits and vegetable. A mixed diet such as the Mediterranean diet may provide intakes of up to 300 mg per day, and this sort of intake has been shown to raise plasma levels to around 75 to 80 µmol/L. Higher intakes are generally only possible with supplement. In vitro evidence suggests that vitamin C at 1000 µmol/L is able to kill cancer cells, some authors and scientists such as Dr Linus Pauling recommended doses of vitamin C in the grams per day for the prevention and treatment of cancer.
Some early scientific evidence has been misrepresented to suggest that saturation of plasma with doses of 80mg of vitamin C demonstrate that higher intakes cannot be absorbed. However, these studies did not measure plasma levels above 80mg and therefore conclusions regarding higher intake are not justified. In fact more recent research has shown that very high intakes of vitamin C are absorbed, non-toxic, and effectively raise plasma levels far above intakes of 80mg. For example, oral intakes of 1.25grams of vitamin C administered to depleted subjects increased peak plasma levels to 134.8 µmol/L, but administration of the same amount intravenously raised levels to 885 µmol/L1. The maximum tolerable oral dose of vitamin C administered to subjects was 3g every 4 hours, and at this intake plasma levels rose to around 220 µmol/L. An intravenous dose of 50g administered once, increase plasma levels to 13 400 µmol/L.
These results suggest that plasma levels do indeed peak with oral vitamin C, but they attain much higher levels than suggested by some. Intravenous administration of vitamin C was able to raise urine levels to much higher concentrations than oral dosing, suggesting that the limitation of plasma levels is absorption through the gastrointestinal tract. Intravenous administration is able to increase plasma vitamin C levels over 70-fold higher than oral dosing. Administration of gram amounts of oral supplements however, increases plasma levels 2 to 3-fold higher than could be realistically achieved by even very high quality diets. However, fruits and vegetables contain many other substances that may confer health benefits, such as fibre, carotenoids and flavonoids. While oral supplements of vitamin C may be useful adjuncts to a healthy diet, they should not replace fruit or vegetables as the cornerstone of good nutrition.
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