In vitro studies show that vitamin C has the ability to modulate the function of immune cells. For example, vitamin C can enhance the motility of neutrophils and monocytes. Vitamin C deficiency in humans causes detrimental effects on the immune system, and administration of vitamin C to deficient individuals can improve immunity for example by increasing the motility of neutrophils. Vitamin C may also alter lymphocyte function and interferon production. The effects of vitamin C on the immune system in humans is therefore fairly well established. However, it is unknown as to the optimal dose required to maximise immunity. Evidence shows that plasma levels of vitamin C do not become saturated until gram amounts (about 2.5 grams per day per 70 kg individual) are ingested. Further work with animals, as reported by Linus Pauling, show that most mammals produce gram amounts of vitamin C per day endogenously when body size is corrected to that of a typical human.
Gram doses of vitamin C are controversial. This controversy is derived mainly from mainstream medical established professionals who continue to believe that the typical Western diet provides all the nutrients required for health. Nutritionists however have generally long believed that additional vitamin C is required for optimal health. Even the best high fruit and vegetable diet can only provide around 600 mg per day of vitamin C, and yet plasma levels continue to increase to doses up to around 2.5 grams. Evidence from studies showing that the severity of symptoms, and the duration of symptoms of a common cold can be reduced by administration of gram amounts of vitamin C add further weight to the contention that supplemental vitamin C benefits immunity. While many studies have shown no effect for vitamin C on immunity, this absence of proof is not proof of absence. Study design and in particular subject selection can bias results in favour of no effect and so care must be taken when interpreting such studies.
A number of well designed studies have shown beneficial effects for vitamin C on immunity. For example, in one study1, researchers assessed the functions of neutrophils and monocytes following the ingestion of steadily increasing doses of vitamin C in healthy volunteers. Subjects ingested 1 gram of vitamin C daily for 1 week, 2 grams daily for a second week, and then 3 grams daily for a third week. Ingestion of the 2 and 3 gram amounts of vitamin C, but not the 1 gram amount, enhanced neutrophil motility in response to a chemotactic stimulus. It is unclear whether the 1 gram doses of vitamin C was too low, or if the dose was not administered for long enough to have an effect. All three doses of vitamin C increased the lymphocyte proliferation to phenohaemogluutanin and concanavalin A, a measure of T-cell function. These results suggest that vitamin C in gram amounts may stimulate immunity. The recommendation to consume 2 to 3 grams of vitamin C for improved immunity therefore seem justified.
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