Does Vitamin C Efficacy Really Exist?

Vitamin C is an essential nutrient that is well known for its ability to neutralize free radical molecules that can cause damage to cells. It is also involved in the body’s immune system by stimulating the activity of white blood cells. In the context of these roles, there is interest in whether a dietary intake higher than the recommended daily allowance (RDA) might protect against certain diseases, e.g. infections such as the common cold and potentially more serious illnesses such as cancer.

However, clinical studies examining these issues have produced conflicting results. The potential reasons for this are complex and include a number of factors. One issue is that vitamin C displays pharmacokinetic properties that make it different from the typical low molecular weight drug. Unlike drugs that display 1st order kinetics (where the plasma and tissue concentration is proportional to the administered dose and independent of the route of administration), vitamin C displays saturation kinetics. This means that the maximum achievable concentration of vitamin C in the plasma is determined by its cellular uptake capacity. The plasma concentration can therefore be quite high even after a very small dose, whereas the tissue concentration remains at a deficient level.

Furthermore, vitamin C rapidly oxidizes upon exposure to air and thus has a limited shelf life. This makes it important to choose products that are stored in a dark/opaque container or that have been packaged in such a way that they are protected from light. This may increase the cost of the product but will help to maintain a more stable and efficacious vitamin C preparation.

Another problem is that the vitamin C status of participants in any given study is affected by their dietary habits and supplementation practices prior to the trial. If, for example, the participants in a study consume large quantities of oral supplements, their baseline vitamin C level will be much higher than that of a placebo group and any intervention effect would likely be obscured. This is a particular problem with observational studies that only assess plasma vitamin C levels rather than taking blood samples from the participants at various time points during the follow-up period.

A final factor that limits the possible effects of vitamin C is that, in contrast to most other species, humans do not have the capacity to increase biosynthesis of vitamin C when deprived of it. Consequently, dietary intake of vitamin C is critical to optimal health and the only practical method for achieving higher than the RDA amounts is through supplemental use. Therefore, the question of whether a dietary supplement higher than the RDA offers any additional health benefits requires careful consideration.