RDAs are based on the assumption that people obtain the required nutrients daily. In reality, many people consume at least some nutrients only sporadically. Reliance on vitamin C supplements, e.g. tablets or highly fortified blended cereal-legume foods, may not be justified, especially if they are not consumed regularly. Data obtained from animal studies indicate that the frequency of nutrient supplementation is a factor that should be considered in establishing dietary recommendations (Snook et al., 1983). Human studies show that after single-dose administration, plasma levels return to their normal values in about 12 to 13 hours, no matter how much vitamin C has been consumed. This suggests that in order to maintain equilibrium in serum C levels, the vitamin should be ingested several times a day.
Snook et al. (1983) carried out a study to determine if subjects given a relatively large nutrient supplement (4 times the RDA) every 4 days maintained blood and urinary levels of the nutrient comparable to those of subjects given small doses (1/3 the RDA) with every meal. Results suggest that subjects adjust to receiving supplements of vitamin C on a periodic basis, which is contrary to the observation that serum levels maintain an equilibrium.