Information from the Boston Nutritional Status Survey of the Elderly estimated that 35 and 44 percent of the males and females, respectively, took some form of vitamin C supplements; while 19 percent of males and 15 percent of females surveyed who took supplements had intakes greater than 1,000 mg (5,680 µmol)/day. Approximately 31 percent of all adults in one 1986 survey reported taking a vitamin C supplement (Moss et al., 1989). Total vitamin C intakes from food plus supplements from the Third National Health and Nutrition Examination Survey (NHANES III) are found in Appendix Table C-2.
The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects in almost all individuals. Although members of the general population should be advised not to exceed the UL routinely, intake above the UL may be appropriate for investigation within well-controlled clinical trials. In light of evaluating possible benefits to health, clinical trials of doses above the UL should not be discouraged, as long as subjects participating in these trials have signed informed consent documents regarding possible toxicity and as long as these trials employ appropriate safety monitoring of trial subjects. Also, the UL is not meant to apply to individuals who are receiving vitamin C under medical supervision.
Many people believe vitamin C to be nontoxic and beneficial to health; therefore, the vitamin is often taken in large amounts. There is no evidence suggesting that vitamin C is carcinogenic or teratogenic or that it causes adverse reproductive effects. Reviews of high vitamin C intakes have indicated low toxicity (Johnston, 1999); adverse effects have been reported primarily after very large doses (greater than 3 g/day). Data show little increase in plasma steady-state concentrations at intakes above 200 mg/day (Figure 5-3), and saturable intestinal absorption and renal tubular reabsorption data suggest that overload of ascorbic acid is unlikely in humans (Blanchard et al., 1997; Levine et al., 1996a). Possible adverse effects associated with very high intakes have been reviewed and include: