Recommended Dietary Allowance for Vitamin C in the United States is Also Applicable to a Population of Young Japanese Women

The recommended dietary allowance (RDA) for ascorbic acid (AA) in Canada and the United States has been set for several years at 75 mg/day for women 19-30 years old. Recently this level was questioned, and an increase to 90 mg/day was suggested. For Japanese women in the same age group, we found that the RDA for AA is currently 100 mg/day. Our goal was to determine which RDA is sufficient for maintaining a serum concentration of AA in young Japanese women above the lower reference limit of 7.0 mg/L. We measured serum AA concentrations by an ascorbate oxidase method in 176 healthy Japanese women (19-26 years old). We also performed an ROC analysis to estimate the optimal cutoff value for oral dosage to distinguish individuals with hypovitaminosis-C (<7.0 mg/L) from those with a normal serum AA. We evaluated the Japanese RDA using the 75 or 90 mg/day U.S. RDA and the weight ratio between Japanese and U.S. women, and discovered that the RDA value ranged between 66 and 79 mg/day. From the ROC analysis, we found that the optimal daily dosage of AA is approximately 75 mg/day. This value gave the highest efficiency, sensitivity, negative predictive value, and positive likelihood ratio, and the lowest negative likelihood ratio. Therefore, an RDA of 100 mg/day may be unnecessarily high for young Japanese women.