The Recommended Dietary Allowance (RDA), by definition, meets the requirements of 97.5 percent of the population. It is set from an Estimated Average Requirement (EAR) that represents an intake amount that will meet the needs of about 50 percent of the population. The RDA represents an intake amount that is 2 standard deviations above the EAR. What this means for vitamin D is that for most of the population, their requirements are met by an intake level that will achieve a serum level of the biomarker of vitamin D status, 25(OH)D of 50 nmol/L (equal to 20 ng/mL), determined to be equivalent to an RDA level of vitamin D intake (based on essentially all vitamin D from diet and minimal sunlight exposure), as illustrated in Figure 1 below.
FIGURE 1: The Reference Value distribution of serum 25(OH)D concentrations ( ) displays the values linked with the Estimated Average Requirement (EAR) that meets the needs of 50% of the generally healthy populations, the Recommended Dietary Allowance (RDA) that meets the needs of 97.5% of the population and the Tolerable Upper Level (UL) above which the risk of adverse effects increases. This Reference Value distribution was derived by using the mean (95th percentile) specified by the Institute of Medicine with calculated SD = 2.0 ng/ml on the basis of normality; the estimated probability function (y-axis) indicates the frequency of each concentration in the sample. Also displayed is the current observed distribution of serum 25(OH)D concentrations () for adults aged 19-70 y in NHANES 2005-2006 (n=3871). For comparison, the shift in the distribution to achieve serum 25(OH)D concentrations above the RDA-linked value of 20 ng/ml in 97.%% of the population () is also shown. Reproduced from Brannon PM, Mayne ST, Murphy SP, Taylor CL. Vitamin D Supplementation in African Americans: Dose-response. American Journal of Clinical Nutrition 2014;100:982-4 with permission from American Journal of Clinical Nutrition.
In its analysis, the IOM committee used a mixed-model approach to estimate the dose-relationship between total dietary intake of vitamin D to achieve the desired serum level of 25(OH)D. In its methodology to determine DRIs for vitamin D, the IOM committee used an estimated dose-response to inform their judgment of the intake needed to achieve serum 25(OH)D levels in the desired range. Because of limitations in the available data, the IOM committee made the judgment to overestimate the dose of dietary vitamin D needed; 600 IU daily would meet the RDA based on a serum 25(OH)D level of 50 nmol/L derived from the estimated dose-response relationship.
In this approach, the Estimated Average Requirement was determined to be equivalent to an intake that produced a serum level of 40 nmol/L (based on essentially all vitamin D from diet and minimal sunlight exposure). Thus, because this level represents the RDA for dietary vitamin D, the 2 standard deviations has, in effect, already been added and therefore no additional adjustment is required or appropriate. Using the distribution to 97.5 percent to assure a low prevalence of inadequacy among the healthy population groups, the RDA value was based on an achieved level of 50 nmol/L (equal to 20 ng/mL). To be clear, the goal is not, and should not be, to assure that 97.5% of the population exceeds the serum value linked to the RDA. Doing so would shift the distribution to a higher level that is associated with increased risk for adverse effects as illustrated in Figure 1 as Distribution to achieve RDA-linked concentration.
The Tolerable Upper Level (UL) is another Dietary Reference Intake (DRI) value that represents the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to most individuals in a population. Based on the committee’s review of the evidence, the UL is 4,000 IU/day. An intake level above 4,000 IU daily exceeds the UL for all age and sex groups.