The RDA is the average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all healthy individuals in a particular life stage and gender group. If the distribution of requirements in the group is assumed to be normal, then the RDA is the value that exceeds the requirements of 97 to 98 percent of the individuals in the group (Figure 1-1). Under the assumption of normality, the RDA can be computed from the EAR and the standard deviation of requirements (SD_REQ) as follows:

RDA = EAR + 2 SD_REQ

If the distribution of requirements is normal, 97 to 98 percent of the individuals in the group will have a requirement that is below the RDA. The RDA is intended for use primarily as a goal for usual intake of individuals. Because the RDA is derived directly from the EAR, if data are insufficient to establish an EAR, no RDA can be set.

FIGURE 1-1 Dietary reference intakes. This figure shows that the Estimated Average Requirement (EAR) is the intake at which the risk of inadequacy is 0.5 (50 percent) to an individual. The Recommended Dietary Allowance (RDA) is the intake at which the risk of inadequacy is very small —only 0.02 to 0.03 (2 to 3 percent). The Adequate Intake (AI) does not bear a consistent relationship to the EAR or the RDA because it is set without being able to estimate the requirement. At intakes between the RDA and the Tolerable Upper Intake Level (UL), the risks of inadequacy and of excess are both close to 0. At intakes above the UL, the risk of adverse effects increases.