The Recommended Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all (97 to 98 percent) apparently healthy individuals in a particular life stage and gender group (see Figure 1-1). The RDA is intended to be used as a goal for daily intake by individuals. The process for setting the RDA is described below; it depends on being able to set an Estimated Average Requirement (EAR). That is, if an EAR cannot be set, no RDA will be set.
The Estimated Average Requirement (EAR) is the daily intake value that is estimated to meet the requirement, as defined by the specified indicator of adequacy, in half of the apparently healthy individuals in a life stage or gender group (see Figure 1-1). A normal or symmetrical distribution (median and mean similar) is usually assumed for nutrient requirements. At this level of intake, the other half of a specified group would not have its nutritional needs met. The general method used to set the EAR is the same for all of the nutrients in this report. The specific approaches, provided in Chapters 5 through 8, differ because of the different types of data available.
The EAR is used in setting the RDA as follows. If the standard deviation (SD) of the EAR is available and the requirement for the nutrient is normally distributed, the RDA is defined as equal to the EAR plus 2 SDs of the EAR:
RDA = EAR + 2 SD EAR.
The definition of EAR implies a median as opposed to a mean, or average. The median and average would be the same if the distribution of requirements followed a symmetrical distribution and would diverge as a distribution became skewed. Three considerations prompted the choice of the term EAR: data are rarely adequate to determine the distribution of requirements, precedent has been set by other countries that have used EAR for reference values similarly derived (COMA, 1991), and the type of data evaluated makes the determination of a median impossible or inappropriate.