How Are the RDAs Determined?

The RDAs are determined based upon in-depth research studies, including those performed to determine “balance.” Balance studies are designed to determine how much of a specific nutrient humans need to eat in order to balance that which is normally lost daily from the body and to maintain appropriate levels of that nutrient in the body. When these studies were performed, scientists observed that there was quite a bit of variability among the balances of different individuals. A hypothetical representation of a particular nutrient’s balance is depicted in Figure 3.1.
 

Based on balance studies, researchers are able to determine estimated average requirements for specific gender and age groups for a given nutrient. Here they add up all of the individual balance levels and divide by the total number of people they assessed. The result is the Estimated Average Requirement or EAR. In this figure we see that the RDA for this nutrient is set well above the EAR. In fact the RDA is set two standard deviations (a measure of statistical variation) above the average. By doing so the RDA would be adequate to meet the needs of 97 to 98 percent of the people in the study. Knowing this, the RDAs will provide more of the nutrient than needed for balance for most individuals. From this we can certainly understand that the RDAs are not really personal recommendations but are more appropriate for making recommendations for populations. For example, the RDA for vitamin C for adult women of all ages is

75 milligrams, which should avoid deficiency and promote general health for about 97 to 98 percent of adult women. It should be noted that the recommendation for energy was not set to include 97 to 98 percent of the population, but only 50 percent (see Figure 3.1). If the recommendation was set to include 98 percent of
the population this might lead to weight gain for many people using the recommendation for energy as a guideline.
 

Beyond balance studies, other research studies involving the relationship between the essential nutrients and the body are reviewed to help determine the RDA. For example, the RDA for many nutrients during the years of rapid growth and during pregnancy must account for balance as well as an additional amount of a nutrient to allow for these periods of rapid growth. Furthermore, RDA determinations do not take into consideration acute disease, medications, or exercise training. Only recently have RDA considerations included chronic diseases such as osteoporosis and heart disease. However, at this time the RDAs and AIs are still considered to be below a level that would optimally support the prevention of several major diseases such as heart disease and cancer.