Data presented in Table B-2, Table B-3, Table B-4 through Table B-5 indicate that it is not possible to use this approach to assess the adequacy of vitamin A, vitamin C, vitamin E, and some other nutrients. In these cases, the distribution of daily intakes cannot be assumed to be normal, and thus observed daily intake cannot be used to carry out the assessment.
Because the distributions of daily intake for many nutrients are nonnormal, more research is needed in order to extend this methodology to all nutrients of interest.
The proposed approach relies also on normality of the requirement distribution. When requirements are not distributed in a symmetrical, approximately normal fashion around the EAR, results may be biased. For example, the confidence with which it can be concluded that intake is adequate may be less than 85 percent even though the observed ratio D/SDD is equal to 1.
Iron is an example of a nutrient for which the distribution of requirements is not normal. Iron requirements in menstruating women are skewed, with a long tail to the right. In this situation, the method described above does not produce reliable results. No alternative can be offered at this time; more research is needed in this area.
Until now, little if any attention has been paid to reliably estimating the variance of nutrient requirement distributions. DRI reports (IOM, 1997, 1998b, 2000) have assumed that the CV of requirements for most nutrients is 10 percent of the EAR, unless other information is known (e.g., niacin is given as 15 percent). Given an EAR and a CV of requirement, an SD of requirement can be calculated as SDr= CV × EAR. For example, if the EAR of a nutrient is 120 units/day and the CV of requirement is 10 percent, then the SD of requirement will be 0.1 × 120 = 12 units/day.
It is not clear that the fixed 10 percent (or 15 percent) CV estimates across nutrients result in reliable estimators of the SD of requirement. Since the SD of requirement is an important component of the SD of D, an inaccurate value of SDr will result in an inaccurate value of SDD and hence an inaccurate value of the ratio D/SDD.
At this time, no better alternatives than using the CV of the requirement as given in the DRI reports have been identified, and thus the results of such analyses should be interpreted with caution.